RESPONSES OF EPILIMNETIC PHYTOPLANKTON TO EXPERIMENTAL NUTRIENT ENRICHMENT IN 3 SMALL SEEPAGE LAKES

This paper describes the responses of three epilimnetic phytoplankton communities to experimental nitrogen and phosphorus enrichment as comp ared to the phytoplankton community in a fourth, unmanipulated, lake. Increased nutrient inputs increased total phytoplankton biomass, prima ry productivity, chlorophytes, cryptomonads and species turnover rates in all three enriched lakes; cyanobacteria increased in two of the th ree enriched lakes. However, nutrient addition also led to declines in previously dominant dinoflagellates and chrysophytes, and in species diversity. At the species level, there were large changes in community composition from year to year in both enriched and reference lakes, s uggesting that phytoplankton community composition is highly dynamic e ven in the absence of enrichment. Overall, changes in total biomass, p roductivity and species diversity were consistent among the enriched l akes, while changes in species composition differed due to variation i n the physical, chemical and biotic environment of each lake. This sug gests that aggregated variates are more useful for quantitative predic tion of nutrient effects, while species responses can be used to signa l qualitative differences in environmental conditions among lakes.