Herbivorous crustacean zooplankton link phytoplankton and other microorganisms with higher trophic levels in pelagic food webs. Crustaceans show little intraspecific variation in somatic C:N:P elemental ratios, which may have implications for the regulation of zooplankton growth and for nutrient cycling.

Nucleic acids were found to constitute the largest pool of P in Daphnia and Eudiaptomus. This proportion of the total P pool decreased with increasing size and decreasing reproductive rate. In daphnids, 14% of the P was allocated to the carapace, which may result in a significant loss of P from the epilimnion during periods of high Daphnia abundance.

Algal P content was an important factor determining the body growth rate of Daphnia, but at low algal C:P ratios, the addition of polyunsaturated fatty acids (PUFA) to the diet resulted in earlier maturation. It is concluded that the two major hypotheses for explaining nutrient-limited growth, i.e. that growth is limited mainly by the availability of P and PUFA, respectively, are not mutually exclusive.

Effects of grazers on the nutrient status and growth of phytoplankton and bacterioplankton were assessed in laboratory and field experiments. Grazing by Daphnia resulted in changes in the stoichiometry of phytoplankton and bacterioplankton. Grazing by Eudiaptomus stimulated bacterial growth. Similar responses have previously been explained as trophic cascades via bacterivorous protozoa, but this study shows that the response may also be explained by zooplankton nutrient regeneration. In a field study, bacterioplankton production was frequently limited by P at the same time as phytoplankton were N-limited. During early summer, when bacteria were co-limited by P and C, zooplankton stimulated bacterioplankton production.

This thesis shows that zooplankton are affected by the stoichiometry of their food, and that zooplankton affect the nutrient status and growth of their food organisms. The feedback mechanisms involved are not yet fully understood, and their importance in natural systems has not been extensively explored. In future food-web research, we need to recognise differences between species in terms of their roles in nutrient cycling. Approaches aimed at improving the understanding of pelagic food webs based on ecological stoichiometry appear to hold promise for such work.