Omnivory in planktonic food webs: a study on the impact of mixotrophic flagellates and microzooplankton on food web dynamics and productivity

Omnivory, feeding on two or more different trophic levels by one consumer, is a common phenomenon in aquatic food webs. A terminal predator is competing with a so-called intermediate consumer for a common basal resource. At the same time, the terminal predator preys upon the intermediate consumer, making the intermediate consumer suffering from competition and predation at the same time. According to recent dynamical models, omnivory should lead to exclusion of the intermediate consumer when productivity of the common basal resource is high. In addition, the presence of an intermediate consumer should reduce the productivity of the top predator, since in its presence, the top predator is feeding on a higher trophic level than in its absence. Mixotrophy is a special case of omnivory: Here, a phototrophic protist is additionally consuming particulate prey (usually small phytoplankton and bacteria) by phagotrophy. Phytoplankton and bacteria compete with the mixotroph on the shared resources nutrients and light (only phytoplankton) and are its potential prey at the same time. Effects of omnivory and mixotrophy were investigated in experimental marine food webs. The results show, that (i) mixotrophic flagellates may strongly reduce their intermediate consumers picoplankton (bacteria and small phytoplankton), and enhance primary production at the same time. (ii) Though microzooplankton represents an intermediate competitor for calanoid copepods, it primarily meliorates the food environment for copepods. (iii) In no experiment, omnivory led to exclusion of an intermediate consumer.