Eutrophication-threatened aquatic ecosystems: sediment biogeochemical processes towards nutrients control

Tese de doutoramento em Engenharia Química e BiológicaEutrophication is the most common reason for lake management and results mainly from anthropogenic activities occurring in their watersheds. Nowadays, eutrophication is also due to internal inputs of phosphorus (P), as is the case of Azoreans lakes. P tends to accumulate in lake sediments leading to an excess of P in the water column and the concomitant proliferation of planktonic algae. In Azores, despite the considerable effort made to describe the phytoplankton growing on the water column, the lack of information regarding the microbial processes in sediments is still high. In that regard, the present work aims to assess the sediment reference conditions (e.g. biogeochemical composition of sediments) of Azorean lakes in order to incorporate sediment issues and knowledge into management strategies enhancing the efficacy of the remediation activities that should be taken to achieve the good ecological status until 2015. Sediment samples were collected and biogeochemical profiles, organic matter (OM), total nitrogen, TP and total iron (TFe), were determined. Lake Azul profiles were quite homogeneous for all parameters, while in lake Furnas TFe profile presented a peak below the aerobic layer. For lake Verde, the concentration of all studied elements (20±2 % OM; 2.10±0.08 mg/g TP; 1.31±0.50 mg/g TN; 8.06±0.13 mg/g TFe) in the uppermost sediment layer was approximately two times higher than the ones in sediments from other lakes, decreasing with sediment depth. The higher amounts of P and OM in lake Verde suggested a higher internal contribution of P to eutrophication. Dominant members were mostly affiliated to Proteobacteria phylum (Alpha-, Delta-, and Gamma-subclasses), group Bacteroidetes/Chlorobi and phylum Chloroflexi. The Cyanobacteria phylum was solely detected in sediments from lake Verde and lake Furnas that presented the highest amounts of N and P both in the water column and sediments, while the other phyla were detected in sediments from the three studied lakes. The combination of quantitative PCR and activity tests suggested that bacteria performing ammonium oxidation (aerobic and anaerobic), nitrite oxidation, heterotrophic nitrate reduction, iron reduction and biological P storage/release were present and active in sediments. Bacteria performing anaerobic ammonium oxidation were the most abundant in sediments from the eutrophic lakes Verde, Azul and Furnas (4.5 % to 16.6 %), followed by nitrifying bacteria (0.8 % to 13.0 %), denitrifying bacteria (0.5 to 6.8 %), iron-reducing bacteria of the Geobacteraceae family (0.2 % to 1.4 %), and phosphorus-accumulating organisms affiliated to Candidatus Accumulibacter (less than 0.3 %). In opposition, denitrifying bacteria dominated in sediments from the oligo-mesotrophic lake Fogo (8.8 %). Lake Furnas sediments presented bacterial stratification (aerobes in upper layers and anaerobes downward) due to the presence oxygen in the hypolimnion all year round. The present work also suggested that biological phosphorus storage/release by denitrifying bacteria in sediments might as well contribute to the release of P from sediments. Moreover, lake Furnas sediment bacterial community was electroactive as demonstrated both by cyclic voltammetry and power generation in a simulated benthic microbial fuel cell.Martins G. 2010 sediments from the eutrophic lakes Verde, Azul and Furnas (4.5 % to 16.6 %), followed by nitrifying bacteria (0.8 % to 13.0 %), denitrifying bacteria (0.5 to 6.8 %), iron-reducing bacteria of the Geobacteraceae family (0.2 % to 1.4 %), and phosphorus-accumulating organisms affiliated to Candidatus Accumulibacter (less than 0.3 %). In opposition, denitrifying bacteria dominated in sediments from the oligo-mesotrophic lake Fogo (8.8 %). Lake Furnas sediments presented bacterial stratification (aerobes in upper layers and anaerobes downward) due to the presence oxygen in the hypolimnion all year round. The present work also suggested that biological phosphorus storage/release by denitrifying bacteria in sediments might as well contribute to the release of P from sediments. Moreover, lake Furnas sediment bacterial community was electroactive as demonstrated both by cyclic voltammetry and power generation in a simulated benthic microbial fuel cell. Finally, a mathematical model for lake Verde water quality was developed and calibrated in order to support the decision making processes in aquatic restoration programmes. Prospective scenarios showed that external measures are not enough to improve water quality in lake Verde (an average concentration of TP of 26 g/L and phytoplankton biomass of 1.4 mg/L could be reached in a 10 years horizon) and that an integrated approach (external and internal measures) need to be designed. In conclusion, the present work suggested that sediment bacteria were active and performed diverse roles on carbon, nutrients and iron cycling. The variability of sediment geochemical profiles as well as the structure and activity of sediment bacterial community indicates that individual lake sediment characterization and site-specific assessments of the efficacy of remediation approaches are required. In this regard, the designing of re-qualification strategies towards the good ecological status prescribed by the Water Framework Directive should include, in addition to the classical procedure, an evaluation of the contribution of biological processes in sediments to the eutrophication problem.A eutrofização é o motivo mais comum para a necessidade de gestão de lagos e, resulta principalmente de actividades antropogénicas que ocorrem nas bacias hidrográficas. Actualmente, a eutrofização é também devida a entradas internas de fósforo (P). O P tende a acumular-se nos sedimentos, levando a um excesso de P na coluna de água e à consequente proliferação de algas planctónicas. Nos Açores, apesar dos consideráveis esforços para descrever o crescimento do fitoplâncton na coluna de água, a falta de informação relativa aos processos microbianos nos sedimentos é ainda muito elevada. Nesse sentido, o presente trabalho teve como objectivo avaliar as condições de referência dos sedimentos (e.g., a composição biogeoquímica dos sedimentos) dos lagos dos Açores, a fim de incorporar as questões e conhecimento sobre sedimentos nas estratégias de gestão, bem como reforçar a eficácia das actividades de remediação que devem ser tomadas para alcançar o bom estado ecológico até 2015. Nesse sentido, foram recolhidas amostras de sedimentos e os perfis biogeoquímicos, matéria orgânica (OM), azoto total (TN), P total (TP) e ferro total (TFe), foram determinados. Os perfis biogeoquímicos para a lagoa Azul revelaram-se bastante homogéneos para todos os parâmetros determinados, enquanto na lagoa das Furnas, o perfil de TFe apresentou um pico abaixo da camada aeróbia. As maiores quantidades de P e OM na lagoa Verde sugeriram uma maior contribuição interna de P para a eutrofização. Relativamente aos membros dominantes da comunidade bacteriana presente nos sedimentos das lagoas Verde, Azul e Furnas, a sua maioria estava afiliada ao filo Proteobacteria, grupo Bacteroidetes/Chlorobi e filo Chloroflexi. O filo Cyanobacteria foi apenas detectado nos sedimentos das lagoas Verde e Furnas, que correspondem às lagoas com maiores teores de N e P, tanto na coluna de água como nos sedimentos. A combinação do PCR quantitativo com os testes de actividade sugeriu que as bactérias que realizam a oxidação do amónio (aeróbia e anaeróbia), a oxidação do nitrito, a redução heterotrófica de nitrato, a redução de ferro e o armazenamento/libertação biológica de fósforo estavam presentes e activas nos sedimentos. As bactérias anaeróbias oxidantes de amónio foram as mais abundantes nos sedimentos das lagoas eutróficas Verde, Azul e Furnas (4.5 % a 16.6 %), seguidas pelas bactérias nitrificantes (0.8 % a 13.0 %), desnitrificantes (0.5 a 6.8 %), redutoras de ferro da família Geobacteraceae (0.2 % a 1.4 %) e organismos acumuladores de fósforo filiados a “Candidatus Accumulibacter” (