Seasonal modulation of bacterioplankton community at a temperate eutrophic shallow lake

At the present time, there is still a lack of information about environmental parameters modulating variations on bacterial diversity in temperate lakes, particularly from Portugal. Fermentelos Lake (Central Portugal) is a shallow water body that sustains an important wetland area. The strong nutrient inputs from agriculture and industrial runoffs have led to its current eutrophic status. The present work aimed to understand which factors modulate the seasonal bacterioplankton diversity at this lake using 16S rRNA PCR-denaturing gradient gel electrophoresis (DGGE) and multivariate analysis. Environmental data demonstrated eutrophic features throughout all samples with nitrate concentrations reaching 12.0 mg N (NO₃ ⁻) l⁻¹ in March 2006, while the highest conductivity (609 μS cm⁻¹), soluble reactive phosphorus (0.37 mg l⁻¹), total suspended solids (87.2 mg l⁻¹) and chlorophyll a (286.6 μg l⁻¹) levels were recorded in August 2007. Over the past two decades there was a general increase in nitrate, pH and conductivity levels at this lake, suggesting the eutrophication process is still in progress. Multivariate analysis showed that summer versus winter DGGE patterns could be established for bacterial assemblages and were mainly defined by water temperature and chlorophyll a. Actinobacteria were dominant throughout the study period although a general preference for higher temperature, pH, total suspended solids, conductivity, soluble reactive phosphorus (SRP) and chlorophyll a levels was observed. The highest concentrations of nitrogen sources were related to Bacteroidetes and phototrophic eukaryote (cryptophycean) dominance. The expansion of Betaproteobacteria, Alphaproteobacteria and Cyanobacteria phylotypes was generally associated to high temperature, pH, conductivity and SRP values.