|Beschreibung:||The impact by freshwater and marine mesozooplankton on phytoplankton particle size and stoichiometry was studied. Additionally, zooplankton d15N were determined as an inidicator of trophic level. Mesocosm studies, using logarithmically scaled zooplankton density gradients, were performed at three locations (lake Schoehsee, the marine Hopavagen lagoon, Norway, and Kiel Fjord) in August 2000, July 2001 and September 2002. In two of the three studies (Schoehsee, Hopavagen), zooplankton shifted phytoplankton size structure predictably to small or large species, respectively, with freshwater zooplankton (copepods, Daphnia) being complementary in their impact. Stoichiometrically, both Daphnia and marine copepods increased the carbon to nutrient ratios (C:P or C:N) of POM, indicative of phytoplankton nutrient limitation. Yet, while the depletion of P in freshwater POM was due to its retention in ‘new’ Daphnia biomass (numerical increase), the mechanism for marine copepods was different: copepods triggered the growth of nanoflagellates via a a trophic cascade (‘copepods-ciliates-nanoflagellates’), which increased at the expense of their intracellular nitrogen pool. Measurements of zooplankton d15N showed that differences were large between Daphnia and freshwater copepods (3.2 to 4.8‰), while negligible between marine copepods and the appendicularian Oikopleura dioica. Moreover, stable isotope analysis revealed different patterns of d15N enrichment for cruising versus stationary suspension feeding copepods, and also traced the (indirect) transfer of isotopically ‘light’ nitrogen from diazotrophic cyanobacteria to zooplankton.