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26Stable isotope and fatty acid analyses were used to study carbon sources for animals in a 27 submerged plant bed. Epiphytes growing on Potamogeton perfoliatus, sand microflora, and 28 alder leaves were the most important carbon sources. The most abundant macrophyte, P. 29 perfoliatus was unimportant as a food source. Modelling (IsoSource) showed that epiphytes 30were the most important food source for the most abundant benthic invertebrates, the isopod 31Asellus aquaticus (annual mean contribution 64%), the amphipod Gammarus pulex (66%), 32 and the gastropod Potamopyrgus antipodarum (83%). The mean annual contributions of sand 33 microflora were respectively 21, 19, and 9% and of alder leaves, 15, 15, and 8% for these 34 three species. The relative importance of carbon sources varied seasonally. The relative 35 contribution of epiphytes was lowest for all three grazer species in July: A. aquaticus 38%, G. 36 pulex 43%, and P. antipodarum 42%. . A decline in epiphyte biomass in summer may have 37 caused this switch to less attractive food sources. P. perfoliatus provided habitat and shelter 38 for consumers, but food was mainly supplied indirectly by providing space for attached 39 epiphytes, which are fast-growing and provide a highly nutritious food source.

In an enclosure study in Scho¨hsee, a small mesotrophic lake in Northern Germany, the impact of copepods and daphniids on the seston community was studied. In general, these two guilds differ in their feeding behaviour. Copepods actively select their food, with a preference for larger particles, whereas most cladocerans are unselective filter-feeders. In this study we investigate how the impact of the two different grazers affects zooplankton growth. We combine results obtained in the laboratory with results measured in situ in the enclosures. Copepods and cladocerans were cultured on seston from enclosures that were inhabited by density gradients of copepods or daphniids. We observed that Daphnia grew faster on seston that was pre-handled by copepods than on seston that was pre-handled by daphniids, and that somatic growth decreased with increasing densities of daphniids in the enclosures. In contrast, we observed no differences in development rates for copepods grown on the different media. The population growth rates of Daphnia in the Daphnia treatments were determined in the enclosures. Growth differences in both somaticand population growth of Daphnia were correlated to food quality aspects of the seston. In the laboratory we found that Daphnia growth was correlated with several fatty acids. The strongest regression was with the concentration of 20:4x3 (r 2 ¼ 0.37). This particular fatty acid also showed the highest correlation with growth after normalisation of the fatty acids to the carbon content of the enclosures (r 2 ¼ 0.33). On the other hand, in the enclosure the population growth correlated most to the particulate nitrogen content (r 2 ¼ 0.78) and only to the N:C ratio, when normalised to carbon (r 2 ¼ 0.51).

We investigated the impact of copepods on the seston community in a mesocosm set-up, and assessed how the changes in food quantity, quality and size affected the condition of the grazers, by measuring the RNA:DNA ratios in different developmental stages of Calanus finmarchicus. Manipulated copepod densities did not affect the particulate carbon concentration in the mesocosms. On the other hand, chlorophyll a content increased with higher copepod densities, and increasing densities had a positive effect on seston food quality in the mesocosms, measured as C:N ratios and x3:x6 fatty acid ratios. These food quality indicators were significantly correlated to the nutritional status of C. finmarchicus. In contrast to our expectations, these results suggest a lower copepod growth potential on higher quality food. However, in concordance with earlier studies, we found that when copepods were in high densities the large particles (>1000 lm 3 ) decreased and that the smaller particles (<1000 lm 3 ) increased in number. These patterns were closely linked to the condition of C. finmarchicus, which were of better condition (RNA:DNA ratios) with increasing biovolumes of large particles, and, conversely, lower RNA:DNA ratios with increasing biovolumes of smaller particles. Consequentially, the selective grazing by copepods stimulated increased biovolumes of smaller plankton, and this increase was responsible for the increased food quality, in terms of C:N and x3:x6 ratios. Thus, we conclude that the decreasing growth potentials of C. finmarchicus were a result of a decrease of favourably sized food particles, induced by copepod grazing.