Planktonic food web in marine mesocosms in the Eastern Mediterranean: Bottom-up or top-down regulation?

A mesocosm experiment was conducted in order to study the structure of the planktonic food web. The dynamics of pico-, nano- and microplankton populations were followed during 40 days in four large (40 m3) enclosures. In three tanks a gradient of added nutrients (nitrogen and phosphorus) was applied, while a fourth tank was used as a control. On day 14, the top predator (sea bream Sparus aurata larvae) was introduced into the tanks and part of the water column in each tank was isolated in a plastic bag without fish larvae, to act as a control for predation. Physical parameters, chlorophyll a and nutrient concentrations, as well as plankton concentrations were monitored. A diatom bloom was observed in all four tanks, in the first phase ending with silicate depletion. Flagellate and dinoflagellate abundance subsequently increased, these organisms being limited by zooplankton grazing. The zooplankton populations were controlled by both resources (mostly flagellates) and predation (by fish larvae) as indicated by the results of the control experiments.