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The aim of this work was the comparison of the impact of copepods and cladocerans, the major guilds within the mesozooplankton in marine and freshwater habitats, respectively, on lower trophic food levels. Differences between copepods and cladocerans were expected because these two zooplankton guilds differ in their feeding behavior; copepods can actively select food particles, while cladocerans are unselective filter-feeders. Copepods and cladocerans are also known for their different stoichiometric N:P ratios, thus the project aimed at comparing the impacts on the nutrient ratios. Furthermore, zooplankton interactions were compared upon stable isotope analysis in order to determine differences between the zooplankton species. Mesocosm experiments were conducted at three different sites in order to compare freshwater (Lake Schöhsee), brackish (Kiel Bight) and marine (Hopavågen) systems. The mesocosm bags were stocked with natural occurring copepods or cladocerans in a logarithmically scaled density gradient in order to examine the effects qualitatively and quantitatively. Whereas copepod and cladoceran treatments were set up in the freshwater experiment, only copepods were used in the marine and in the brackish experiment due to very low abundances of cladocerans.
The first objective of this study was the change of the phytoplankton community induced by copepods and cladocerans. By conducting the experiments in spring, we aimed at investigating if the zooplankton guilds are able to reduce the phytoplankton spring bloom independent of their initial density. In freshwater, copepods were not able to reduce the phytoplankton bloom efficiently, no matter of their initially stocked abundance. This finding might be due to a strong decrease in copepod density, either caused by crowding effects, unsuitable food availability or, most likely, predation of cyclopoid copepods. In contrast, Daphnia were able to increase rapidly in abundance and graze intensively on the phytoplankton. The main reason for this impact was the relatively small size of the phytoplankton present, suitable for the filter-feeder Daphnia. After around 22 days, Daphnia in all different density bags managed to decrease phytoplankton abundances markedly and induce a clear water phase within the enclosure bags. The opposite was found in saltwater, where copepods of all density bags were able to considerably decrease the spring bloom, consisting mainly of long chained diatoms. This indicates that natural occurring densities of cladocerans in Lake Schöhsee and calanoid copepods in Hopavågen fjord might be able to decrease the spring phytoplankton bloom.
However, in contrast to freshwater cladocerans, marine copepods did not induce a clear water phase, due to restricted grazing on large phytoplankton, simultaneously increasing small cells. In brackish water, the phytoplankton bloom occurred previous to the experiment, but similar results as in Hopavågen were found. In confirmation, copepods in the freshwater experiment also increased phytoplankton biomass (chl a), mainly small species,either be due to released grazing pressure of copepods on small species while feeding on larger cells, and/or due to an indirect effect via a trophic cascade. While copepods decreased ciliates density-dependently in all experiments (see also Zöllner, 2004) and partly fed on nauplii, ciliates and nauplii released grazing pressure on small cells. Results showed that the most important factors determining the feeding behavior of zooplankton were size and quantity of phytoplankton.
Another aim of this study was the investigation of zooplankton impacts on the nutrient stoichiometry. By different nutritional demands, copepods and cladocerans were expected to induce changes upon the dissolved, sestonic as well as sediment fraction. As copepods are relatively rich in nitrogen, I indeed found a copepod density-dependent decrease of N:P ratios in the dissolved fraction. This decrease of dissolved N:P was supposedly induced by preferential retention of nitrogen compared to phosphorus by copepods, resulting in excretion of lower N:P ratios than ingested. However, the pattern could not be confirmed within the sestonic fraction, but the decrease of copepod abundances and decomposition of dead animals probably added nitrogen to the particulate fraction. Contrastingly, Daphnia had a lower body N:P ratio than copepods. Daphnia preferentially retain phosphorus and thus increased the dissolved N:P ratio as well as the sestonic N:P ratio density-dependently. By these effects, Daphnia showed a feedback mechanism on their food sources, affecting phytoplankton quality, revealing intriguing seston C:P ratios above 1000. Thus, Daphnia affected their own growth by changing the quality of their potential food. Still, Daphnia increased in abundance even at these high ratios, indicating daphniids’ extremely efficient capability of P retention. To my knowledge, Daphnia growth under these high C:P ratios was not reported before.
A comparison of the nutritional differences between freshwater and saltwater copepod C:N and the seston and sediment C:N within the different enclosure bags, revealed differences between the sites. While freshwater copepods contain relatively more nitrogen and show a lower C:N ratio than seston and sediment values, marine copepods seemed to be in balance with the particulate matter. However, this balance was caused by including a lipid rich copepod species (Calanus finmarchicus) in the calculations. All other species confirmed freshwater copepod results, showing a preferential retention of nitrogen. Thus, the interaction of copepods and seston did not differ qualitatively between a freshwater and a saltwater system.
The third aim, 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 δ13C values. Increasing abundances of high nucleic acid bacteria in the copepod bags may account for the trend in Daphnia δ13C 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 other zooplankton in the sea, but show fast changes in their δ13C which might result from a high flexibility to adjust to available carbon sources. In general, the study highlighted the usefulness of complementing conventional plankton study techniques with stable isotope analyses by showing complex species interactions of zooplankton, which would not be revealed by conventional techniques.
