Influence of hydrological seasonality on bacterioplankton in two neotropical floodplain lakes

Seasonal fluctuation in river stage strongly affects the ecological functio ning of tropical floodplain lakes. This study was conducted to assess the i nfluence of hydrological seasonality on bacterial production and abundance in two floodplain lakes of the Autana River, a blackwater river in the Midd le Orinoco basin, Venezuela. Water samples for nutrient chemistry, chloroph yll 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 w ater when river and lakes were well connected but became different during t he period of low water when the interaction was minimal. Higher values of b acterial 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 n umbers followed bacterial production in the floodplain lakes, reaching high er 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 loss es of bacterial cells could be determining the observed seasonal patterns i n bacterial production and abundance. The Autana lakes exhibited a strong s easonal pattern in the chemical and biological conditions, showing higher p roductivity during the lentic phase that lasted between 5 and 6 months.