Trophic interactions between zooplankton and the microbial community in contrasting food webs: the epilimnion and deep chlorophyll maximum of a mesotrophic lake

Trophic interactions between the mesozooplankton and the microbial web (bac teria, protozoa) were studied in 2 in situ grazing experiments in the epili mnion and the deep chlorophyll maximum (DCM) of a mesotrophic lake. We comb ined the Landry-Hassett dilution technique, whereby growth and grazing rate s can be determined simultaneously, with the presence or absence of mesozoo planktun and additional nutrients. The epilimnion was cladoceran-dominated, and had relatively high ciliate, and low heterotrophic nanoflagellate (HNF ) abundance, while the DCM was rotifer-dominated, with fewer ciliates, but higher HNF abundance. Temperature differed by 13 degreesC (22 degreesC in t he epilimnion; 9 degreesC in the DCM). The community differences between th e epilimnion and DCM and the experimental manipulations had their greatest effects on ciliates, lesser effects on HNF, and essentially no net effect o n bacteria. Bacteria growth rates and grazing losses did not differ between the epilimnion and DCM, despite the large differences in chlorophyll a and abiotic parameters, and were independent of nutrient addition or mesozoopl ankton removal. In contrast, the experimental manipulations produced relati vely moderate changes in HNF growth rates and grazing losses in both the ep ilimnion and DCM. Although HNF densities were 3 times higher in the DCM tha n the epilimnion, epilimnetic HNF growth rates were approximately twice the rate found in the DCM. This was consistent with both higher temperature an d a release from predation through indirect predatory effects on alternativ e HNF predators like ciliates. In the DCM, crustacean zooplankton were less abundant and therefore had smaller direct and indirect effects on the HNF, while the metazoan microzooplankton (mainly rotifers) were more important. Ciliate growth rates were lower in the epilimnion than in the DCM, but onl y in the epilimnion were ciliate growth rates enhanced by the addition of n utrients. The cladoceran-dominated community of the epilimnion was able to reduce ciliate growth rates to negative values, while in the rotifer-domina ted DCM, ciliate growth rates were always positive along a predator density gradient. These results demonstrate that while ciliates, and to a lesser e xtent heterotrophic flagellates, respond quickly to changes in predator com munity composition, there are so many other direct and indirect pressures o n bacteria that changes in the mesozooplankton community composition have n o net effect on their adundance.