ZOOPLANKTON CONTRIBUTION TO PARTICULATE PHOSPHORUS AND NITROGEN IN LAKES

Based on the recognition of rather constant species-specific element to dry weight ratios in freshwater zooplankton, pools of metazoan zooplankton P and N were calculated for 45 Norwegian lakes of varying trophy. On the average zooplankton constituted 20.4 +/- 12.3% and 4.6 +/- 4.3% of particulate P and N respectively. The fraction was considerably higher in many oligotrophic lakes, at most >50 and 20% respectively, but declined to approximately 15% (P) and approximately 5% (N) in meso- to eutrophic lakes. In general, phytoplankton contribution to particulate P was <50%, leaving a large share of particulate P to bacteria and microzooplankton. The zooplankton proportion of particulate P was only weakly influenced by the predation pressure in terms of fish community structure. Zooplankton P is important in the overall lake metabolism, and knowledge of this pool may be used to estimate zooplankton-mediated loss and regeneration of P and improve total nutrient element budgets in lakes.