Relationship between bacterial density and abiotic factors at different sediment depths of lakes in the Upper Paraná River floodplain / Relação entre densidade bacteriana e fatores abióticos em diferentes profundidades do sedimento em lagoas da planície de inundação do Alto Rio Paraná

River-floodplain systems are known for their heterogeneity of habitats and the hydrological pulse, the main driving force, which increases nutrient concentrations at the onset of the flood due to leaching from the littoral region and decomposition. This decaying organic matter tends to be deposited in the sediment, where occur biogeochemical processes associated with microorganisms. This study aimed to characterize the vertical distribution of bacterial density in the sediment strata of six environments of the Upper Paraná River floodplain, checking similarity as for bacterial density and physical and chemical conditions. To this end, we analyzed the following factors: total phosphorus, total Kjeldahl nitrogen, levels of organic and inorganic material, oxidation potential and particle size. The results evidenced a gradient of vertical distribution of bacterial density, with predominance of cocci, which possibly indicates no nutrient limitation in this limnic compartment. The analysis of variance was applied to determine significant differences between the layers of the sediment and environments. It can be observed a tendency of deposition of total-P and MI within the upper layers of all environments. More reducing potentials in the initial layers indicate a higher bacterial activity, since this region possesses a greater availability of most easily decomposable material.

Sistemas rios-planície de inundação são conhecidos pela sua heterogeneidade de habitats e pelo pulso hidrológico, principal função de força atuante, que aumenta as concentrações de nutrientes no início devido à lixiviação da região litorânea e à decomposição. Essa matéria orgânica tende a ser depositada no sedimento, onde ocorrem processos biogeoquímicos associados a microrganismos. O objetivo deste estudo foi caracterizar a distribuição vertical da densidade bacteriana em estratos do sedimento de seis ambientes da planície de inundação do Alto Rio Paraná, verificando a similaridade entre os mesmos em relação à densidade bacteriana e condições físicas e químicas. Foram analisados os seguintes fatores: Fósforo total (P-total), nitrogênio total Kjeldahl (NTK), material orgânico (MO) e inorgânico (MI), potencial oxidativo e granulometria. Os resultados mostraram um gradiente de distribuição vertical da densidade bacteriana, com predomínio da forma cocos, possivelmente indicando que não há limitação de nutrientes nos estratos. Análise multivariada mostrou grupamentos por ambientes, ressaltando a grande heterogeneidade de condições limnológicas e de habitats. Pôde-se observar uma tendência de deposição de P-total e MI nas ultimas camadas de todos os ambientes. Potenciais mais redutores nas camadas iniciais indicam maior atividade bacteriana, visto que nesta região há maior disponibilidade material mais facilmente decomponível.

Large scale distribution of bacterial communities in the upper Paraná River floodplain

A bacterial community has a central role in nutrient cycle in aquatic habitats. Therefore, it is important to analyze how this community is distributed throughout different locations. Thirty-six different sites in the upper Paraná River floodplain were surveyed to determine the influence of environmental variable in bacterial community composition. The sites are classified as rivers, channels, and floodplain lakes connected or unconnected to the main river channel. The bacterial community structure was analyzed by fluorescent in situ hybridization (FISH) technique, based on frequency of the main domains Bacteria and Archaea, and subdivisions of the phylum Proteobacteria (Alpha-proteobacteria, Beta-proteobacteria, Gamma-proteobacteria) and the Cytophaga-Flavobacterium cluster. It has been demonstrated that the bacterial community differed in density and frequency of the studied groups. And these differences responded to distinct characteristics of the three main rivers of the floodplain as well as to the classification of the environments found in this floodplain. We conclude that dissimilarities in the bacterial community structure are related to environmental heterogeneity, and the limnological variables that most predicted bacterial communities in the upper Paraná River floodplain was total and ammoniacal nitrogen, orthophosphate and chlorophyll-a.