ABSTRACT: We performed laboratory experiments with natural seawater communities of the Northwestern Mediterranean to test whether turbulence could affect bacterial abundance and activity. There was no direct effect of turbulence on bacteria when they were uncoupled from the remainder of the microbial community. In the presence of the microbial community, bacteria showed higher activity and maintained high abundances for a longer time under turbulence than in still water. Thus, turbulence sufficiently altered some microbial component or process in the water samples that indirectly affected bacteria. The population dynamics of bacteria 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 which are able to prey on larger and more nutritious cells when the encounter rates with these cells are increased through the shear present under turbulence. The result is a higher control on phytoplankton and a relaxation of grazing on bacteria. Hence, episodic high turbulence events in coastal systems could accelerate nutrient recycling.

KEY WORDS: Bacteria · Turbulence · Bacterial activity · Phototrophic pico- and nanoplankton · Grazing · Shift of preferred prey size

Published in MEPS Vol. 172 (1998) on October 22
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1998