SEASONAL PATTERN IN NUTRIENT LIMITATION AND GRAZING CONTROL OF THE PHYTOPLANKTON COMMUNITY IN A NONSTRATIFIED LAKE

1. The relative importance of zooplankton grazing and nutrient limitat ion in regulating the phytoplankton community in the non-stratified La ke Kvie, Denmark, were measured nine times during the growing season. 2. Natural phytoplankton assemblage bioassays showed increasing import ance of nutrient limitation during summer. Growth rates at ambient nut rient concentrations were continually below 0.12 per day, while co-enr ichment with nitrogen (N) and phosphorus (P) to above concentration-sa turated conditions enhanced growth rates from May to the end of July. 3. Stoichiometric ratios of important elements in seston (C:N, C:P, N: P), in lake water (TN:TP) in external loading (TN:TP) and in internal loading (DIN:DIP) were measured to determine whether N or P could be t he limiting nutrient. TN:TP molar ratio of both lake water, benthic fl uxes and external loading suggested P limitation throughout the growin g season. However, seston molar ratios suggested moderate P-deficiency only during mid-summer. 4. Abundance and community structure of the z ooplankton varied considerably through the season and proved to be imp ortant in determining the responses of algal assemblages to grazing. H igh abundance of cladocerans and rotifers resulted in significant graz ing impact, while cyclopoid copepods had no significant effect on the phytoplankton biomass. 5. Regeneration of ammonium and phosphate by zo oplankton were periodically important for phytoplankton growth. A comp arison of nutrient regeneration by zooplankton with nutrient inputs fr om sediment and external sources indicated that zooplankton may contri bute significantly in supplying N and P for the growth of phytoplankto n.