Influence of hydrological seasonality on bacterioplankton in two neotropical floodplain lakes

Seasonal fluctuation in river stage strongly affects the ecological functioning of tropical floodplain lakes. This study was conducted to assess the influence of hydrological seasonality on bacterial production and abundance in two floodplain lakes of the Autana River, a blackwater river in the Middle Orinoco basin, Venezuela. Water samples for nutrient chemistry, chlorophyll a, and microbiological determinations were collected in two floodplain lakes and in the mainstem of the river during 1997-98. DOC and chlorophyll a concentrations were similar between mainstem and lake sites during high water when river and lakes were well connected but became different during the period of low water when the interaction was minimal. Higher values of bacterial production were observed in the floodplain lakes (0.62-1.03 mug C l(-1) h(-1)) compared to the mainstem sites (0.17- 0.19 mug C l(-1) h(-1)) during the period of low water, while during the period of high water river and lake sites showed similar levels (0.04 mug C l(-1) h(-1)). Bacterial numbers followed bacterial production in the floodplain lakes, reaching higher numbers during the period of low water (1.41-2.40 x 10(6) cells ml(-1)). Availability of substrate and inorganic nutrients, pH, and inputs and losses of bacterial cells could be determining the observed seasonal patterns in bacterial production and abundance. The Autana lakes exhibited a strong seasonal pattern in the chemical and biological conditions, showing higher productivity during the lentic phase that lasted between 5 and 6 months.