Carbon fluxes from the microbial food web to mesozooplankton. An approach in the surface layer of a pelagic area (NW Mediterranean Sea)

The microbial food web structure, the carbon flux between mesozooplankton and the microbial community, and the importance of heterotrophs in the diet of marine zooplankton were investigated in the surface layer of a pelagic area of the NW Mediterranean Sea in five 24-h shipboard experiments: Heterotrophic flagellates grazed 65 % to 91 % of the bacterial production. However, the protozoan community seems also to utilize alternative carbon sources, such as small autotrophs. In most of the experiments, the estimated production of the protozoan > 7 mu m was sufficient to supply mesozooplankton carbon demand. Copepods stimulated bacterial growth, presumably through excretion and preference for heterotrophic flagellates. Variations of the experimentally estimated metabolic O/N ratio (oxygen consumption through respiration, relative to nitrogen excretion) indicated that the mesozooplankton nutrition varied with the type of food web found on each occasion. Mesozooplankton directly interacted with the microbial food web by grazing upon heterotrophic protozoa, especially when small phytoplankton dominated the autotrophic community or under oligotrophic conditions.