Seasonal variability in plankton community structure, productivity and food web transfer along the salinity gradient of the Baltic Sea

Abstract

The brackish water environment in the semi‐enclosed Baltic Sea causes changes in plankton community structure in close relation to large‐scale circulation patterns. In addition, seasonal changes in nutrients levels and stoichiometric ratios, light and stratification successively modified it. The analysis of comprehensive data sets on environmental properties, micro and meso‐plankton communities, their metabolic activity and productivity basing on classical and enzymatic methods allows verifying the interplay between community structure and food web transfer from Kattegat to the Gulf of Finland in the ninety nineties. Aspects, like the ratio between new and regenerated production, the percentage of primary productivity utilized by heterotrophs of different size, and the stoichiometry in nutrient regeneration by mesozooplankton were investigated. Regions where new (primary) production was detectable were restricted to areas with river discharge after the spring bloom, but did never exceed some percentages of gross production. Nitrogen and phosphorous were excreted by mesozooplankton in N/P=6 in May and N/P=10 in August in the same region because of the larger percentage of parthenogenetically reproducing cladocerans in summer. Herbivorous plankton