IMPACT OF PLANKTONIC FOOD-WEB STRUCTURE ON NUTRIENT RETENTION AND LOSS FROM A LATE SUMMER PELAGIC SYSTEM IN THE COASTAL NORTHERN BALTIC SEA

A large-scale mesocosm experiment was carried out in the coastal area of the northern Baltic Sea in order to study limiting (bottom up) and controlling (top-down) factors of the late summer pelagic community. T he experiment was conducted in 5 floating 30 m(3) mesocosms manipulate d with nutrient (N and P) enrichments and fish (stickleback fry) accor ding to a cross-over experimental design with rotating treatments. Con centrations of particulate organic C, N, P, and chlorophyll a as well as the development of bacteria, phytoplankton, protozoa, and mesozoopl ankton biomass were followed for 21 d, and sedimentation was measured. Nutrient enrichments induced phytoplankton blooms with equal biomass peak levels in all mesocosms. However, the timing of the enrichment an d the effect of the top-down manipulation resulted in diversified stru cture of planktonic communities in each mesocosm. Basically 2 kinds of system emerged: (1) mesocosms that had received nutrients immediately after the start of the experiment developed towards more regenerating systems where both N and P were retained to greater extent; (2) mesoc osms that received nutrients after a 5 d lag-period developed towards a 'new production' type of system. In the latter kind, accumulation an d loss of N followed closely the development of autotrophic biomass. I n all mesocosms, N-limitation was maintained due to greater sedimentar y loss of N, while P was retained more effectively within the detrital pool of the pelagic system. The cascading effect of top-down manipula tion influenced the grazer community and resulted in a different funct ional response in each manipulated mesocosm. These results indicate th at during the process of eutrophication, the food web structure, timin g of the fertilization, and alternative grazing/predation strategies o f the planktonic heterotrophs have a crucial impact on the retention a nd loss of nutrients from the pelagic system.