Microbial and classical food webs: A visit to a hypertrophic lake

Plankton community structure and fluxes of carbon for bacteria (production and bacterivory) were investigated in the urban, hypertrophic Lake Rod6 (Uruguay) using a short time interval for sampling (5-15 d) during one year. The lake sustains a high phytoplankton biomass (up to 335 pg I- ’ chlorophyll a) always dominated by the filamentous cyanobacteria PZunktot/zrti agardhii. The zooplankton community was numerically dominated by rotifers and ciliates; cladocerans were rare during most of the year. The rotifer abundance was very high (up to lo5 individual 1 _ ’ ), the bacterivorous Anurueopsis fissa being the most abundant species. Predation rates of heterotrophic nanoflagellates (HNF) on bacteria (range: 31-130 bacteria HNF-1 h- ‘) were higher than those reported in the literature for field studies. A carbon budget showed that HNF can consume on average 91 and 76% of the bacterial carbon production in summer and winter, respectively. Bacterial turnover times are the lowest reported until now from field conditions (5 to 42 h). Consequently, bacterial carbon production was extremely high (72 to 1071 pg C 1-l d- ‘). Bacterial production was positively correlated to bacterial abundance but the relationship was significantly improved by the inclusion of temperature (82% variability explained). My results support the general trend for increased bacterial production with increasing trophic status, and suggest a lower energy transfer efficiency to higher trophic levels in hypertrophic lakes due to the many trophic interactions involved.