PHYTOPLANKTON AND ZOOPLANKTON COMMUNITY STRUCTURE AFTER NUTRIENT ADDITIONS TO THE OLIGOTROPHIC LAKE HECKLAN, SWEDEN

Lake Hecklan, in central Sweden, was fertilized with phosphorus and nitrogen during thermal stratification (late May-early Oct) 1984-1987. The nutrient additions were relatively small and raised the total phosphorus concentrations from 6 to 10 mug l-1. The working hypothesis was that this moderate increase In the phosphorus concentration could increase the phytoplankton biomass without adverse changes in the planktonic community structure. The fertilization increased the phytoplankton biomass from 0.1 to a maximum of 2 mm3 l-1. Chrysophyceae and Cryptophyceae dominated throughout the experimental period. Thus, the phytoplankton composition remained typical for a Swedish forest lake and provided a potential for increased zooplankton growth. An increased growth of zooplankton was indicated by increased biomass of Cladocera and Copepoda in 1984 and 1985, and by increased fecundity of herbivorous zooplankton.