EFFECTS OF DAPHNIA ON THE RESPONSE OF MESOTROPHIC LAKES TO EXPERIMENTAL ENRICHMENT

We are currently involved in a whole-lake experiment designed to asses s how Daphnia affect the response of mesotrophic lakes to increased nu trient loading. In the first year of the experiment we wished to demon strate that there are P loading rates which eutrophy lakes without Dap hnia but not lakes with Daphnia. In order to do this, we needed to cho ose an appropriate level and schedule for the experimental enrichment. This paper describes bow we made that choice, then evaluates whether our choice was successful. Based on the literature, a simulation model , and a mesocosm experiment, we decided that enriching at a rate of 1 mu g P L(-1) d(-1) would create the desired contrast between lakes wit h and without Daphnia. Model simulations indicated that mean algal res ponse to enrichment would be comparable under monthly, weekly, or cont inuous additions. We chose to add nutrients continuously at ambient N: P ratios. Experimental results from 1993 suggest that we achieved a nu trient loading rate consistent with our goal: chlorophyll responded le ss to enrichment in the lake with Daphnia than in the lake without Dap hnia. The modeling, mesocosm, and whole-lake studies summarized here s upport the idea that Daphnia reduce chlorophyll at P loads less than o r equal to 1 mu g P L(-1) d(-1). However, cyanobacteria may escape con trol by all grazers at relatively low P loading rates.