Growth and cytometric diversity of bacterial assemblages under different top-down control regimes by using a size-fractionation approach

Zooplankton communities in tropical inland waters are generally characterized by small bodied individuals and the absence of large daphnids. However, the effects of this peculiar food web configuration on microbial compartments have not been tested experimentally. To establish which predator could be responsible for most bacterial loss in a tropical shallow lake, we performed a predation experiment manipulating consumer size fractions. We found that protists had an effect more than four times greater (-86%) than the one exerted by microcrustaceans (-20%), whereas rotifers and nauplii had a minimum effect (-8%). Thus, our results indicate that predation was a crucial factor controlling bacterial abundance and that protists (mainly ciliates) were responsible for most of this loss. Moreover, bacterial community structure was also affected by predation, with a change in the relative proportion of cytometric subpopulations (high-nucleic acid and low-nucleic acid) as a function of different degrees of predation pressure and a decrease in community evenness (assessed by cytometric diversity) with the removal of predators. Therefore, protists play an important role in controlling the abundance and maintaining prokaryotic diversity in warm regions, where zooplankton is present and controlled by juvenile fish throughout the year. (AU)