The relationship in lake communities between primary productivity and species richness

An understanding of the relationship between species richness and productiv ity is crucial to understanding biodiversity in lakes. We investigated the relationship between the primary productivity of lake ecosystems and the nu mber of species for lacustrine phytoplankton, rotifers, cladocerans, copepo ds, macrophytes, and fish. Our study includes two parts: (1) a survey of 33 well-studied lakes for which data on six major taxonomic groups were avail able; and (2) a comparison of the effects of short- and long-term whole-lak e nutrient addition on primary productivity and planktonic species richness . In the survey, species richness of all six taxa showed a significant quadra tic response to increased annual primary productivity (C-14 estimate, g C.m (-2).yr(-1)) when lake area is taken into account. However, the richness-pr oductivity relationship for phytoplankton and fish was strongly dependent o n lake area. The relationship for phytoplankton, rotifers, cladocerans, cop epods, and macrophytes was significantly unimodal. Species richness general ly peaked at levels of primary productivity in the range of 30-300 g C.m(-2 ).yr(-1). For the average lake size, the highest biodiversity tended to occ ur in lakes with relatively low primary productivity, such as those found i n the Northern Temperate Lakes Long-Term Ecological Research (LTER) site in the upper Midwest (United States) and in the Experimental Lakes Area of On tario (Canada). Based on short-term (3 yr) and long-term (21-24 yr) experiments, we tested whether individual lakes respond to whole-lake enrichment experiments in th e manner suggested by analyses of survey data. Experimental addition of nut rients produced varied and unpredictable responses in species richness, pro bably due to transient dynamics and time lags. Responses to nutrient additi on were taxon and lake specific. Phytoplankton showed a variety of relationships between species richness an d pelagic primary productivity (PPR), depending on the history of enrichmen t and recovery. No significant effect of primary productivity on rotifer ri chness occurred in any of the experimental lakes, whereas richness of crust acean zooplankton was negatively correlated with primary productivity in bo th the short- and long-term experiments.