The microbial food web structure of a hypertrophic warm-temperate shallow lake, as affected by contrasting zooplankton assemblages

The composition of zooplankton is known to affect the structure of the microbial trophic web. The zooplankton of the hypertrophic Laguna Chascomús (Argentina) is generally dominated by rotifers and cyclopoids copepods. An unusual dominance by small-cladocerans was observed after a massive winter fish kill in 2007. We hypothesized that small-cladocerans would increase the grazing pressure on heterotrophic flagellates (HF), reducing the degree of coupling between HF and picoplankton. The aim of this study was to investigate the microbial food web structure under two contrasting zooplankton assemblages. The lake was sampled every other week between 2007 and 2009. The abundances of heterotrophic bacteria (HB) and picocyanobacteria (Pcy) laid among the highest values reported for aquatic systems (>10 8 and 10 7 cells ml −1 , respectively). Pcy averaged 53% of total picoplanktonic biomass. When small-cladocerans dominated zooplankton HF reached the higher abundance (>10 5 cells ml −1 ) and picoplankton showed the opposite pattern, while the proportion of grazing resistant morphologies (i.e. microaggregates of Pcy) was higher. In contrast, when rotifers dominated, HF abundance decreased and picoplankton increased. Our data suggest that the degree of HF–HB coupling was affected by changes in zooplankton dominance. In contrast to our initial hypothesis, the present results suggest that large numbers of rotifers (>5,000 ind. l −1 ) are more efficient than small-cladocerans at controlling HF populations.