Differential impacts of copepods and cladocerans on lake seston and resulting effects on zooplankton growth
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Becker C, Feuchtmayr H, Brepohl D, Santer B, Boersma M
Daphnia, Copepod, stoichiometry, homeostasis, feeding, growth, PUFA, mesocosm, Schosee, Germany, 1.6 m3, freshwater
In an enclosure study in Scho¨ hsee, a small mesotrophic lake in Northern Germany, the impact of copepodsand daphniids on the seston community was studied. In general, these two guilds differ in their feedingbehaviour. Copepods actively select their food, with a preference for larger particles, whereas most cladoceransare unselective filter-feeders. In this study we investigate how the impact of the two differentgrazers affects zooplankton growth. We combine results obtained in the laboratory with results measured insitu in the enclosures. Copepods and cladocerans were cultured on seston from enclosures that wereinhabited by density gradients of copepods or daphniids. We observed that Daphnia grew faster on sestonthat was pre-handled by copepods than on seston that was pre-handled by daphniids, and that somaticgrowth decreased with increasing densities of daphniids in the enclosures. In contrast, we observed nodifferences in development rates for copepods grown on the different media. The population growth rates ofDaphnia in the Daphnia treatments were determined in the enclosures. Growth differences in both somaticandpopulation growth of Daphnia were correlated to food quality aspects of the seston. In the laboratorywe found that Daphnia growth was correlated with several fatty acids. The strongest regression was with theconcentration of 20:4×3 (r2 ¼ 0.37). This particular fatty acid also showed the highest correlation withgrowth after normalisation of the fatty acids to the carbon content of the enclosures (r2 ¼ 0.33). On theother hand, in the enclosure the population growth correlated most to the particulate nitrogen content(r2 ¼ 0.78) and only to the N:C ratio, when normalised to carbon (r2 ¼ 0.51).
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