Mesozooplankton impacts on lower trophic levels from freshwater, marine and brackish systems in spring : a comparative study
In order to compare the impact of copepods and cladocerans on phytoplankton biomass, phytoplankton species and nutrient stoichiometry, and to investigate trophic interactions, mesocosm experiments were carried out in freshwater (Schöhsee), saltwater (Hopavågen fjord) and brackish water (Kiel Bight). While freshwater cladocerans decreased the spring phytoplankton bloom and phytoplankton biomass considerably, copepods in salt- and brackish water increased phytoplankton biomass. Thus, even by a high reduction of the spring diatom bloom by copepods in saltwater, no clear water phase could be induced due to growth of small phytoplankton species. The dissolved nutrients (TDN:TDP) were impacted according to stoichiometric theory: copepods decreased the dissolved N:P ratios, while freshwater cladocerans increased the dissolved and particulate N:P ratios. Cladocerans even increased the C:P ratio of the particulate fraction up to extremely high values of ~1000, however, cladocerans did not seem to be limited in phosphorus, they still increased in abundance. By stable isotope analyses of carbon and nitrogen it could be shown that cyclopoid copepods are not only able to prey on cladocerans, but can also change the food source of cladocerans probably by an indirect increase of bacterial abundances. In the saltwater experiment, results suggest that the calanoid copepod Calanus finmarchicus can be more predatory than generally assumed.