EFFECTS OF TURBULENCE ON BACTERIAL-GROWTH MEDIATED THROUGH FOOD-WEB INTERACTIONS

We performed laboratory experiments with natural seawater communities of the Northwestern Mediterranean to test whether turbulence could aff ect bacterial abundance and activity. There was no direct effect of tu rbulence on bacteria when they were uncoupled from the remainder of th e microbial community. In the presence of the microbial community, bac teria showed higher activity and maintained high abundances for a long er time under turbulence than in still water. Thus, turbulence suffici ently altered some microbial component or process in the water samples that indirectly affected bacteria. The population dynamics of bacteri a and pigmented eukaryotes suggests that, under turbulence, there is a community grazing shift from smaller to larger prey sizes. This shift can be explained in terms of the advantage to protozoan predators whi ch are able to prey on larger and more nutritious cells when the encou nter rates with these cells are increased through the shear present un der turbulence. The result is a higher control on phytoplankton and a relaxation of grazing on bacteria. Hence, episodic high turbulence eve nts in coastal systems could accelerate nutrient recycling.