Could bacterivorous zooplankton affect lake pelagic methanotrophic activity?

We tested the hypothesis that grazing by bacterivorous zooplankton might suppress methanotrophic activity in small stratified, humic lakes. During two series of replicated laboratory experiments, densities of methane-oxidising bacteria (MOB) in water from a small, polyhumic lake were measured in the presence of different densities (range 0-200 individuals ](-1)) of a large bacterivorous cladoceran, Daphnia longispina. Grazing by Daphnia and by nanoflagellates reduced the proportion of MOB in the microbial community and the methanotrophic activity decreased significantly at higher Daphnia densities. Thus, the hypothesis arising from previous field observations was supported experimentally, but field studies are required to quantify the influence of grazers on methanotrophic activity and CH4 effluxes of lakes.