MICROBIAL AND CLASSICAL FOOD WEBS - A VISIT TO A HYPERTROPHIC LAKE

Plankton community structure and fluxes of carbon for bacteria (produc tion and bacterivory) were investigated in the urban, hypertrophic Lak e Rodo (Uruguay) using a short time interval for sampling (5-15 d) dur ing one year. The lake sustains a high phytoplankton biomass (up to 33 5 mu g l(-1) chlorophyll a) always dominated by the filamentous cyanob acteria Planktothrix agardhii. The zooplankton community was numerical ly dominated by rotifers and ciliates; cladocerans were rare during mo st of the year. The rotifer abundance was very high (up to 10(5) indiv idual l(-1)), the bacterivorous Anuraeopsis fissa being the most abund ant species. Predation rates of heterotrophic nanoflagellates (HNF) on bacteria (range: 31-130 bacteria HNF-1 h(-1)) were higher than those reported in the literature for field studies. A carbon budget showed t hat HNF can consume on average 91 and 76% of the bacterial carbon prod uction in summer and winter, respectively. Bacterial turnover times ar e the lowest reported until now from field conditions (5 to 42 h). Con sequently, bacterial carbon production was extremely high (72 to 1071 mu g C l(-1) d(-1)). Bacterial production was positively correlated to bacterial abundance but the relationship was significantly improved b y the inclusion of temperature (82% variability explained). My results support the general trend for increased bacterial production with inc reasing trophic status, and suggest a lower energy transfer efficiency to higher trophic levels in hypertrophic lakes due to the many trophi c interactions involved.