Turbulence and the microbial food web: Effects on bacterial losses to predation and on community structure

Abstract

Changes in picoplankton population abundance and growth under turbulence have been suggested to be the consequence of turbulence affecting larger trophic levels and hence the grazing pressure. We designed a laboratory set-up to assess the effects of turbulence on plankton assemblages, and tested the degree of food-web complexity needed to produce cascading effects on picoplankton and the interactions with nutrient enrichment. Grazing pressure on bacteria was relaxed under turbulence and we show that one trophic link is enough to produce effects at the picoplankton level. Nutrient enrichment increased the effect of turbulence as there was more biomass to act upon. The organisms responsible for driving the grazing pressure shifts could not be identified since they seemed to change depending on initial conditions and experimental treatment. A trend of increased heterotrophic biomass under turbulence was found in all cases, which can have important implications in community metabolism dynamics.