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UNIVERSIDADE FEDERAL DO RIO GRANDE DO NORTE
CENTRO DE BIOCIÊNCIAS
DEPARTAMENTO DE ECOLOGIA
PROGRAMA DE PÓS-GRADUAÇÃO EM ECOLOGIA
Adição de policloreto de alumínio e remoção de
peixes bentívoros como técnicas de restauração
de lagos rasos do semiárido brasileiro
Fabiana Oliveira de Araújo Silva
Orientador: Prof. Dr. José Luiz Attayde, UFRN
Co-orientadora: Profa. Dra. Vanessa Becker, UFRN
Natal – RN
2015
2
Adição de policloreto de alumínio e remoção de
peixes bentívoros como técnica de restauração de
lagos rasos do semiárido brasileiro
Fabiana Oliveira de Araújo Silva
Tese apresentada ao Programa de Pós-graduação em
Ecologia da Universidade Federal do Rio Grande do
Norte como parte integrante dos requisitos para
obtenção do grau de Doutora em Ecologia.
Orientador: Prof. Dr. José Luiz Attayde
Co-orientadora: Profa. Dra. Vanessa Becker
Natal – RN
2015
Catalogação da Publicação na Fonte. UFRN / Biblioteca Setorial do Centro deBiociências
Silva, Fabiana Oliveira de Araújo.Adição de policloreto de alumínio e remoção de peixes bentívoros
como técnicas de restauração de lagos rasos do semiárido brasileiro /Fabiana Oliveira de Araújo Silva. – Natal, RN, 2015.
88 f.: il.
Orientador: Prof. Dr. José Luiz Attayde.Coorientadora: Profa. Dra. Vanessa Becker.
Tese (Doutorado) – Universidade Federal do Rio Grande do Norte.Centro de Biociências. Programa de Pós-Graduação em Ecologia.
1. Biomanipulação. – Tese. 2. Fósforo. – Tese. 3. Restauração. –Tese. I. Attayde, José Luiz. II. Becker, Vanessa. III. Universidade Federaldo Rio Grande do Norte. IV. Título.
RN/UF/BSE-CB CDU 574
4
Adição de policloreto de alumínio e remoção de peixes bentívoros como técnica derestauração de lagos rasos do semiárido brasileiro
Fabiana Oliveira de Araújo Silva
Orientadores: Dr. José Luiz Attayde & Dra. Vanessa Becker
Tese apresentada ao Programa de Pós-graduação em Ecologia da Universidade Federaldo Rio Grande do Norte como parte integrante dos requisitos para obtenção do grau deDoutora em Ecologia.
Aprovada por:
________________________Presidente Prof. Dr. José Luiz Attayde, UFRN
________________________Profa. Dra. Vanessa Becker, UFRN
_______________________Prof. Dr. André Megali Amado, UFRN
________________________Profa. Dra. Renata Panosso, UFRN
_______________________Prof. Dr. José Etham de Lucena Barbosa, UEPB
_______________________Prof. Dr. André Luis Calado Araújo, IFRN
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Agradecimentos
Primeiramente gostaria de agradecer ao meu amigo e orientador Coca (Prof. JoséLuiz Attayde) pela oportunidade e aprendizado ao longo dos últimos oito anos.Agradeço imensamente pelo carinho, pela confiança, conselhos, amizade, por tudo!
Agradeço também à Profa. Vanessa Becker, uma amiga e orientadora que tive aoportunidade de conhecer pouco antes de ingressar no doutorado, que sempre meincentivou e acreditou em mim. Obrigada pelo carinho, atenção, cuidado eensinamentos. Sua força e determinação me inspiram.
Agradeço ao Programa de Pós-Graduação em Ecologia, ao Conselho Nacionalde Desenvolvimento Científico e Tecnológico (CNPq) pelo financiamento da pesquisa eà Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) pelaconcessão de bolsa de doutorado e ao Projeto CAPES-Universidade de Wageningenpela concessão da bolsa de estágio de doutorado no exterior.
Agradeço ao Prof. Miquel Lurling, orientador na Universidade de Wageningen,pelos ensinamentos e oportunidade concedida. Agradeço também ao amigo Frank pelaimensa ajuda e orientação com os experimentos em laboratório, sem esquecer da ajuda edo carinho da Wendy e do John. Muito obrigada!
Agradeço aos amigos brasileiros que fiz na Holanda: Marina, Felipe, Thadeu e,em especial, a Luciana Rangel. A amizade de vocês fez toda a diferença nesse períodoque morei longe da minha família e amigos. Muito obrigada pela companhia, passeios,risadas, conselhos e amizade. Lu, agradeço imensamente a Deus por ter colocado vocêem minha vida num momento tão difícil pra mim. Obrigada pelo seu carinho e amizade!
Muito obrigada aos meus queridos amigos Mariana, Marcolina, Gabi, Pablo eDanyhelton – muito obrigada pela ajuda com os experimentos, com a tese, conselhos,pelo apoio, por sempre me incentivarem, pelas reuniões na universidade ou na mesa dobar... Muito obrigada pela amizade de vocês! A batalha é grande, mas juntos somosmais fortes!
Agradeço também aos amigos do LEA (Laboratório de Ecologia Aquática):Elinez, Rosemberg, Leonardo, Alex, Bárbara, e aos amigos do LARHISA (Laboratóriode Recursos Hídricos e Saneamento Ambiental): Jurandir, Ângela, Aline, Laíssa, pelaajuda nos experimentos ou com a tese, e pela amizade.
Agradeço aos meu queridos amigos (ex)biólogos e afins: Cabeça, Ricardo,Tiego, Pan, Débora, Kívia, Mary, Fernanda e Nara. Muito obrigada pela amizade devocês!
Agradeço a minha querida amiga Esther, um anjo que Deus colocou em minhavida, pela sua amizade, carinho, cuidados, apoio, incentivo, conselhos, por me ouvir ecompartilhar bons momentos. Você é muito especial pra mim! Muito obrigada!
Quero agradecer aos Professores André Megali e Renata Panosso pelascontribuições na qualificação.
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Agradeço também ao Professor Hélio Rodrigues pela parceria em um doscapítulos desta tese. Muito obrigada pelas contribuições e por compartilhar seuconhecimento.
Por fim, gostaria de agradecer a Deus pela minha família, meu alicerce: meuspais Josemar e Fátima, meu irmãos Janaína e Júnior, e ao meu marido Rodrigo. Muitoobrigada pelo amor, por dividir alegrias e pelo apoio nos momentos difíceis. Se chegueiaté aqui, foi graças a vocês! Agradeço também a Rodrigo pelo apoio, companheirismo,dedicação, e por abdicar de muita coisa e me acompanhar no meu estágio no exterior.Foi muito importante ter você ao meu lado durante esse período importante da minhavida.
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Sumário
Resumo ............................................................................................................................. 8
Abstract............................................................................................................................. 9
Introdução....................................................................................................................... 10Dinâmica do Fósforo em Lagos.................................................................................. 11Restauração de Lagos Eutrofizados através de Métodos Físicos ............................... 12Restauração de Lagos Eutrofizados através de Métodos Químicos ........................... 13Restauração de Lagos Eutrofizados através da Biomanipulação ............................... 17O semiárido e a Problemática da Qualidade da Água ................................................ 18Objetivos..................................................................................................................... 20
CAPÍTULO I. Effects of polyaluminium chloride and lanthanum modified bentonite onthe growth rates of three Cylindrospermopsis raciborskii strains.................................. 22
Abstract................................................................................................................... 221. Introduction ........................................................................................................ 232. Materials and methods........................................................................................ 253. Results ................................................................................................................ 284. Discussion........................................................................................................... 295. Conclusions ....................................................................................................... 32References .............................................................................................................. 33
CAPÍTULO II. Shallow lake restoration by the combined effects of polyaluminiumchloride addition and benthivorous fish removal: a field mesocosm experiment. ......... 43
Abstract................................................................................................................... 431. Introduction ........................................................................................................ 442. Material and methods ......................................................................................... 463. Results ................................................................................................................ 474. Discussion........................................................................................................... 48Rerences.................................................................................................................. 49
CAPÍTULO III. The use of polyaluminium chloride as a restoration measure to improvewater quality in tropical shallow lakes ........................................................................... 58
Abstract................................................................................................................... 581. Introduction ........................................................................................................ 592. Material and methods ......................................................................................... 603. Results ................................................................................................................ 624. Discussion........................................................................................................... 63References .............................................................................................................. 65
Considerações Finais ...................................................................................................... 73
Referências ..................................................................................................................... 75
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Resumo
A eutrofização cultural é a causa mais comum de deterioração da qualidade da
água no mundo. Este processo se dá pela entrada excessiva de nutrientes, especialmente
nitrogênio e fósforo, nos corpos aquáticos causando florações de algas e cianobactérias.
Em lagos rasos esses efeitos são mais acentuados devido a uma maior interação do
corpo aquático com o entorno, com o ar e o sedimento. Existem várias técnicas de
restauração de lagos eutrofizados, com uma vasta gama de resultados bem sucedidos,
mas no Brasil há apenas um único caso de restauração bem sucedida: o lago Paranoá em
Brasília. A região semiárida brasileira possui milhares de lagos artificiais,
regionalmente chamados de açudes, em sua maioria rasos e eutróficos. A eutrofização
desses corpos aquáticos é documentada e o fitoplâncton desses ambientes é
frequentemente dominado por cianobactérias potencialmente produtoras de toxinas. O
principal objetivo deste trabalho é testar diferentes técnicas de restauração da qualidade
da água que possam ser facilmente aplicadas em lagos rasos do semiárido brasileiro.
Resultados de um experimento em laboratório sugerem que a aplicação de argila
adsorvente de fósforo associada a um coagulante à base de alumínio é uma técnica
efetiva na remoção do fósforo solúvel reativo e na diminuição da taxa de crescimento da
Cylindrospermopsis raciborskii, cianobactéria potencialmente tóxica que domina nos
reservatórios do semiárido brasileiro; porém esse efeito é dependente da biomassa no
momento da aplicação da técnica. Os resultados de um experimento de campo realizado
em mesocosmos num lago raso eutrofizado demonstraram que a aplicação de
coagulante à base de alumnínio em conjunto com a da remoção de peixes bentívoros é
mais eficiente na remoção de fósforo total e clorofila-a da coluna de água do que a
aplicação isolada de apenas uma dessas técnicas. Por fim, testes de laboratório
demostraram que o coagulante à base de alumínio apresentou um bom desempenho em
remover turbidez e fósforo total em testes de bancada com água de seis reservatórios do
semiárido, sendo a eficiência reduzida com o aumento da biomassa algal e do pH. Os
resultados deste estudo mostram que é possível melhorar a qualidade da água de
reservatórios eutrofizados no semiárido através do controle da carga interna de
nutrientes seja pela precipitação e inativação do fósforo no sedimento, como também
pela inibição da liberação do fósforo no sedimento por peixes bioturbadores, e que os
resultados são aditivos quando as técnicas são aplicadas em conjunto.
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Abstract
Eutrophication is the most common cause of water quality degradation in the
world. This process occurs by excessive nutrients inputs, nitrogen and phosphorus, to
the aquatic systems resulting in algal and cyanobacterial blooms. In shallow lakes these
effects are pronounced due to the higher interaction of the lake with watershed, air and
sediment. There are innumerous restoration techniques of eutrophied lakes with a range
of successful results but there is only one case of successful lake restoration in Brazil:
Paranoá Lake in Brasília city. The Brazilian semiarid region has many artificial lakes,
named açudes, which are mostly eutrophic and shallow lakes. The eutrophication in
these lakes is reported and the phytoplankton community is dominated by potentially
toxic cyanobacteria species, mainly Cylindrospermopsis raciborskii. The aim of this
thesis is to test techniques for water quality management which can be easily applied in
Brazilian semiarid lakes. Results from a laboratory experiment suggest that the addition
of a phosphorus sorbent clay associated with an aluminium based coagulant is an
effective technique in removing soluble reactive phosphorus and reducing C. raciborskii
growth rate – cyanobacteria potentially toxic dominant in reservoirs of Brazilian
semiarid – but this effect is dependent on the biomass in the application moment.
Results from a field experiment in mesocosm in a eutrophied lake showed that the
addition of aluminium based coagulant and removal of benthivorous fish is more
efficient in removing total phosphorus and chlorophyll-a from water column than the
isolated application of one of the techniques. Lastly, laboratory tests showed that
aluminium based coagulant exhibited good performance in removing turbidity and total
phosphorus from water of six reservoirs but the efficiency was reduced by algal biomass
and pH. The results of this study showed that the improvement in water quality of
eutrophied reservoirs in semiarid region is possible through internal loading control by
phosphorus precipitation and inactivation in sediments or inhibition of phosphorus
release by benthivorous fishes, and also that these results show are additives in water
quality improvement.
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IntroduçãoAo longo dos anos, o crescimento da população humana, o consequente
desenvolvimento industrial e a expansão da agricultura, têm gerado uma forte tensão
sobre os ecossistemas de água doce disponíveis no mundo. O uso de fertilizantes pela
agricultura, a produção intensiva de animais e outros usos inadequados da terra têm
alterado os ciclos biogeoquímicos e acelerado a entrada de nutrientes para os sistemas
aquáticos (Vitousek et al., 1997). Além disso, os sistemas aquáticos funcionam como
receptores de água não tratada (esgotos domésticos e industriais) em muitos lugares no
mundo (Carpenter et al., 1998). A eutrofização, processo natural de enriquecimento do
sistema aquático pela entrada de nutrientes, principalmente o nitrogênio (N) e o fósforo
(P), têm sido acelerada pelas atividades antropogênicas, levando à deterioração da
qualidade das águas superficiais. A eutrofização é considerada o problema de qualidade
da água mais importante no mundo (Smith & Schindler, 2009).
Lagos eutrofizados experimentam diversos problemas que inviabilizam o seu
uso múltiplo. O enriquecimento por nitrogênio e fósforo resulta no aumento da
biomassa fitoplanctônica, levando à redução da transparência da água (Smith, 1998). A
decomposição dessa grande quantidade de matéria orgânica resulta em baixos níveis de
oxigênio dissolvido, o que pode levar à morte de peixes. Muitas vezes a eutrofização
está associada ao desenvolvimento de florações de cianobactérias potencialmente
produtoras de toxinas que são nocivas a muitos animais, inclusive ao homem (Chorus &
Bartram, 1999), representando assim uma ameaça aos recursos aquáticos e a saúde
pública (Smith & Schindler, 2009; Paerl et al., 2011). Além disso, essas florações
liberam outras substâncias químicas que causam gosto e odor na água (Chorus &
Bartram, 1999) aumentando assim os custos com o tratamento. A eutrofização resulta
na degradação da qualidade da água e na perda dos serviços que este recurso provém
(Smith, 2003), podendo esta ser a principal causa de escassez de água no mundo (UN-
Water, 2007).
O manejo e a recuperação de corpos aquáticos eutrofizados se apresenta como
urgência frente ao atual quadro de eutrofização no mundo, principalmente onde há
escassez de água. Diante da baixa disponibilidade, da má distribuição no espaço e no
tempo e da crescente demanda frente ao crescimento populacional, se faz necessário o
uso racional deste recurso para que se possa assegurar quantidade, qualidade e acesso a
toda a população. A perda deste recurso, associado à perda dos serviços por ele
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oferecidos (Postel & Carpenter, 1997), tem levado a criação de diversas estratégias de
restauração de lagos eutrofizados ao longo dos últimos anos.
Dinâmica do Fósforo em LagosA principal medida para controle da eutrofização e as consequentes florações de
cianobactérias é a redução da carga externa de nutrientes, principalmente o fósforo, a
níveis que possam limitar a produção primária (Cooke et al., 2005; Carpenter, 2008;
Schindler et al., 2008; Schindler, 2012). As fontes pontuais de poluição, representadas
pelas descargas diretas de efluentes domésticos e industriais, são facilmente controladas
quando comparadas com as fontes difusas (Carpenter et al., 1998). Estas, por sua vez,
referem-se ao escoamento de nutrientes oriundos da agricultura, do pasto e de outros
locais na bacia de drenagem, através da lixiviação do solo e consequente destino final
no corpo aquático, e representam um desafio no que diz respeito ao monitoramento e
redução das cargas. Em muitos casos a redução da entrada externa de nutrientes para os
sistemas aquáticos resultam na redução do estado trófico e produção primária, e
aumento na transparência da água (Marsden, 1989; Jeppesen et al., 2005). No entanto,
muitos lagos rasos apresentam retardo ou falha no processo de restauração após a
redução da entrada externa de nutrientes (Marsden, 1989; Jeppesen et al., 1991; Van der
Molen & Boers, 1994; Søndergaard et al., 2000), fato que tem sido atribuído
principalmente à carga interna de fósforo (Søndergaard et al., 1999, 2003; Cooke et al.,
2005).
Quando o fósforo entra nos lagos, parte é consumida e incorporada nos
organismos e a outra parte é retida no sistema pela sedimentação (Søndergaard et al.,
2001). O fósforo incorporado pelos organismos eventualmente será sedimentado e
depositado no sedimento dos lagos. Com o tempo, a decomposição de matéria orgânica
rica em fósforo leva ao enriquecimento do sedimento. Vários processos químicos e
biológicos resultam na retenção do fósforo neste compartimento do lago. O ferro,
alumínio, cálcio, manganês, partículas de argila e matéria orgânica estão envolvidos na
retenção química e na adsorção do fósforo no sedimento (Søndergaard, 2007). A
capacidade do sedimento em reter o fósforo está diretamente relacionado à presença e
concentração desses compostos, como também das condições químicas, pH e potencial
de oxi-redução, no sedimento (Jensen & Andersen, 1992; Jensen et al., 1992;
Søndergaard et al., 2003; Søndergaard, 2007). O balanço entre o que é retido e o que é
liberado do sedimento representa a carga interna de fósforo para o sistema. Esse balanço
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varia entre lagos e é determinado principalmente pelo histórico do lago, a composição e
a capacidade do sedimento de reter o fósforo (Pettersson, 1998). Lagos que receberam
por muito tempo uma alta carga externa de fósforo possuem sedimentos ricos em
fósforo o que permite sustentar a eutrofização por muito tempo, mesmo quando cessam
as entradas externas, o que explica o atraso na recuperação (Marsden, 1989; Jeppesen et
al., 1991; Van der Molen & Boers, 1994; Søndergaard et al., 2000).
Lagos eutrofizados que experimentam processos de estratificação tem a
liberação de fósforo do sedimento relacionada às condições de anoxia na superfície do
sedimento. Na ausência de oxigênio na interface água-sedimento, o ferro e o alumínio
tornam-se solúveis liberando assim o fosfato. Lagos rasos geralmente são bem
misturados e possuem a coluna de água bem oxigenada, e a liberação de fósforo do
sedimento ocorre em condições aeróbicas (Boström et al., 1988; Jensen & Andersen,
1992; Jeppesen et al., 1997). Neste caso, outros fatores são determinantes na liberação
do fósforo do sedimento: pH, disponibilidade de nitrato, atividade de bactérias,
processos de mineralização, ressuspensão e mistura do sedimento por eventos físicos ou
organismos bioturbadores (Søndergaard et al., 2001; Hupfer & Lewandowski, 2008).
Em lagos rasos, a liberação do P do sedimento é particularmente importante uma vez
que há uma maior interação sedimento-água quando comparados com lagos profundos
(Søndergaard et al., 2003), podendo constituir uma fonte significante deste nutriente e
ainda exceder a carga externa de P (Boers et al., 1998; Søndergaard et al., 1999).
Portanto, na restauração de lagos rasos eutrofizados é extremamente importante levar
em consideração a carga interna de fósforo. Durante as últimas décadas, estratégias
adicionais à redução da carga externa de fósforo vêm sendo desenvolvidas e testadas na
restauração de lagos rasos eutrofizados. Essas estratégias envolvem métodos físicos,
químicos e biológicos, apresentados a seguir.
Restauração de Lagos Eutrofizados através de Métodos FísicosA redução da concentração de nutrientes na água visando a limitação da
produção primária pode ser feita através da diluição ou pelo aumento da taxa de troca
(flushing) de água do lago (Welch, 1981). Em lagos estratificados, a remoção da camada
hipolimnética de água rica em nutrientes e pobre em oxigênio (Nurnberg, 1987a)
representa uma solução, sendo dependente da manutenção da termoclina (Nurnberg,
1987b). Essas técnicas apresentam resultados positivos em muitos casos (Cooke et al.,
2005). Contudo, tais técnicas necessitam de um grande volume de água com baixa
13
concentração de nutrientes para que seja possível a diluição ou a troca pela água
eutrofizada, para que o nível da água do lago permaneça relativamente constante. A
adição/remoção de água está associada à entrada de água com baixas concentrações de
nutrientes para a manutenção do nível de água do lago, sendo inviável e de elevado
custo em regiões que apresentam a escassez desse recurso. Além disso, essa técnica
apresenta problemas quanto ao descarte da água de má qualidade (baixas concentrações
de oxigênio e altas concentrações de nutrientes e metais) removida (Kumar, 2008) e
altos custos com o tratamento desta água para uso.
Outra estratégia de restauração envolve a oxigenação mecânica da camada
hipolimnética anóxica, em lagos estratificados, ou a circulação de toda camada de água
(Cooke et al., 2005), o que permite a retenção de fósforo no sedimento (Nurnberg,
1987b) e boas condições para os peixes. Ambas as técnicas tem sido empregadas com
sucesso em muito lagos (Cooke et al., 2005), mas há casos em que o aumento da
concentração de oxigênio não apresentou efeito significante na carga interna de fósforo
(Gächter & Wehrli, 1998).
A remoção do sedimento rico em nutrientes pode ser uma alternativa em lagos
que sofrem com a contínua liberação de fósforo do sedimento. Embora efetiva em
reduzir a liberação de fósforo do sedimento, esta técnica é pouco aplicada por ser cara e
apresentar problemas com a ressuspensão de partículas e matéria orgânica que
promovem aumento da turbidez e depleção de oxigênio, respectivamente (Jeppesen et
al., 2007), remoção de alimento para peixes bentívoros e habitat para organismos
bentônicos, além dos problemas com a disposição adequada para o sedimento removido
(Cooke et al., 2005). Além disso, a nova superfície do sedimento pode apresentar baixa
capacidade de retenção do fósforo (Søndergaard et al., 2007).
Restauração de Lagos Eutrofizados através de Métodos QuímicosOs métodos químicos para a restauração de lagos eutrofizados envolvem a
melhoria ou o aumento da capacidade do sistema em reter fósforo e a diminuição da
carga interna desse nutriente no lago. Este objetivo pode ser alcançado pela precipitação
de fósforo presente na água do lago e inativação do fósforo no sedimento (Cooke et al.,
2005), ou indiretamente através dos processos envolvidos na retenção do fósforo no
sedimento.
A capacidade de retenção do fósforo no sedimento poder ser aumentada pela
adição de nitrato visando o aumento da mineralização da matéria orgânica e a prevenção
14
da depleção de oxigênio no sedimento (Foy, 1986; Søndergaard, 2007; Jiang et al.,
2008). Foi observado que adição de nitrato pode inibir a liberação do fósforo de
sedimentos anóxicos e óxicos (Andersen, 1982). Contudo, tais medidas devem ser
consideradas em situações onde a liberação do fósforo do sedimento está associada às
condições de anoxia.
A adição de sais de metais (alumínio e o ferro) e outros compostos (argilas
naturais ou modificadas) adsorventes de fosfato ao lago promovem a precipitação do
fósforo presente na água e a inativação deste no sedimento. Coagulantes à base de ferro
e alumínio são comumente utilizadas no tratamento de água e efluentes (Jiang &
Graham, 1998a) e tem sido utilizada como medida de restauração de lagos eutrofizados
através da precipitação do fósforo (Cooke et al., 2005). Ao serem adicionados à água, o
ferro e alumínio reagem com o fosfato formando precipitados, pelo processo de
coagulação e floculação, que então são removidos da coluna de água pelo processo de
sedimentação. Além da remoção do fósforo dissolvido, a coagulação/floculação pelo
ferro e alumínio remove a matéria orgânica e inorgânica (Jiang et al., 1993; Jiang &
Graham, 1998b; Drikas et al., 2001; Hullebusch et al., 2002), e reduzem a concentração
de fósforo total presente nas partículas suspensas após a sedimentação dos flocos
(Reitzel et al., 2003; Auvray et al., 2006).
Coagulantes à base de ferro (sulfato férrico [Fe2(SO4)3] ou cloreto férrico
[FeCl3]) tem sido utilizado para a remoção de fósforo (Yamada et al., 1986; Deppe &
Benndorf, 2002) ou para a remoção da biomassa de cianobactérias (Chow et al., 1998)
pela precipitação dos flocos formados. Para a inativação do fósforo sedimentado, o
sedimento precisa manter-se oxidado para evitar a redução do ferro e a consequente
liberação do fósforo (Nurnberg, 1994).
Coagulantes à base de alumínio apresentam vantagem sobre os coagulantes à
base de ferro, pois a efetividade não é dependente das condições redox. Em virtude
disso, o alumínio tem sido amplamente usado na restauração de lagos (Cooke et al.,
2005). Dentre os coagulantes à base de alumínio, o sulfato de alumínio (Al2(SO4)3) é o
mais comumente utilizado e sua efetividade em remover fósforo tem sido reportada
baseada em experimentos em laboratório e em aplicações em lago inteiro (Welch &
Schrieve, 1994; Hullebusch et al., 2002; Lewandowski et al., 2003; Reitzel et al., 2003,
2005).
Quando o sulfato de alumínio e outros coagulantes são adicionados à água, uma
série de reações químicas ocorre levando a liberação de íons de hidrogênio e,
15
consequentemente, ao declínio do pH da água. Em baixos valores de pH, o alumínio
apresenta toxicidade a peixes (Baker & Schofield, 1982; Poléo et al., 1997),
macroinvertebrados (Havens, 1993) e anfíbios (Freda, 1991), o que limita a quantidade
de coagulante que pode ser adicionada. Baseado nisso, a dose de alumínio a ser aplicada
no lago pode ser determinada levando em consideração a alcalinidade da água (Cooke et
al., 2005), ou seja, quanto pode ser adicionado sem que se atinja o pH onde o alumínio
apresenta toxicidade. Contudo, essa abordagem não leva em consideração o pool de
fósforo no sedimento que poderá ser mobilizado e fazer parte da carga interna. Em lagos
com baixa ou moderada alcalinidade, este problema pode ser resolvido adicionando-se
um tampão (hidróxido de sódio, hidróxido de cálcio ou carbonato de sódio) durante a
aplicação do coagulante.
A fim de solucionar problemas com a acidificação da água após a aplicação de
coagulantes à base de alumínio, compostos pré-hidrolizados foram desenvolvidos, como
o policloreto de alumínio (PAC). Coagulantes pré-hidrolizados são feitos a partir da
hidrólise parcial do ácido em condições controladas, o que permite o não consumo de
hidroxilas (OH-), não reduzindo o pH da água após a aplicação. O policloreto de
alumínio tem recebido atenção especial por possuir uma desempenho de coagulação
superior devido a sua atuação em um amplo espectro de pH, menor sensitividade a
baixas temperaturas, necessidade de menor dose e menores concentrações de alumínio
residual (Jiang & Graham, 1998b), e vem sendo usado como uma alternativa para o
sulfato de alumínio. Estudos em laboratório e em lagos tem mostrado o desempenho do
PAC em remover fósforo e turbidez da água (Reitzel et al., 2003, 2005; Gao et al.,
2005; Lopata & Gawrońska, 2008; Chen & Luan, 2010; Julio et al., 2010; Yang et al.,
2010; Egemose et al., 2011; Jancula & Maršálek, 2012). Contudo, uma vez que
coagulação/floculação é diretamente afetada pela presença de partículas e matéria
orgânica dissolvida presentes na água (Edzwald, 1993) e também pela química da água
– pH e alcalinidade (Pernitsky & Edzwald, 2006), a performance do PAC pode
depender da turbidez e do estado trófico da água, variando de acordo com as
características da água.
Compostos naturais ou modificados como argilas capazes de adsorver e
precipitar o fósforo tem ganhado interesse nos últimos anos (Spears et al., 2013), dentre
os quais se destacam as argilas naturais ou enriquecidas com alumínio (Gibbs &
Özkundakci, 2010), ferro (Zamparas et al., 2012, 2013) e lantânio (Douglas, 2002).
Esses compostos são capazes de se ligar ao fósforo presente na água, bloqueando a
16
liberação do fósforo presente no sedimento para a água após a precipitação. O objetivo é
cobrir o sedimento e atuar como uma barreira química ativa (Jacobs & Förstner, 1999)
que inibe a liberação de fósforo do sedimento. As argilas apresentam vantagem na
aplicação em lagos, pois não levam a acidificação da água, não necessitando do uso de
tampões em lagos com baixa alcalinidade. Além disso, tem sido relatada a eficiência de
argilas na remoção por sedimentação de florações cianobactérias potencialmente
produtoras de toxinas (Pan et al., 2006a, 2006b; Verspagen et al., 2006).
Dentre esses compostos, a argila bentonita modificada com adição de lântanio
(Phoslock®) se destaca por ser bem testada com sucesso em lagos para controle do
fósforo e florações (Spears et al., 2013). O lantânio adicionado a bentonita possui uma
extrema afinidade ao fosfato dissolvido na água (Johannesson & Lyons, 1994; Liu &
Byrne, 1997) e tem sido considerado um método promissor para mitigar a eutrofização
em lagos onde a maior parte do fósforo se encontra em sua forma dissolvida (e.g.
Douglas et al., 1999; 2004; Robb et al., 2003; Ross et al., 2008; Haghseresht et al.,
2009, van Oosterhout and Lurling 2013).
Em lagos eutrofizados com elevada biomassa de algas, boa parte do fósforo
encontra-se presente nos organismos, principalmente na biomassa fitoplanctônica.
Cianobactérias como a Cylindrospermopsis raciborskii, por exemplo, possuem uma alta
capacidade de estocar fósforo (Padisák, 1997), podendo ser uma importante fonte deste
nutriente para o sistema. Esse fósforo pode então ser removido através de coagulação-
floculação. Em seguida, argilas adsorventes de fósforo podem ser aplicadas para
auxiliar no processo de sedimentação, aumentando o peso dos flocos, e capturando o
fósforo dissolvido da água. Ao atingir o fundo do lago, a argila cobre todo o sedimento
e a matéria sedimentada pelo coagulante. A combinação desses compostos tem sido
bem testada apresentando resultados positivos (Sengco & Anderson, 2004; Beaulieu et
al., 2005; Hagström & Granéli, 2005; Sengco et al., 2005; Pan et al., 2006a, 2006b,
2012; Zou et al., 2006; Wang et al., 2012) e recentemente denominada de técnica ‘Flock
and Lock’ (Lürling & van Oosterhout, 2013).
Embora a técnica ‘Flock and Lock’ pareça ser promissora em remover fosfato e
controlar florações de cianobactérias, não existem informações sobre a eficácia deste
método em controlar florações de C. raciborskii. Esta cianobactéria contêm vesículas de
gás o que permite o acúmulo dos organismos na superfície da água (Walsby, 1994),
como estratégia para evitar a sedimentação. Outro ponto desconhecido é se a eficácia do
método em reduzir o crescimento é afetado pela quantidade de biomassa presente no
17
momento da aplicação da técnica. Tem sido sugerido que a concentração celular afeta a
eficiência de remoção pelo complexo coagulante-argila uma vez que o aumento na
quantidade de partículas no sistema aumenta a taxa de floculação (Hagström & Granéli,
2005; Sengco et al., 2005).
Restauração de Lagos Eutrofizados através da BiomanipulaçãoA biomanipulação de peixes é uma técnica bastante utilizada na recuperação de
lagos eutrofizados (Meijer et al., 1994; Perrow et al., 1997; Hansson et al., 1998;
Drenner and Hambright, 1999; Meijer et al., 1999; Mehner et al., 2002; Jeppensen et al.,
2007; Jeppensen et al., 2012). O objetivo desta técnica é a redução da biomassa algal e o
aumento da transparência da água do lago. Peixes planctívoros diminuem a abundância
do zooplâncton de maior porte devido a predação sobre esses organismos, o que resulta
no aumento da biomassa de fitoplâncton (Carpenter et al., 1985). Logo, a redução da
biomassa de peixes planctívoros, através da remoção seletiva dessas espécies ou pela
introdução de espécies piscívoras, se apresenta como estratégia de restauração.
Contudo, esta teoria não aplica a lagos e reservatórios tropicais como nas regiões
temperadas devido a diferenças nas interações biológicas (Jeppesen et al., 2007). Lagos
tropicais apresentam mais onivoria (Lazzaro, 1997), com peixes pequenos e de
reprodução contínua ao longo do ano e uma comunidade zooplanctônica geralmente
dominada por organismos de pequeno tamanho (Fernando, 1994), o que dificulta a
manipulação da estrutura da teia trófica.
Peixes bentívoros, por sua vez, ressuspendem o sedimento durante a
alimentação, aumentando a turbidez da água e translocando o fósforo depositado no
sedimento para a coluna de água, aumentando assim a biomassa fitoplanctônica
(Andersson et al., 1978; Meijer et al., 1990; Breukelaar et al., 1994; Cline et al., 1994;
Lougheed et al., 1998; Schaus & Vanni, 2000; Volta et al., 2013). Dessa forma, esses
peixes afetam a qualidade da água através de mecanismos ascendentes na teia trófica,
desempenhando um papel importante em lagos rasos. No semiárido brasileiro, a
comunidade de peixes é composta por espécies de peixes nativas e introduzidas (Gurgel
& Fernando, 1994), sendo a Prochilodus brevis uma espécie bentívora, nativa e
abundante nos reservatórios nessa região (Chellappa et al., 2009; Nascimento et al.,
2014). Experimentos em escala de mesocosmo em um reservatório raso no semiárido
mostraram uma redução na concentração de fósforo total e clorofila-a na água
18
associados com a remoção de curimatã (P. brevis), ressaltando a importância dessa
espécie em manter a eutrofização nesses ambientes (Dantas, 2015).
O semiárido e a Problemática da Qualidade da ÁguaOs açudes do semiárido brasileiro são lagos rasos artificiais construídos para o
armazenamento de água para abastecimento público, como alternativa para solução de
problemas com a escassez deste recurso na região e estima-se a existência de 70.000
açudes de mais de 1.000 m² nesta região (Molle, 1994). Devido à escassez de água, os
reservatórios no semiárido apresentam vazões reduzidas e consequentemente um
elevado tempo de retenção da água, além de um balanço hídrico negativo durante a
maior parte do ano devido às altas taxas de evapotranspiração, contribuindo assim, para
o acúmulo e concentração de sais e nutrientes, tornando esses ambientes mais
vulneráveis à eutrofização (Barbosa et al., 2012). As previsões para os próximos anos
apontam o agravamento dos períodos secos (Marengo et al., 2009; Roland et al., 2012).
Muitos desses reservatórios recebem ainda uma elevada carga externa de nutrientes em
função da alta susceptibilidade dos solos à erosão, da falta de saneamento básico e de
padrões inadequados de uso e ocupação do solo. Como consequência, muito deles
sofrem com a eutrofização e as persistentes florações de cianobactérias, sendo a
Cylindrospermopsis raciborskii uma espécie importante nas florações do semiárido
brasileiro (Bouvy et al., 1999; Lazzaro et al., 2003; da Costa et al., 2006; Sant’Anna et
al., 2006; Panosso et al., 2007; Costa et al., 2009; Soares et al., 2013). A cianobactéria
C. raciborskii tem recebido uma atenção especial devido a sua potencial toxicidade e a
problemáticas florações de alta densidade em lagos e reservatórios tropicais (Bouvy et
al., 2000; McGregor & Fabbro, 2000; Soares et al., 2013). O seu sucesso tem sido
atribuído principalmente a sua capacidade de captar e estocar fósforo (Padisák, 1997;
Isvánovics et al., 2000; Posselt et al., 2009), representando assim uma importante fonte
deste nutriente para o sistema. Além disso, esta cianobactéria filamentosa possui a
habilidade de regular sua posição na coluna de água através de vacúolos de gás
(Walsby, 1994), oferecendo assim vantagens sobre as outras espécies (Reynolds et al.,
1987; Dokulil & Teubner, 2000; Burford & Davis, 2011).
A redução da entrada externa de nutrientes é, sem dúvida, de extrema
necessidade e importância na restauração de lagos eutrofizados. Contudo, em regiões
semiáridas, com nenhuma ou pouca influência de fontes pontuais de poluição, a entrada
nutrientes para o corpo aquático está relacionada ao uso e ocupação do solo na bacia de
19
drenagem. A principal fonte de nutrientes relatada para sete reservatórios inseridos em
diferentes bacias hidrográficas no semiárido potiguar é a pecuária, seguido de
agricultura (Vasconcelos, 2011). Logo, o controle da entrada de nutrientes envolve uma
complexidade de medidas sociais e econômicas que muitas vezes podem ser
consideradas inexequíveis, como por exemplo, a redução da criação de animais no
entorno dos reservatórios. Portanto, esforços devem ser voltados para estratégias de
restauração in situ.
No Brasil, um único caso de restauração de lago é reportado para o Lago
Paranoá, em Brasília/DF, onde uma massiva mortandade natural de peixes levou a
redução da biomassa fitoplanctônica e a melhoria da transparência da água (Starling et
al., 2002). As diferenças entre lagos temperados e tropicais dificultam a aplicação em
lagos tropicais de métodos de restauração que apresentam resultados positivos em lagos
temperados. Em lagos tropicais o aporte interno de nutrientes parece ser mais
importante do que em lagos temperados, onde a carga interna é relevante apenas durante
alguns meses do ano (verão) (Søndergaard et al., 2013). Além disso, a entrada difusa de
nutrientes para os lagos tropicais representa uma menor contribuição em regiões que
sofrem com a escassez de chuvas. Logo, o manejo da carga interna de nutrientes e o
controle ascendente da produção primária deve ser o foco da restauração de lagos rasos
eutrofizados em regiões tropicais (Jeppesen et al., 2007; Beklioglu et al., 2011). Diante
disto, este estudo preenche a lacuna existente sobre a eficiência de métodos de controle
da carga interna de nutrientes e de florações de cianobactérias no semiárido brasileiro.
Tem sido relatada a eficiência de argilas na remoção por sedimentação de florações de
Microcystis (Pan et al., 2006a, 2006b), e que a cepa pode afetar a eficiência da
coagulação (Verspagen et al., 2006). O caráter inovador deste trabalho se dá pela
investigação da eficiência de coagulantes e argilas em remover florações de C.
raciborskii, cianobactéria dominante na região em questão. A sedimentação de
filamentos de C. raciborskii pode ser afetada pela presença de vacúolos de gás e, até o
momento, não se sabe se a eficácia da técnica em reduzir o crescimento de C.
raciborskii é afetada pela cepa ou pela biomassa de algas no lago momento da
aplicação.
Embora a eficiência da biomanipulação e da aplicação de compostos adsorventes
de fósforo como técnica de restauração em regiões temperadas seja bem documentada,
os efeitos de ambas as técnicas combinadas se restringe a um único caso na literatura
(Jeppesen et al., 2012). Devido ao acesso ao sedimento para a alimentação, peixes
20
bentívoros podem dificultar a consolidação do sedimento (Scheffer et al., 2003) e
reduzir a efetividade da cobertura do sedimento com compostos que adsorvem o fósforo
(Lewandowski et al., 2003). Como consequência, o fósforo ligado a estes compostos
pode retornar a coluna de água e tornar-se disponível para o fitoplâncton (Jeppesen et
al., 2007). Em lagos em que o sedimento representa uma importante fonte de fósforo e
que a comunidade de peixes é dominada por espécies que possuem o hábito de
ressuspender o sedimento, contribuindo assim para aumentar a carga interna de fósforo,
verifica-se a necessidade de se investigar a combinação de técnicas de biomanipulação e
remoção de fósforo.
ObjetivosDiante deste contexto, o objetivo geral desta tese é testar diferentes técnicas de
restauração da qualidade da água que possam ser facilmente aplicadas nos açudes do
semiárido brasileiro, visando à recuperação desses corpos aquáticos em uma região que
sofre com a escassez de água de boa qualidade.
No primeiro capítulo dessa tese, objetivou-se avaliar o efeito da técnica “Flock
and Lock” na sedimentação e crescimento da cianobactéria C. raciborskii, em testes de
laboratório. Neste capítulo foi testada a hipótese de que a combinação do floculante
policloreto de alumínio (PAC) e da argila bentonita modificada com lantânio (LMB) irá
sedimentar efetivamente a C. raciborskii em tubos testes, independente da cepa
utilizada. Em seguida, foi testada a hipótese que a combinação de uma baixa dose de
PAC com a dose recomendada pelo fabricante de LMB irá inibir o crescimento da C.
raciborskii independente da biomassa e da cepa.
A partir dos resultados do capítulo um, objetivou-se aplicar esta técnica em um
lago raso eutrófico inserido na região semiárida brasileira, em escala de mesocosmos.
Contudo, em testes pilotos de bancada utilizando a água do lago, observou-se que a
eficiência do uso do coagulante sozinho em remover fosfato da água foi a mesma se
aplicado em conjunto com a argila adsorvente de fósforo. Além disso, experimentos
realizados neste mesmo lago mostraram que a ressuspensão do sedimento feita pelo
peixe bentívoro dominante, curimatã (Prochilodus brevis), aumenta a concentração de
nutrientes, estimula a produção primária, e reduz a transparência da água neste lago
(Dantas, 2015). Portanto, o objetivo do segundo capítulo desta tese foi então testar os
efeitos isolados e combinados da adição do coagulante policloreto de alumínio e da
21
remoção de peixe bentívoro sobre a qualidade da água de um lago raso tropical em
escala de mesocosmo.
Diante dos resultados da eficiência do uso do coagulante policloreto de alumínio
em mesocosmos (capítulo dois), foi testado o desempenho deste coagulante em remover
fósforo e turbidez na água de seis reservatórios do semiárido brasileiro através de testes
em laboratório. Sabendo que a coagulação pelo PAC é afetada pela presença de
partículas na água, além do pH e alcalinidade, o objetivo do último capítulo desta tese
foi avaliar a eficiência do policloreto de alumínio em melhorar a qualidade da água de
diferentes reservatórios no semiárido brasileiro.
22
CAPÍTULO I. Effects of polyaluminium chloride and
lanthanum modified bentonite on the growth rates of three
Cylindrospermopsis raciborskii strains.
Araújo, F.a, van Oosterhout, F.b, Becker, V.c, Attayde, J. L.d, Lürling, M.b,f
a Programa de Pós-Graduação em Ecologia. Universidade Federal do Rio Grande do Norte, 59078970
Natal, RN, Brazil.b Aquatic Ecology and Water Quality Management Group, Department of Environmental Sciences,
Wageningen University, P.O. Box 47, 6700 AA Wageningen, The Netherlands.c Laboratório de Recursos Hídricos e Saneamento Ambiental, Departamento de Engenharia Civil, Centro
de Tecnologia. Universidade Federal do Rio Grande do Norte, 59078970 Natal, RN, Brazil.d Departamento de Ecologia, Centro de Biociências. Universidade Federal do Rio Grande do Norte,
59078970 Natal, RN, Brazil.f Department of Aquatic Ecology, Netherlands Institute of Ecology (NIOO-KNAW), P.O. Box 50, 6700 AB
Wageningen, The Netherlands.
Abstract
In tropical and subtropical lakes, eutrophication often leads to nuisance blooms
of the filamentous cyanobacteria Cylindrospermopsis raciborskii. In this study, we
tested the combined effects of the coagulant polyaluminium chloride (PAC) and the
lanthanum modified bentonite (LMB, Phoslock®) on the sinking and growth rates of
three C. raciborskii strains in laboratory experiments. We tested the hypothesis that the
combination of PAC and LMB would (1) effectively sink C. raciborskii in a test tube
experiment and (2) impair C. raciborskii growth irrespective of the inoculum (bloom)
biomass and the strain in a 5 days growth experiment. For the test tube experiment the
strains were incubated at the biomass of 100 µg L-1 and the isolated and combined
addition of 1 mg L-1 PAC and 0.1g L-1 LMB for 20-24h. In the growth experiment each
one of the strains were incubated at four different initial biomass (40 µg L-1, 80 µg L-1,
180 µg L-1 and 380 µg L-1) in WC culture medium with and without addition of 1 mg L-
1 PAC and LMB. LMB was applied at one (LMB1) and three (LMB3) times the
recommended dose. Results show that the combined addition of PAC and LMB
enhanced sedimentation of all C. raciborskii strains. Moreover PAC and LMB1 reduced
the growth rate of all three strains but the efficacy was dependent on the biomass and
strain. The combined addition of PAC and LMB3 inhibited the growth of all three
23
strains independently of the biomass and strain. We conclude that the addition of a low
dose of PAC in combination with the recommended dose of LMB reduces C.
raciborskii blooms and that the efficiency of the technique is dependent on the bloom
biomass and intraspecific composition. Finally, a higher dose of LMB is needed to have
a more efficient control of C. raciborskii blooms.
Keywords: cyanobacteria; bloom control; Phosphorus removal; coagulant; Phoslock®.
1. Introduction
Eutrophication of lakes and reservoirs often leads to blooms of cyanobacteria
and is considered the most important water quality problem worldwide (Smith and
Schindler, 2009; Paerl et al., 2011). Due to its potential toxicity, cyanobacteria blooms
and associated surface scums render water from eutrophic freshwater ecosystems unfit
for human use (Chorus and Bartram, 1999). The filamentous cyanobacteria
Cylindrospermopsis raciborskii has received great attention because of its potential
toxicity and problematic high densities in many eutrophic warm-water lakes and
reservoirs (Bouvy et al. 2000; McGregor and Fabbro, 2000; Soares et al., 2013). The
success of C. raciborskii has been attributed to its high uptake and storage capacity of
phosphorus (Padisák, 1997, Isvánovics et al., 2000; Posselt et al., 2009; Wu et al., 2009)
among others factors. Many reservoirs in Brazil used for water supply suffer persistent
blooms of C. raciborskii (Bouvy et al., 1999, 2003; Costa et al., 2006, 2009; Panosso et
al., 2007; Soares et al., 2013). Such blooms pose a health risk to both human and life
stock for which mitigating methods are highly wanted.
Eutrophication control, hence mitigation of cyanobacteria blooms, primarily
focuses on phosphorus (P) control (Schindler et al., 2008). The reduction of the external
P loading is a prerequisite for water quality improvement (Cooke et al., 2005; Hilt et al.,
2006). However, lakes often show little signs of recovery in response to the reduction
of external P load, which is attributed to internal P loading from the P-rich sediments.
(Søndergaard et al., 1999, 2001; Cooke et al., 2005). Not dealing with this legacy P is
one of the causes of failing mitigation attempts (Gulati and van Donk, 2002).
Therefore, additional actions are often needed to reduce this internal P loading and to
accelerate lake recovery (Cooke et al., 2005; Hilt et al., 2006). Internal P-loading can be
24
reduced by removal of P rich sediments - dredging (Peterson, 1982), or by applying a P-
fixative as an in-lake treatment which is often a far cheaper option than dredging
(Cooke et al., 2005; Welch and Cooke, 2005). Aluminium-, calcium- and iron salts have
long been applied as P-fixative in lakes (Cooke et al., 2005). Recently, solid phase P
sorbents (SPB) have gained interest (Spears et al., 2013), among which are modified
clays – i.e. clays enriched with Aluminium (Gibbs et al., 2011), iron (Zamparas et al.,
2012) and lanthanum (Douglas, 2002). Among these SPB, the lanthanum modified
bentonite (LMB; Phoslock®) is the most widely used and tested (Spears et al., 2013).
The LMB is reported to remove dissolved P from the water column and block P release
from the sediment after settling onto the lake bottom (Spears et al., 2013). The LMB
contains 5% lanthanum (Haghseresht, 2005) and has extreme affinity to bind P
(Johanneson and Lyons, 1994; Liu and Byrne, 1997). Thus, whole-lake application of
the LMB is considered a promising method to mitigate eutrophication (e.g. Douglas et
al., 1999; 2004; Robb et al., 2003; Ross et al., 2008; Haghseresht et al., 2009, van
Oosterhout and Lurling 2013).
Because the LMB only targets phosphates, it does not directly affect the
phosphorus present in biota. As cyanobacteria such as C. raciborskii have a high P
uptake and storage capacity (Padisák, 1997), a bloom may prevail after the application
of the LMB. Lurling and Van Oosterhout (2013) combined the LMB with a low dose of
flocculent (polyaluminium chloride, PAC) to instantaneously achieve a durable
mitigation of persistent blooms of cyanobacteria in a Dutch lake. This ‘Flock and Lock’
treatment removes total P from the water column through flocculation, using the LMB
as both sinking weight and sediment capping P fixative (Lurling and Van Oosterhout,
2013). Cyanobacteria contain gas vesicles which provide positive buoyancy allowing
them to accumulate at the water surface (Walsby, 1994). Therefore, the added sinking
weight is quite essential as with a low dose of flocculent to effectively sink buoyancy
controlled cyanobacteria. The ‘Flock and Lock’ method yielded good flocculation and
sinking of cyanobacteria in a short term (2 hours) laboratory experiment and a whole
lake application in a 15 m deep lake (Lurling and Van Oosterhout, 2013). Although the
‘Flock and Lock’ technique seems promising in removing and controlling cyanobacteria
through P limitation there is no report on the efficacy of this method to control C.
raciborskii blooms. It has been suggested that cell concentration seems to affect the
removal efficiency by clay-flocculant once the increasing of particles in the system
increases flocculation rates (Hagstrom ang Granéli, 2005; Sengco et al., 2005).
25
However, it is unknown if the efficacy of the method to reduce growth is affected by the
amount of cyanobacterial biomass present at the moment of application. Here, we first
tested the hypothesis that the combination of PAC and LMB would effectively sink the
positive buoyant C. raciborskii to the bottom of test tubes regardless of the strain used.
We then tested the hypothesis that the combination of a low PAC dose and the
recommended LMB dose by Phoslock manufacturer would impair C. raciborskii growth
irrespective of the inoculum (bloom) biomass and the strain in a 5 days growth
experiment.
2. Materials and methods
2.1 Chemicals
The flocculent PAC (polyaluminium chloride, with the general formula
Aln(OH)mCl3n-m; Ekofix) was provided by Sachtleben Wasserchemie GmbH (Germany).
The lanthanum-modified bentonite (LMB) - Phoslock (5% La) was supplied by
Phoslock Europe GmbH (Ottersberg, Germany).
The manufacturers recommend a LMB dose of LMB (g): P (g) =100: 1. In the
growth experiment described below, the LMB is applied at one (LMB1) and three
(LMB3) times the recommended dose – as based on the filterable reactive phosphorus
(FRP) concentrations measured at the start of each experiment.
2.2 Organisms
Three clonal non-axenic strains of the cyanobacterium Cylindrospermopsis
raciborskii (Woloszynska) Seenaya and Subba Raju were used. Two originated from a
temperate area and one from the tropics. The German C. raciborskii strain G75 was
provided by Dr. Jutta Fastner (Federal Environmental Agency, Berlin, Germany). The
French (PMC 124.12) strain was obtained from the Museum National d’Histoire
Naturelle (Paris, France). The Brazilian strain CYRF1 was obtained from the
Laboratório de Ecofisiologia e Toxicologia de Cianobactérias, Federal University of Rio
de Janeiro (Brazil).
26
The strains were cultured in slightly modified WC medium (Lürling and
Beekman, 1999), in an incubator at 27°C, under constant orbital shaking of 60 rpm and
a photoperiod of 14:10 h light:dark. Day-night transitions were simulated through
gradual increase (or decrease) of the light intensity from complete darkness up to an
approximate 130 µmol photons m-2 s-1. Stock cultures were transferred to fresh sterile
medium every three-four weeks.
2.3 Sinking experiment
The experiment was done according to a complete 2 x 2 factorial design with PAC
and LMB concentrations as factors and 3 replicates per cell. Factor PAC had two levels:
no addition (control) and addition of 1 mg Al l-1. Factor LMB had two levels: no
addition (control) and addition of 0.1 g l-1 LMB. Aliquots of stock cultures of each C.
raciborskii strain were diluted in freshly prepared WC medium. This dilution aimed at
an approximate 100 µg L-1 chlorophyll-a per strain. From each of the diluted C.
raciborskii suspensions, 125 mL aliquots were distributed over 18 glass tubes. PAC was
first added in a solution to achieve the final concentrations of 1 mg Al l-1 in treatments
with PAC and then the suspensions were mixed. After this, the LMB was added by
making slurry with 5 mL water from the tube, which was then sprayed on the top of the
tube using a pipette. All tubes were incubated at room temperature (around 20° C) and
no shaking during one day (24h for CYRF and 20h for G75 and PMC124.12).
As C. raciborskii is a buoyancy controlled cyanobacteria, this experiment may
result in accumulation (e.g. scum formation) in the top of the tubes rather than in their
bottom as a result of sinking. Hence, at the end of the incubation periods chlorophyll-a
(CHL-a) concentrations and turbidity were measured in the top 10 mL and in the
bottom 10 mL samples from each tube. CHL-a was measured with the PHYTO-PAM
phytoplankton analyzer and the turbidity was determined using a Hach 2100P turbidity
meter.
2.4 Growth experiment
A five days growth experiment was done for each strain using a 4 x 3 factorial
design where 4 initial biomasses were combined with 3 “Flock and Lock” treatments
with 3 replicates per cell. The factor ‘initial biomass’ simulated different bloom
27
conditions measured as the CHL-a concentrations (µg L-1) at the beginning of the
experiment – this factor had levels CHL-a: B1 ≈ 40 µg L-1, B2 ≈ 80 µg L -1, B3 ≈ 180 µg
L-1, and B4 ≈ 380 µg L-1. Samples were taken from each diluted cultures to assess the
FRP concentration in the medium and to determine the quantity of LMB to be applied.
The levels of the factor ‘treatment’ were: control (no addition), combined addition of 1
mg L-1 PAC and 100g LMB: 1g FRP (LMB1) and combined addition of 1 mg L-1 PAC
and 300g LMB: 1g FRP (LMB3). Initial biomasses were achieved through appropriate
dilution of the stock cultures using WC medium. When applying the treatments, each
experimental unit first received the PAC in order to achieve the final concentration of 1
mg Al L-1. Then to improve flock formation, the pH was adjusted to 6.5 (±0.2) using
Hydrochloric acid (HCl 0.01N) or Sodium hydroxide (NaOH 0.01N) after the
experimental units were mixed. Finally the LMB was added by making slurry with 5
mL water from the experimental units and the experimental units were mixed again. The
experiments were done in 100 mL erlenmeyers with 100 mL of the diluted
cyanobacteria cultures. The incubation was done under the same conditions described
for the stock cultures used in the experiment.
To quantify the FRP depletion, samples were collected just before and 5 days
after the application of PAC and LMB. Filtered samples were analyzed for their FRP
concentration (Murphy and Riley, 1962) in an auto-analyzer (SKALAR SA40). The
FRP removal (%FRP) was calculated as the percentage of initial FRP removed by the
formula % FRP = 100 x ((P0 – Pt)/ P0 ), where Pt and P0 are the FRP concentration at the
end and at the start of the experiment respectively. Growth was assessed by daily CHL-
a measurement. The growth rate µ (d-1) was computed by the formula µ = (lnBt –
lnB0)/t, where Bt and B0 are the biomass at the end and at the start of the experiment
respectively, while t is the duration of the experiment (5 days). Growth rate was
estimated only for the control and LMB1 dose where the strains showed an exponential
growth. In view of it, we compare the effect of LMB3 dose in reducing biomass with
the LMB1 dose by calculating the percentage of biomass (%) reduced by treatment in
relation to control by the formula % B = 100 x ((BC – BT)/BC), where BC and BT are the
biomass concentration at the control and treatment respectively.
28
2.5 Statistics
To evaluate the isolated and combined effects of PAC and LMB on CHL-a
concentration and turbidity in the first experiment we performed a two-way ANOVA.
Likewise, we used a two-way ANOVA to evaluate the isolated and combined effects of
biomass and ‘Flock and Lock’ treatments on FRP concentration and C. raciborskii
biomass and growth rates in the second experiment. The Tukey post-hoc test was
performed when ANOVA showed significant effects of the ‘Flock and Lock’ treatments
in the second experiment. The tests were done for each strain separately.
3. Results
3.1 Sinking experiment
Before applying the treatments the mean CHL-a concentration in the tubes was
85.26 µg L-1 (± 1.01) for Brazilian strain, 102.70 µg L-1 (± 1.09) for French strain and
112.69 µg L-1 (± 0.53) for German strain, and the mean turbidity in the tubes was 31.23
NTU (± 1.56) for Brazilian strain, 34.67 NTU (± 1.76) for French strain and 4.90 NTU
(± 0.44) for German strain. After the incubation period the LMB treatment did not
reduced CHL-a concentration of the top samples (Figure 1 a-c). The PAC treatment
caused a substantial increase in CHL-a of the top samples of the Brazilian and French
strains (Figure 1 a-b) while a minor increase was observed for German strain (Figure
1c). When added in concert, PAC and LMB caused a reduction in CHL-a concentration
in the top samples and a marked increase in CHL-a concentration of the bottom samples
(Figure 1 a-c). A similar pattern was observed for turbidity. The combined addition of
PAC and LMB resulted in a decreased turbidity at the top samples and an increase in
turbidity at the bottom samples (Figure 1 d-f). However, the observed increase in
turbidity of the bottom samples occurred when LMB were added either in isolation or in
combination with PAC. The two-way ANOVA results showed a significant interaction
between LMB and PAC effects in increasing CHL-a in the bottom samples and
decreasing the turbidity in the top samples of all strains (Figure 1).
29
3.2 Growth experiment
During the experiment, we observed a reduction of 16-74% the initial FRP
concentrations in the control treatment and a reduction of more than 90% the initial FRP
concentrations in the two flock and lock treatments (Figure 2). The two-way ANOVA
results showed that PAC + LMB addition had significant effect on FRP reduction
(Table 1) and that this effect does not depend on C. raciborskii biomass. The growth
curves revealed an exponential growth of C. raciborskii in the control and LMB1
treatments during the 5 days of the experiment (Figure 3). In the LMB3 treatment, C.
raciborskii did not sustain an exponential growth until the end of the experiment (Figure
3). Therefore, the growth rate was only estimated for the control and LMB1 treatments.
The growth rates were higher in the control than in the treatment with PAC +
LMB addition, decreasing with increasing the initial biomass (Figure 4). We observed a
reduction of 22-29% for biomass B1, 30-39% for biomass B2 in all strains and 40-44%
for biomass B3 and 40-49% for biomass B4 in Brazilian and German strains. The two-
way ANOVA results showed that the initial biomass and PAC + LMB addition had
significant effects on the growth rates of all strains, and that there was a significant
interaction between these effects (Table 1). No significant difference was found in
growth rate between treatments in biomass 3 and 4 for French strain.
The LMB1 dose reduced up to 61% of biomass while LMB3 dose reduced up to
94% of biomass for all strains (Figure 5). In general, the reduction decreased with the
increasing in biomass.
4. Discussion
Our results from laboratory experiments show that the combined application of
PAC and LMB is an efficient technique to effectively sink the buoyant cyanobacteria
Cylindrospermopsis raciborskii. The clay ability in removing cells can be enhanced by
flocculant addition as a result of an increase in clay adhesiveness (Sengco and
Anderson, 2004). Some studies show that the combination of clay and flocculant can
efficiently remove marine algal cells and blooms (Anderson, 1997; Sengco et al., 2001,
2005; Hagström and Granéli, 2005) and freshwater cyanobacteria bloom (Lurling and
van Oosterhout, 2013) from water column. Clay alone had been reported to effectively
remove Microcystis cells by sinking (Pan et al., 2006a, b, 2011; Verspagen et al., 2006).
30
We did not found the same result for C. raciborskii. LMB alone did not result in a
decrease of CHL-a concentration at the top of our experimental tubes. The increasing
turbidity at the bottom of the tubes in the LMB treatment is explained by the clay
settling to the bottom, because there was no reduction in CHL-a at the top of the tubes
or increase in CHL-a at the bottom of the tubes in the LMB treatment. This reveals the
low aggregation efficiency of C. raciborskii filaments with LMB. Aggregation with
clay may depend on the cyanobacteria used due to variability in extracellular
polysaccharide (EPS) composition, as has been found for Microcystis (Verspagen et al.,
2006). In fact the colonies of Microcystis are embedded in mucilage, as a strategy for
avoid sedimentation, which is formed mainly by polysaccharide (Reynolds, 2006). It
was found the production of EPS by Raphidiopsis brookii, closely related genera to C.
raciborskii (Yunes et al., 2009), but we are not conscious of studies about EPS
production by C. raciborskii. Conversely, we observed that the addition of PAC resulted
in good flock formation as we could see by the increasing in CHL-a concentration at the
top of the tubes of PAC treatment but we did not observe sedimentation of those flocks.
Therefore our results shows that the addition of sinking weight in combination with a
low dose of flocculent is fundamental to effectively sink the filaments of the buoyancy
controlled cyanobacteria C. raciborskii.
In the growth experiment, the application of PAC in combination with the
recommended or a 3 times higher dose of LMB equally resulted in a strong reduction of
FRP concentrations as expected. However, the inhibition of C. raciborskii growth was
stronger at the LMB3 than at the LMB1 dose. As cell removal efficiency by flocculation
process increases with increasing clay concentration (Sengco et al., 2001; Pan et al.,
2006b), the higher flocculation in the LMB3 could also have contributed to the faster
growth inhibition of C. raciborskii strains in this treatment when compared to the
LMB1 treatment.
On the other hand, the C. raciborskii strains showed an exponential growth
during the 5 days of experiment in the LMB1 treatment, but growth rates decreased with
initial biomass as a result of density dependent growth (Reynolds, 2006). This is
because at higher biomass the availability of resources per individual is lower resulting
in stronger intraspecific competition for resources. The combined addition of PAC with
the recommended dose of LMB reduced C. raciborskii growth rate for all the three
strains investigated. The decrease in growth rates can be explained as a result of
phosphorus limitation caused by the binding of FRP to PAC and/or LMB. As PAC +
31
LMB addition removed more than 90% the initial FRP concentrations, we expected
growth would dramatically decrease due to P limitation. However, growth persisted but
at lower rates. This may be explained by the fact that under conditions of P limitation,
C. raciborskii can regulate its physiological response to this stress by reducing growth
rate and photosynthetic activity and an increase of extracellular phosphatase activity
(Wu et al., 2012). Moreover it is known that C. raciborskii is a P storage specialist and
it has a rapid phosphate uptake rate (Padisák, 1997, Isvánovics et al., 2000; Posselt et
al., 2009; Wu et al., 2009). Under ideal circumstances they are able to uptake
phosphorus more than is needed (Istvanovics et al., 2000) and store it. This luxury
uptake allows growth of two or three generations before this element become a limiting
factor (Reynolds, 2006). As the cultures were not submitted to P starvation before the
treatment, P might have been stored by the cells before the experiment, allowing them
to grow even at lower rates with the intracellular P after the combined addition of LMB
+ PAC. Indeed, light limitation due to increasing turbidity in water and flocculation of
filaments caused by PAC and LMB addition can also explains the reduction in growth
rate (Van Oosterhout and Lurling, 2013).
The efficacy of the method to reduce P was not affected by the biomass of C.
raciborskii present at the moment of application or the C. raciborskii strain used.
However, we found that the efficacy of this technique in reducing C. raciborskii
growth rates depends on the biomass and strain of this cyanobacterium. The effects of
PAC + LMB additions in reducing C. raciborskii growth rates increased with
increasing biomass of the Brazilian strain and to a less extent of the German strain. By
contrast, PAC + LMB additions had no effect in reducing growth at higher biomass
(B3 and B4) of the French strain. These differences in response suggest that Brazilian
strain is more sensitive to P and maybe light limitation than the others strains.
Intraspecific variation in response to resource availability has been reported to
cyanobacteria (Kaardinal et al., 2007; Briand et al., 2008; Wilson et al., 2005).
Physiological differences among C. raciborskii strains in response to temperature, light
intensity (Briand et al., 2004) and critical requirements for phosphorus and light
(Marinho et al., 2013) has been found. Also the existence of different physiological
strains or ecotypes in C. raciborskii populations was proposed (Chonudomkul et al.,
2004; Piccini et al., 2011). Therefore, the reduction of growth in response to resource
limitation may be dependent on the ability of the strains in optimize the uptake and the
critical requirements of resources.
32
Our findings have significant implications for restoration of lakes and reservoirs
dominated by C. raciborskii. The combined application of PAC with the recommended
dose of LMB can be a good management strategy to sinking and to reduce the growth of
C. raciborskii population as result of phosphorus limitation, leading to the control of
bloom in a long-term perspective, but the technique tend to be dependent of the bloom
stage and the strain. However, to have a more efficient and faster control of C.
raciborskii blooms may be necessary the use of a combined dose of PAC with a higher
dose of LMB than the recommended by the manufacturers.
5. Conclusions
The flocculent PAC or the lanthanum modified bentonite (LMB) Phoslock®
alone could not effectively sink filaments of different strains of the positive buoyant
cyanobacteria C. raciborskii;
PAC combined with LMB effectively sank the filaments of C. raciborskii
strains;
PAC combined with the recommended LMB dose reduced FRP and growth
rates of C. raciborskii strains tested;
The efficacy of PAC combined with the recommended LMB dose in reduce
growth rate depends on the cyanobacteria biomass and strain.
Acknowledgements
This research is resulted of PhD sandwich programme conducted under the auspices of
the Coordenação de Aperfeiçoamento Pessoal de Nível Superior (CAPES/Brazil) –
WAGENINGEN (The Netherlands) Project “Cyanobacterial Blooms: A growing threat
in freshwater ecosystems” (004/2008).
33
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40
Tables and Figures
Table 1 – F-ratios and P-values of two-way ANOVA to test for the effects of
the initial biomass (B), PAC+LMB addition (D) and their interactions (B x
D) on % FRP removed from cultures and growth rates of
Cylindrospermopsis raciborskii strains. Values were considered significant
assuming P ≤ 0.05.
% FRP removed
Brazil Germany France
B F = 0.85 P = 0.48 F = 1.06 P = 0.39 F = 0.93 P = 0.44
D F = 160.43 P < 0.01 F = 139.37 P < 0.01 F = 117.87 P < 0.01
B x D F = 1.78 P = 0.15 F = 1.78 P = 0.24 F = 3.41 P = 0.01
Growth rate
Brazil Germany France
B F = 671.70 P < 0.01 F = 31.37 P < 0.01 F = 403.58 P < 0.01
D F = 599.20 P < 0.01 F = 27.58 P < 0.01 F = 760.61 P < 0.01
B x D F = 3.22 P = 0.05 F = 3.24 P = 0.05 F = 10.14 P < 0.01
41
Figure 1. Chlorophyll-a concentrations and turbidity at the end of the incubation
in the top 10 mL (white bars) and bottom 10 mL (black bars) of experimental
tubes containing 100 mL of three different Cylindrospermopsis raciborskii strains.
Control without and treatments with addition of 0.1 g L-1 LMB and 1 mg L-1PAC
in isolation (LMB, PAC) or in combination (PAC+LMB). F-ratios and P-values of
two-way ANOVA to test for the effects of PAC (P), LMB addition (L) and their
interactions (P x L). Values were considered significant assuming P ≤ 0.05.
42
Figure 2. Percentage of filterable reactive phosphorus concentration (FRP)
removed after the incubation period (5 days) of experimental cultures containing
100 mL of each of three different Cylindrospermopsis raciborskii strains.
Treatments: Control (white bars), addition of 1 mg L-1 PAC + 100g LMB: 1g FRP
(black bars) and the combination of 1 mg L-1 PAC + 300g LMB: 1g FRP (gray
bars) (LMB: lanthanum modified bentonite; PAC: polyaluminium chloride);
Initial biomass of the strains in chlorophyll-a concentration are B1 ≈ 40 µg L-1, B2
≈ 80 µg L-1, B3 ≈ 180 µg L-1, and B4 ≈ 380 µg L -1.
43
CAPÍTULO II. Shallow lake restoration by the combined
effects of polyaluminium chloride addition and benthivorous
fish removal: a field mesocosm experiment.
Araújo, F.1, Becker, V.2 and Attayde, J. L.3
1 Programa de Pós-Graduação em Ecologia. Universidade Federal do Rio Grande do Norte (UFRN),
Natal – RN – Brazil.2 Departamento de Engenharia Civil, Centro de Tecnologia. Universidade Federal do Rio Grande do
Norte (UFRN), Natal – RN – Brazil.3 Departamento de Ecologia, Centro de Biociências. Universidade Federal do Rio Grande do Norte
(UFRN), Natal – RN – Brazil.
Abstract
Internal loading can be an important source of phosphorus (P) in eutrophic shallow
lakes increasing the resilience of the turbid stable state and delaying the effects of lake
restoration by the reduction of external loading. Therefore, additional actions are often
needed to reduce internal P loading such as the addition of P-sorption agents and the
manipulation of the fish communities. Polyaluminium chloride (PAC) has been used for
P precipitation by coagulation, sedimentation and P inactivation in the lake sediment.
However, benthivorous fish may increase the flux of P from the sediment back to the
water column reducing the efficiency of PAC addition as a management tool to improve
water quality. Therefore, we hypothesized that PAC addition combined with removal of
benthivorous fish would interact synergistically to improve water quality of eutrophic
shallow lakes. To test this hypothesis, we performed a field experiment with a 2 x 2
factorial design during 53 days in 20 mesocosms (6 m³), where four treatments were
randomly allocated: combining the presence and absence of PAC (2 mg Al.l-1) with the
presence and absence of benthivorous fish (Prochilodus brevis). A two-way repeated
measure ANOVA was performed to test the effects of PAC, fish and time and their
interactions on total phosphorus and chlorophyll concentrations as well as on water
transparency. PAC addition significantly decreased total phosphorus concentrations in
water and increased water transparency. Fish removal significantly decreased total
44
phosphorus and chlorophyll a concentrations and increased water transparency. No
significant interaction was observed between the effects of PAC addition and fish
removal. In conclusion, our results suggest that the effects of PAC addition and
benthivorous fish removal are additive and that lake restoration can be better achieved
by the combination of both techniques.
Keywords: P-sorption agent; coagulant; biomanipulation; tropical lake.
1. Introduction
Eutrophication is a serious environmental problem related to degradation of water
quality worldwide. The main measure to restore eutrophic lakes is to reduce the input of
nutrients to water bodies. Although some lakes have recovered after decreasing in the
external input of nutrients, a delay or fail in recovery has been observed in many
shallow lakes (Marsden, 1989; Jeppesen et al., 1991; Van der Molen & Boers, 1994;
Søndergaard et al., 2000) and it has been attributed mainly to the phosphorus (P)
internal loading from P-rich sediments (Søndergaard et al., 1999, 2003; Cooke et al.,
2005).
The internal P loading can be reduced by adding P fixative chemicals into lake
(Welch & Cooke, 2005). Precipitation and inactivation of phosphorus by aluminium
(Al) salts (mainly as aluminium sulphate - alum) have been commonly used as a lake
restoration measure (Cooke et al., 2005; Welch & Cooke, 2005). When Al salts are
added to water, Al+3 reacts with PO4-3 and forms a precipitate, by coagulation and
flocculation process, and then the flocks can be removed after subsequent
sedimentation. Besides this, the flocculation by aluminium salts precipitates organic and
inorganic matter resulting in the improvement of water transparency (Jiang & Graham,
1998b; Hullebusch et al., 2002) and removing total phosphorus concentrations present
in suspended particles after flocks settling (Reitzel et al., 2003; Auvray et al., 2006).
Many successful cases of Al application as restoration measure have been reported
(Cooke et al., 2005). Nonetheless the P bound to the Al may return to the water column
by flocks resuspension and become available to the phytoplankton (Jeppesen et al.,
2007). In addition, the suspended flocks might have impacts on filter-feeding organisms
(Beaulieu, Sengco and Anderson, 2005).
45
Polyaluminium chloride (PAC) has been used as an alternative for alum because
it has a higher efficiency of turbidity reduction based on the same equivalent alum dose
and has a wider working pH range aside from lower coagulant cost to achieve the same
efficiency (Jiang and Graham, 1998).
Other commonly used technique for lake restoration is the removal of
planktivorous and/or benthivorous fishes (Meijer et al., 1994; Perrow et al., 1997;
Hansson et al., 1998; Drenner and Hambright, 1999; Meijer et al., 1999; Mehner et al.,
2002; Jeppensen et al., 2007; Jeppensen et al., 2012). The ultimate goal of such
biomanipulation is the reduction of algal biomass and the increasing of water
transparency. The removal of planktivorous fish increases the abundance of large
zooplankton which in turn is able to suppress phytoplankton biomass (Carpenter et al.,
1985; Meijer et al., 1994). Conversely, fish with benthic-feeding habit play an important
role in shallow lakes by disturbing the sediment, increasing turbidity and translocating P
from the sediment to the water column (Andersson et al., 1978; Meijer et al., 1990;
Breukelaar et al., 1994; Cline et al., 1994; Lougheed et al., 1998; Schaus & Vanni,
2000; Volta et al., 2013). Although attractive, the long-term effectiveness of fish
manipulation is uncertain (Jeppensen et al., 2007). Besides the resuspension of the
sediment by fishes, release of phosphorus from P-rich sediments can be affected by
temperature, pH, iron:phosphorus ratio and mineralization processes in the sediment
(Jensen & Andersen, 1992; Olila & Reddy, 1997; Pettersson, 1998; Søndergaard et al.,
1999, 2003; Jeppesen et al., 2005) and contribute with phytoplankton growth, showing
the need for additional measures in combination with biomanipulation to reduce P
internal loading (Jeppensen et al., 2012).
Although the efficiency of biomanipulation and Al application in improving
water quality are well documented, study of the effects of both techniques combined is
so far limited mainly in warm lakes (Jeppensen et al., 2012). Benthivorous fish may
prevent sediment consolidation (Scheffer et al., 2003) and sediment mixing processes
reduce the effectiveness of sediment capping with P-sorption agents (Lewandowski et
al., 2003). As a consequence the P bound to the Al return to the water column by flocks
resuspension and turn back to be available to the phytoplankton (Jeppesen et al., 2007).
Therefore, we hypothesized that aluminium is more efficient in improving water quality
when benthivorous fish is removed. The aim of this study was to test the isolated and
combined effects of Polyaluminium chloride (PAC) addition (2 mg Al.l-1) and
46
benthivorous fish (Prochilodus brevis) removal on the water quality of a tropical
shallow lake.
2. Material and methods
2.1 Study Area and experimental design
The experiment was performed in an eutrophic shallow man-made lake located
at Seridó Ecological Station, Serra Negra do Norte, Rio Grande do Norte, Brazil
(6º34’49,3” S; 37º15´20” W). The experiment was carried out in 20 mesocosm of 6 m³
(4 m² x 1.5m) placed side by side in the littoral zone of the reservoir. The mesocosms
were open to the atmosphere at the top and to the sediment at the bottom, but were
isolated from the adjacent lake water by a film of transparent plastic.
The experiment consisted of a 2 x 2 factorial design where four treatments with
five replicates were randomly allocated to the 20 mesocosms. The treatments were:
presence of fish with (+Al+Fish) or without Al addition (-Al+Fish) and absence of fish
with (+Al-Fish) or without Al addition (-Al-Fish). The fish used in the experiment,
curimatã (Prochilodus brevis), is a common benthivorous fish in the Brazilian semi-arid
region (Chellappa et al., 2009; Nascimento et al., 2014) and the most abundant fish in
the studied lake. Four fishes 352.6g (± 65.7) were collected from the studied lake and
added to each mesocosm of the two fish treatments just after initial sampling resulting
in a density of 0.67 fish m-3. This is within the range of natural densities found in the
lakes of the region. Aluminium as polyalumnium chloride (PAC; PANFLOC TE1018 –
Pan-Americana S/A) was the coagulant used at dose 2 mg Al.L-1 and was added one day
after fish stocking. The chosen dose was based on an experimental jar test in the
laboratory using the water from the lake (data not shown).
2.2 Sampling and samples analysis
Water samples were collected at 13 days intervals for eight weeks totalizing 6
samples for each variable. Sampling was performed between 06h00 and 10h00 a.m.
Water transparency was measured by a Secchi disc in each mesocosm. Water samples
were collected with a 1.5m length of 5 cm diameter PVC tube at different points in each
mesocosms and integrated in a 30L bucket. Subsamples were taken for turbidity and
nutrient analysis. Turbidity was measured with a Turbidimeter AP2000 and total
47
phosphorus (TP) concentration was measured with a spectrophotometer by the acid
ascorbic method after persulphate digestion (Valderrama, 1981; Murphy and Riley,
1962). Samples were filtered using a 1.2 µm glass fiber filter (VWR 696) and the filters
were used to determine chlorophyll-a (chla) after ethanol extraction according to
Jespersen and Christoffersen (1988). Water samples were also collected outside the
mesocosms in two different points of the lake for the analysis of the same variables. The
fishes were caught and weighed after the end of the experiment.
2.3 Statistical analysis
A two-way repeated measures ANOVA was performed to test the isolated
effects of benthivorous fish removal (F), PAC addition (Al) and time (T) on turbidity,
water transparency, total phosphorus and chlorophyll-a concentrations. The data were
log transformed to attend the assumptions of ANOVA. To compare the initial and final
weight of fish in each treatment a t test for dependent samples was used. A t test for
independent samples was used to compare the weight of fish between the treatments at
the start and end of the experiment. The significance level assumed was α = 0.05.
3. Results
Initial water conditions were equal between treatments for all variables (data not
shown). The two-way repeated measures ANOVA showed that PAC addition and
benthivorous fish removal had significant effects on the measured variables but some
effects were time dependent (Fig. 1). PAC had pronounced effect at the beginning of the
experiment while benthivorous fish had a pronounced effect latter. Overall, PAC
addition significantly decreased total phosphorus concentrations and water turbidity,
while increased water transparency (Fig. 2). Fish removal significantly decreased water
turbidity, total phosphorus and chlorophyll a concentrations, while increased water
transparency (Fig. 2). We observed no significant interaction between the effects of
PAC addition and benthivorous fish removal (Fig. 2).
The curimatã weight did not increase during the experiment within -Al+F (F =
3.76; P = 0.23) and +Al+F treatments (F = 1.65; P = 0.71). No difference was found in
the initial (t = 2.23; P = 0.09) or final (t = 0.21; P = 0.84) weight between the treatments
(Fig. 3).
48
4. Discussion
Our results show the additive effects of aluminium addition and benthivorous
fish removal to improve water quality of shallow eutrophic lakes. The addition of
aluminium in the absence of benthivorous fish showed the best improvement in water
quality. The marked reduction in turbidity make possible to see the bottom of the lake in
some mesocosms of this treatment and the Secchi-disk depth was three times higher
than the initial value. Besides, this was the only treatment in which we could observe
reductions in chlorophyll-a concentration compared to the initial values. As far as we
know, dual treatment in a lake is reported only for the shallow Kollelev Lake, Denmark
(Jeppesen et al., 2012). In this lake aluminium application resulted in a reduction of
total phosphorus concentrations but no changes were observed in water clarity. After a
biomanipulation (one year later) including cyprinid removal and perch stocking, an
immediate and strong improvement in water clarity in aluminium treated basins,
coinciding with a gradual reduction in total phosphorus concentrations (Jeppesen et al.,
2012). In agreement with the experience in Kollelev Lake, our experiment shows that
the combination of biomanipulation and chemical treatment for P inactivation is a good
way to achieved rapid and probably long-term lake restoration.
The PAC addition had marked effects in decreasing turbidity, total phosphorus
and chlorophyll-a concentration just after its addition to the water as expected. The
application of PAC in whole lake experiments has shown its efficacy in removing
phosphorus from the water column (Reitzel et al., 2005; Lopata & Gawrońska, 2008;
Egemose et al., 2011; Jancula & Maršálek, 2012). Nevertheless, the PAC effects in our
experiment may have been underestimated due to the low Al dose applied. If the
amount of the mobile P pool in lake water and in the sediment had been taken into
account to calculate the dose of Al to be applied (Rydin & Welch, 1998), the Al dose
should be higher than the used in the experiments, increasing the potential of PAC in
improve water quality.
The benthivorous fish removal also had the expected results in improving water
quality. However, our experiment shows results from a complete removal of
benthivorous fish, overestimating the effects fish removal. For whole lake treatment the
complete removal of benthivorous fish is hard to reach. It has been suggested the
reduction of 70-85% of benthivorous fish biomass (Meijer et al., 1990; Hosper &
Meijer, 1993; Perrow et al., 1997; Hansson et al., 1998) and the maintenance of their
49
population depressed (Godwin et al., 2011) in combination with the harvesting of the
small size fishes (Schaus et al., 2013) for effective results in water quality improvement
and to achieve long term results.
Although no significant difference was found between and within treatments,
there was a trend to curimatã fish decreased biomass in the treatment without PAC
addition. However in the treatment with PAC addition curimatã maintained the initial
biomass, suggesting that fish may have been favored by the organic matter precipitated
on the bottom of the mesocosms with PAC addition. This is an important assumption
that needs to be investigated with more details.
In conclusion, our results from a mesocosm scale show that the effects of PAC
addition and benthivorous fish removal are additive and that lake restoration can be
better achieved by the combination of both techniques.
Acknowledgements
We thank Maria Marcolina L. Cardoso, Pablo Rubim, Mariana R. A. Costa, Bárbara
Bezerra, Caroline G. B. de Moura, Leonardo Teixeira e Alexander Ferreira for field and
laboratory assistance. Funding was given by CNPq through the Project 562676/2010-4.
Fabiana Araújo was supported by a fellowship from CAPES.
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Søndergaard, M., E. Jeppesen, J. P. Jensen, & T. Lauridsen, 2000. Lake restoration in
Denmark. Lakes and Reservoirs: Research and Management 5: 151–159.
Van der Molen, D. T., & P. C. M. Boers, 1994. Influence of internal loading on
phosphorus concentration in shallow lakes before and after reduction of the external
loading In Mortensen, E., E. Jeppesen, M. Sondergaard, & L. K. Nielsen (eds), Nutrient
Dynamics and Biological Structure in Shallow Freshwater and Brackish Lakes. Springer
Netherlands.: 279–389.
Volta, P., E. Jeppesen, B. Leoni, B. Campi, P. Sala, L. Garibaldi, T. L. Lauridsen, & I.
J. Winfield, 2013. Recent invasion by a non-native cyprinid (common bream Abramis
54
brama) is followed by major changes in the ecological quality of a shallow lake in
southern Europe. Biological Invasions 15: 2065–2079.
Welch, E. B., & G. D. Cooke, 2005. Internal Phosphorus Loading in Shallow Lakes:
Importance and Control. Lake and Reservoir Management 21: 209–217.
55
Tables and Figures
Figure 1. Average values (±1 SD) of turbidity, Secchi depth, total phosphorus and
chlorophyll-a concentrations in the four treatments and in the reservoir during the
experiment. F-ratios and P-values of the two-way repeated measures ANOVA
interactions terms of time (T) with fish removal (F), PAC addition (Al) and the
interaction of the two factors (F x Al) are shown inside each graph. Values were
considered significant assuming α = 0.05.
56
Figure 2. F-ratios and P-values of the two-way repeated measures ANOVA to test
for the effects of fish removal (F), PAC addition (Al) and their interaction (F x Al)
on water turbidity, Secchi depth, total phosphorus and chlorophyll-a
concentrations. Means (dots) and standard deviation (bars) of each variable in the
treatments with (black) and without (white) PAC addition are shown in the graph.
Values were considered significant assuming α = 0.05.
57
Figure 3. Average (+ 1 SD) fish biomass at the start and end of the experiment in
treatments without (-AL+F) and with PAC addition (+Al+F).
58
CAPÍTULO III. The use of polyaluminium chloride as a
restoration measure to improve water quality in tropical
shallow lakes
Araújo, F.1, Santos, H. R.2, Becker, V.2 and Attayde, J. L.3
1 Programa de Pós-Graduação em Ecologia. Universidade Federal do Rio Grande do Norte (UFRN),
Natal – RN – Brasil.2 Departamento de Engenharia Civil, Centro de Tecnologia. Universidade Federal do Rio Grande do
Norte (UFRN), Natal – RN – Brasil.3 Departamento de Ecologia, Centro de Biociências. Universidade Federal do Rio Grande do Norte
(UFRN), Natal – RN – Brasil.
Abstract
The internal phosphorus loading is considered the major cause of delay in shallow lake
restoration after reduction of external P loading. The most common and used technique
to reduce the internal loading is the precipitation and inactivation of phosphorus by
coagulants, especially those based on aluminium. Polyaluminium chloride (PAC) has a
good coagulation performance due to its wider pH range, lower sensitivity to low water
temperature, lower doses required and lower residual Al concentrations in comparison
to non-polymerized aluminium-based coagulants. Direct application of PAC into lakes
has been proposed as a cheap tool for water quality management. The aim of this study
was to evaluate the performance of PAC in water quality improvement of six eutrophic
shallow lakes in Brazilian semiarid region through laboratory jar tests. The results
showed that PAC had a good performance in reducing total phosphorus concentrations
and turbidity, with a reduced efficiency in removing chlorophyll-a and humic
substances by sedimentation of flocks formed. Addition of PAC is a potential tool for
water quality improvement of eutrophic shallow lakes in Brazilian semiarid region but
its efficiency depends on the pH and particulate and dissolved organic matter
concentration in the lake or reservoir water.
59
Key-words: polyaluminium chloride, phosphorus removal, turbidity removal, lake
restoration, semiarid region
1. Introduction
The internal phosphorus (P) loading from P-rich sediments is considered the
major cause of delay in shallow lake restoration after reduction of external P loading
(Marsden, 1989; Jeppesen et al., 1991; Van der Molen & Boers, 1994; Søndergaard et
al., 2000, 2003). As a result, several chemical methods have been applied to control P
internal loading worldwide (Welch & Cooke, 1999; Reitzel et al., 2005; Gibbs et al.,
2011; Lürling & van Oosterhout, 2013; Spears et al., 2013). The most common and
used technique is the precipitation and inactivation of phosphorus by coagulants,
especially those based on aluminium (Al) (Cooke et al., 2005). When Al salts are added
to water, Al+3 preferable reacts with PO4-3 and forms a precipitate which can be
removed from the water column after coagulation, flocculation and subsequent
sedimentation. Besides this, coagulation and flocculation are also able to remove
inorganic and organic suspended particles (Jiang and Graham, 1998), turbidity and total
phosphorus from the water column. Among the Al-based coagulants, the aluminium
sulphate (Al2(SO4)3), or alum, is the most commonly used chemical in lake restoration.
Its effectiveness in removing phosphorus has been reported in several laboratory and
whole-lake experiments (Welch & Schrieve, 1994; Hullebusch et al., 2002;
Lewandowski et al., 2003). However alum may result in high concentration of residual
Al and is strongly affected by temperature (Van Benschoten & Edzwald, 1990) and pH.
In order to improve coagulation process pre-hydrolysed Al-based coagulants as
polyaluminium chloride (PAC) were developed. Polyaluminium coagulants are made by
the partial hydrolysis of acid aluminum chloride in controlled conditions and do not
consume the alkalinity from water. Thus, PAC has a superior coagulation performance
than alum due to its wider pH range, lower sensitivity to low water temperature, lower
doses required and lower residual Al concentrations (Jiang & Graham, 1998b). A
number of laboratory experimental studies has shown the superior performance of PAC
in both turbidity and phosphorus removal (Reitzel et al., 2003; Gao et al., 2005; Chen &
60
Luan, 2010; Julio et al., 2010; Yang et al., 2010). The application of PAC in whole lake
experiments has shown its efficacy in removing phosphorus from the water column
(Reitzel et al., 2005; Lopata & Gawrońska, 2008; Egemose et al., 2011; Jancula &
Maršálek, 2012) and turbidity even at low dose (1.5 mg Al.L-1) in shallow lakes
(Hullebusch et al., 2002), and it has been suggested as a lake restoration measure.
Coagulation-flocculation process is directly affected by the presence of particles
and dissolved organic matter present in the water (Edzwald, 1993) and also by water
chemistry (pH and alkalinity) (Pernitsky & Edzwald, 2006). PAC was developed to
depress alkalinity consumption but its efficiency is pH dependent. The effectiveness of
PAC coagulation is affected by aluminium speciation after its application in water
which in turns is determined by pH (Edzwald, 1993). At pH of 6.0-7.0 the chemical
species of hydrolyzed aluminium are highly charged and very efficient in particles and
dissolved organic matter removal (Yan et al., 2008a, 2008b). Algae also can affect
coagulation due to characteristics such as morphology, motility, surface charge and
algogenic organic matter (Henderson et al., 2008a, 2010).
In the tropical semi-arid region of Northeastern Brazil there are thousands of
eutrophic man-made lakes that are used for water supply despite of constant blooms of
toxic cyanobacteria. Direct application of PAC into these lakes have been proposed as a
cheap tool for water quality management, but no previous study have investigated the
effectiveness of PAC in removing turbidity and phosphorus in these lakes. The aim of
this study was to evaluate the performance of polyaluminium chloride in water quality
improvement of six eutrophic shallow lakes in Brazilian semiarid region through
laboratory jar tests. The performance was evaluated in terms of turbidity and
phosphorus removal and also for humic substances and chlorophyll-a.
2. Material and methods
Raw water
Water samples were collected from the pelagic region of six reservoirs in Rio
Grande do Norte State, Brazil: Gargalheiras, Passagem das Traíras, Boqueirão,
Dourados, Cruzeta and Timbaúba reservoir. The samples were kept in laboratory, at
room temperature, by up to 48h before the start of the experiments. The turbidity (NTU;
Turbidimeter AP2000), concentrations of chlorophyll-a (Jespersen & Christoffersen,
61
1988) and concentrations of total phosphorus (Valderrama, 1981; Murphy & Riley,
1962) were measured to caracterize the raw water.
Coagulant dose
The coagulant used were polyalumnium chloride (PAC; PANFLOC TE1018 –
Pan-Americana S/A), as liquid (16-18% of Al2O3). A stock solution were prepared at a
concentration of 1 g Al.L-1. Six doses were tested: 0, 2, 4, 6, 8 e 10 mg Al.L-1.
Jar Test
Standard jar test equipment (PoliControl – FlocControl III with 6 probes of two
liters capacity each) was used in a conventional assay method: rapid mixing,
flocculation and sedimentation (Table 1). Two liters of raw water were transferred to
each 2 L probes. The coagulant was dosed just after starting the rapid mixing step. All
experiments were carried out in room temperature at 24°C (±1°). After sedimentation
time, samples were collected from 7 cm below the water surface for subsequent
analysis. Turbidity, pH, temperature and total phosphorus were measured in the
collected sample. A subsample was filtrated (Whatman GF/C) to measure chlorophyll-a
and also UV254 absorbance (1 cm quartz cell; Shimadzu spectrophotometer). UV254 was
measured to indicate the content of dissolved organic matter (DOM), mainly as humic
substances (Leenheer and Croué, 2003).
Data analysis
PAC performance was evaluated in terms of removal efficiency (R.E.) by
sedimentation of flocks, as percentage reduction of chlorophyll-a and total phophorus
concentration, turbidity and UV254 absorbance. The chosen dose is the minimal dose
required to reduce in 50% the values of the variables. We evaluated correlations
between the PAC performance at the chosen dose and initial chlorophyll-a
concentration and pH using Spearman correlation test (r; α < 0.05).
62
3. Results
Initial Conditions of raw water
All reservoirs were classified as eutrophic according to Thornton and Rast
(1993) as they had chlorophyll-a concentrations > 15 µg L-1 and total phosphorus
concentration > 50 µg L-1(Table 2). Gargalheiras and Passagem das Traíras showed the
highest chlorophyll-a and total phosphorus concentrations.
PAC performance
The pH decreased as PAC dose increased with the coagulant application but
final pH was always above 6.5 (data not shown). In general, the removal efficiency
increased sharply from dose 2 to 4 mg Al.L-1, achieving higher values between the dose
4 to 6 mg Al.L-1 for all variables (Fig. 1). The increasing in the dose from 6 to 10 mg
Al.L-1 did not cause a substancial increasing in the efficiency for total phosphorus,
chlorophyll-a and turbidity removal. However, the efficiency increased continuously
with the increasing of the Al dose for UV254 removal where only the highest doses
showed an efficiency above 50%. All doses tested removed less than 50% of total
phosphorus, turbidity, UV254 and chlorophyll-a for Passagem das Traíras reservoir (Fig.
1).
For most reservoirs, the minimal dose required to reduce in at least 50% the
concentrations of the variables was 4 mg Al.L-1 (Fig. 2). This dose resulted in turbidity
values ≤ 10 NTU, total phosphorus concentration ≤ 50 µg L-1 and chlorophyll-a
concentration ≤ 15 µg L-1 for Cruzeta, Timabúba e Dourados reservoirs. Boqueirão
reservoir water achieved a turbidity of 10.3 NTU and had total phophorus concentration
reduced to values below 50 µg L-1 but clorophyll-a concentration remained above 15 µg
L-1. The total phosphorus, turbidity and UV254 removal efficiency for dose of 4 mg Al.L-
1 was significantly negatively correlated with the initial chlorophyll-a concentration and
pH (Fig. 3). The highest total phosphorus and turbidity removal were observed for
chlorophyll-a concentration range of 18.8-39.9 µg L-1 and pH range of 6.8-7.9. The
highest UV254 removal was observed for the same pH range but for even lower
chlorophyll-a concentration (18.8-27.3 µg L-1).
63
4. Discussion
In general, PAC showed good performance in removing total phosphorus
concentrations and turbidity, but its efficiency was affected by chlorophyll-a and pH.
We suggested that 4 mg Al.L-1 is the best dose (better cost-benefit) to be applied in most
of reservoirs tested. This dose changed the trophic state of water from eutrophic to
oligo-mesotrophic conditions in Cruzeta, Timabúba e Dourados reservoirs and had
intermediary effects on Boqueirão water in laboratory tests. The efficiency in total
phosphorus removal is reported for in-lake PAC application (Reitzel et al., 2005; Lopata
& Gawrońska, 2008; Egemose et al., 2011; Jancula & Maršálek, 2012). However, PAC
showed a low efficiency in improving water quality in Gargalheiras and Passagem das
Traíras reservoirs.
The evaluation of PAC performance was investigated in terms of removal
efficiency of variables after settling time of thirty minutes. Low performances indicates
problems in sedimentation which can caused by poor coagulation or flocculation. High
chlorophyll-a concentration, pH and humic substances in initial conditions probably are
the causes of the low efficiency removal of flocks formed by PAC in Gargalheiras and
Passagem das Traíras water. Our correlations showed that total phosphorus, turbidity
and UV254 removal efficiency are correlated with higher values in initial pH and
chlorophyll-a concentrations. Initial pH has been reported to affect coagulation
performance of PAC (Yang et al., 2010). In the maximal PAC performance pH was
found to be around the neutral (Hu et al., 2006; Julio et al., 2010). Coagulation is
favorable for a pH range of 6.0-7.0 due to presence of positively charged Al species
promoting flock formation (Pernitsky & Edzwald, 2006) which determines the
coagulation performance (Yan et al., 2008a). After the dose application of 4 mg Al.L-1
the pH remain above 8.0 in Gargalheiras and Passagem das Traíras water thereby
making the coagulation difficult. It has been suggested that high alkaline waters requires
a higher PAC dose to achieve pH values favorable to coagulation (Hu et al., 2006).
The coagulation process induces the formation of flocks with differents size,
charge and density, factors that influence directly in flock sedimentation. Flocks formed
by algae cells have low density making them difficult to settle (Edzwald, 1993;
Henderson et al., 2008b). Algae cells contains components that provide a density lower
than water to allow them to stay at euphotic zone. Lipid accumulation, mucilage
production, ionic regulation and gas vesicules are components of algal cell to avoid
64
sedimentation (Reynolds, 2006). Extracellular and surface-retained organic matter
produced by algae are reported to inhibit floc formation ((Henderson et al., 2008a,
2010). Lipopolysaccharide on cell surface of Microcystis aeruginosa produced by the
excess of growth exhibited inhibitory effects on PAC coagulation (Takaara et al., 2010),
which can be one important cause of the increase in coagulant demand in algae-rich
waters (Takaara et al., 2007). Also the flocculation process is negatively affected by the
presence of dissolved organic matter present in the water (Edzwald, 1993), particularly
humic susbtances. Humic substances are highly negatively charged (Yan et al., 2008a),
which increases with increasing the pH and by adsorbing onto the surfaces of natural
particles (Pernitsky & Edzwald, 2006). Maximal UV254 removal was observed to be
found around pH 6.0 (Yan et al., 2008b; Yang et al., 2010). The aquatic humic
substances form complexes with dissolved aluminium species which are removed by
adsorbing onto a solid, making the coagulation difficult in waters with both algae and
humic susbtances (Pernitsky & Edzwald, 2006).
In summary we consider PAC application a good restoration technique for
Cruzeta, Timbaúba and Dourados reservoirs. For Gargalheiras and Passagem das
Traíras reservoirs, with high pH and particulate and dissolved organic matter
concentration, we suggest PAC dose > 10 mg Al.L-1 to reduce the trophic state.
Addition of polyaluminium chloride is a potential tool for water quality
improvement of eutrophic shallow lakes in Brazilian semiarid region but its efficiency
depends on the pH and particulate and dissolved organic matter concentration in the
lake or reservoir water.
Acknowledgements
We thank the participants of the MEVEMUC/FINEP and ESEC/CNPq projects for the
provision of water from the reservoirs to the tests.
65
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Tables and Figures
Table 1. Jar test conditions.
Table 2. pH, Turbidity, absorbance at UV 254nm, total phosphorus (TP) and chlorophyll-
a concentrations in raw water used for jar tests.
70
Figure 1. Removal efficiency (%) for total phosphorus (TP), turbidity (NTU), humic
substances (UV254) and chlorophyll-a (Chl-a) in raw water from Dourados (DOU),
Timbaúba (TIM), Cruzeta (CRU), Boqueirão (BOQ), Gargalheiras (GAR) and Passagem
das Traíras (PTR) reservoirs, after coagulation-flocculation with different concentrations
of aluminium (0, 2, 4, 6, 8 and 10 mg Al L-1) and 30 minutes of sedimentation.
71
Figure 2. Removal efficiency (%) for total phosphorus (TP), turbidity (NTU), humic
substances (UV254) and chlorophyll-a (Chl-a) in raw water from Dourados (DOU),
Timbaúba (TIM), Cruzeta (CRU), Boqueirão (BOQ), Gargalheiras (GAR) and Passagem
das Traíras (PTR) reservoirs, after coagulation-flocculation with the dose of 4 mg Al L-1
and 30 minutes of sedimentation.
72
Figure 3. Correlations between chlorophyll-a concentration and pH with total phosphorus
(TP), turbidity and UV254 removal efficiency (%) based on the dose of 4 mg Al L-1.
73
Considerações Finais
Os resultados desta tese mostraram que uma melhoria na qualidade da água dos
reservatórios da região semiárida podem ser alcançada através da precipitação do
fósforo e da remoção de peixes bentívoros, sendo os efeitos somados quando as técnicas
são combinadas.
A combinação do floculante policloreto de alumínio (PAC) e da argila bentonita
modificada com lantânio (LMB) foi efetiva em sedimentar florações de diferentes cepas
de C. raciborskii, resultado este que não foi obtido quando PAC ou LMB foi aplicado
sozinhos. Esse resultado ressalta a importância de adicionar ‘peso’ aos flocos formados
a partir de florações de cianobactérias que são capazes de regular sua flutuabilidade na
água. Em seguida, foi observado que esta técnica é capaz de controlar o crescimento
desta cianobactéria a partir da coagulação e limitação do crescimento por fósforo, mas
que esse efeito é dependente da dose, da cepa e da biomassa da cianobactéria no
momento da aplicação.
Quando aplicado em um lago raso eutrofizados (escala de mesocosmo), o PAC
apresentou resultado positivos, reduzindo a carga interna de fósforo e aumentando a
transparência da água, resultado semelhante a remoção do peixe bentívoro dominante no
lago. O experimento também mostrou que esses efeitos são aditivos e que estas técnicas
juntas apresentaram um melhor resultado.
O policloreto de alumínio é um composto químico de baixo custo e de fácil
aplicação, quando comparado a outros compostos. A remoção seletiva de peixes se
apresenta com uma técnica simples e pode ser realizado através de parcerias com
pescadores, beneficiando assim o ecossistema aquático e a economia local. A
combinação dessas técnicas aumentam as chances de sucesso, possibilitando assim a
aplicabilidade na região. A avaliação da eficiência do PAC em melhorar a qualidade da
água de outros seis reservatórios eutrofizados do semiárido mostraram que este
coagulante apresenta uma boa performance em remover turbidez e fósforo total, sendo a
eficiência reduzida com o aumento da biomassa de clorofila e pH. Contudo, o PAC
apresentou baixa eficiência em melhorar a qualidade da água em ambientes onde a
biomassa fitoplanctônica foi muito elevada. Baseado nos resultados dos experimentos
de sedimentação (capítulo 1), podemos supor que o uso de uma argila natural da região
pode ser necessário para a efetiva sedimentação dos flocos formados.
74
Os resultados desta tese foram obtidos a partir de experimentos de curta duração
em laboratório e em escala de mesocosmo. A longevidade desses resultados deve ser
investigada em experimentos com lago inteiro e numa maior escala de tempo. Outro
ponto relevante é a implementação de um plano de manejo contínuo que envolva
aplicações periódicas do coagulante associada à manutenção da população de peixes
bentívoros em baixa densidade em reservatórios eutrofizados. Com o passar do tempo,
nova matéria orgânica e inorgânica será depositada sobre a cobertura do coagulante no
sedimento, que será então degradada, liberando os nutrientes do sedimento para a água.
Além disso, novos experimentos são necessários para se determinar a densidade mínima
de peixe necessária para que os efeitos da ressuspensão de nutrientes e sedimento sejam
minimizados.
Os resultados deste estudo mostram que é possível melhorar a qualidade da água
de reservatórios eutrofizados no semiárido brasileiro através do controle da carga
interna de nutrientes, seja pela precipitação e inativação do fósforo no sedimento, como
também pela inibição da liberação do fósforo estocado no sedimento por peixes
bioturbadores, e que os resultados se somam quando as técnicas são aplicadas em
conjunto.
75
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