To compare the different impacts of 2 taxa of crustacean mesozooplankton, copepods and cladocerans, in fresh-, salt- and brackish water food webs, 6 large-scale mesocosm experiments were preformed in collaboration with the Leibniz Institute of Marine Sciences at the University of Kiel. Copepods and cladocerans differ in feeding behaviour and nitrogen to phosphorous ratios. To address the effects of copepods and cladocerans on the lower food web and nutrient availability, we generated in situ experiments with different zooplankton densities in freshwater (Lake Schöhsee), brackish (Kiel Bight) and marine (Hopavågen) systems. Furthermore, stable isotope analysis was applied to determine zooplankton interactions and species-specific differences.
One of the main objectives of this study was to analyse changes within the phytoplankton community and the ability of copepods and cladocerans to reduce the phytoplankton spring bloom independent of their initial density. Freshwater copepods were not able to reduce the phytoplankton bloom efficiently, irrespective of the initially stocked abundance. In contrast, Daphnia abundances increased rapidly, independent of their initial abundance, and grazed intensively on the phytoplankton community ultimately inducing a clear water phase. The opposite was found in saltwater, where copepods were able to considerably decrease the spring bloom irrespective of initial abundance. However, marine copepods did not induce a clear water phase.
The explanation for the different top-down control can be found in the phytoplankton size structure and nutrient stoichiometry. Whereas freshwater phytoplankton was mainly composed of small species, suitable to be grazed upon by cladocerans, marine phytoplankton was dominated by large species. Copepods preferentially graze on large cells, simultaneously increasing abundances of small phytoplankton species or taxa. In addition, Daphnia proved to be extremely efficient in phosphorus retention, inducing a C:P ratio in the seston (particles <100µm) above 1000, leaving very low, growth limiting concentrations of phosphorous. Copepods, however, are relatively rich in nitrogen. They did not induce changes in the seston stoichiometry hence did not confine phytoplankton growth.
The application of stable isotope analyses to this study clearly revealed insights of the relative trophic positions of different copepod species, and differences between copepods and cladocerans. In Schöhsee, daphniids from enclosures stocked purely with differing densities of Daphnia showed little variability in stable isotope values, but when cladocerans developed in a copepod-mediated environment, a change in carbon food source occurred for Daphnia. Copepods modified the lower trophic level food web components and abundances, and daphniids that thrived in enclosure bags together with copepods exhibited a density dependent depletion in 13C values. Increasing abundances of high nucleic acid bacteria in the copepod bags may account for the trend in Daphnia 13C via increased respiratory release of isotopically light CO2 into the water column of the bags. Cyclopoid copepod stable isotope signatures from Lake Schöhsee suggest that cyclopoids preyed on the available zooplankton. In Hopavågen, stable isotope analysis revealed that calanoid copepods can feed upon cladocerans in the sea, but marine zooplankton show fast changes and high flexibility to adjust to available carbon sources.