The role of crustacean zooplankton in pelagic nutrient cycling

Abstract: Herbivorous crustacean zooplankton link phytoplankton and other microorganisms withhigher trophic levels in pelagic food webs. Crustaceans show little intraspecificvariation in somatic C:N:P elemental ratios, which may have implications for theregulation of zooplankton growth and for nutrient cycling.Nucleic acids were found to constitute the largest pool of P in Daphnia andEudiaptomus, This proportion of the total P pool decreased with increasing size anddecreasing 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 highDaphnia abundance.Algal P content was an important factor determining the body growth rate ofDaphnif, 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 theavailability of P and PUFA, respectively, are not mutually exclusive.Effects of grazers on the nutrient status and growth of phytoplankton and bacterio-plankton were assessed in laboratory and field experiments. Grazing by Daphniaresulted in changes in the stoichiometry of phytoplankton and bacterioplankton.Grazing by Eudiaptomus stimulated bacterial growth. Similar responses havepreviously been explained as trophic cascades via bacterivorous protozoa, but this studyshows that the response may also be explained by zooplankton nutrient regeneration. Ina field study, bacterioplankton production was frequently limited by P at the same timeas phytoplankton were N-limited. During early summer, when bacteria were co-limitedby 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. Thefeedback mechanisms involved are not yet fully understood, and their importance innatural systems has not been extensively explored. In future food-web research, weneed to recognise differences between species in terms of their roles in nutrient cycling.Approaches aimed at improving the understanding of pelagic food webs based onecological stoichiometry appear to hold promise for such work.