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 microplankt on populations were followed during 40 days in four large (40 m(3)) en closures. In three tanks a gradient of added nutrients (nitrogen and p hosphorus) was applied, while a fourth tank was used as a control. On day 14, the top predator (sea bream Sparus aurata larvae) was introduc ed into the tanks and part of the water column in each tank was isolat ed in a plastic bag without fish larvae, to act as a control for preda tion. 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 de pletion. Flagellate and dinoflagellate abundance subsequently increase d, these organisms being limited by zooplankton grazing. The zooplankt on populations were controlled by both resources (mostly flagellates) and predation (by fish larvae) as indicated by the results of the cont rol experiments.