MEPS 145:195-208 (1996)  -  doi:10.3354/meps145195

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. The experiment was conducted in 5 floating 30 m³ mesocosms manipulated with nutrient (N and P) enrichments and fish (stickleback fry) according to a cross-over experimental design with rotating treatments. Concentrations of particulate organic C, N, P, and chlorophyll a as well as the development of bacteria, phytoplankton, protozoa, and mesozooplankton 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 and the effect of the top-down manipulation resulted in diversified structure 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) mesocosms that received nutrients after a 5 d lag-period developed towards a 'new production' type of system. In the latter kind, accumulation and loss of N followed closely the development of autotrophic biomass. In all mesocosms, N-limitation was maintained due to greater sedimentary loss of N, while P was retained more effectively within the detrital pool of the pelagic system. The cascading effect of top-down manipulation influenced the grazer community and resulted in a different functional response in each manipulated mesocosm. These results indicate that during the process of eutrophication, the food web structure, timing of the fertilization, and alternative grazing/predation strategies of the planktonic heterotrophs have a crucial impact on the retention and loss of nutrients from the pelagic system.

Planktonic food web · Carbon · Nitrogen · Phosphorus · Sedimentation · Eutrophication · 'Top-down' and 'bottom-up' manipulations · Baltic Sea