Species turnover and richness of aquatic communities in north temperate lakes
I estimated annual species turnover rates for three groups of aquatic organisms in relatively undisturbed north temperate lakes. Apparent turnover rates (i.e. measured turnover rates) were high, averaging 18\% for phytoplankton, 16\% for zooplankton, and 20\% for fishes. Based on life history characteristics and dispersal abilities, I expected phytoplankton to have higher turnover rates than zooplankton, which would have higher turnover rates than fishes. Results were contrary to my expectations; apparent turnover was high and similar for each of the taxonomic groups. Comparison of apparent turnover rates, however, was problematic because sampling error could account for much of the apparent turnover. Because the turnover that could be attributed to sampling error was so high, it should be taken into consideration when assessing species turnover. I have developed a new and unique method for quantifying potential sampling error in which I calculate the species turnover that could be attributed to failing to detect species that were present but at low abundance. Sampling error was influenced by sampling intensity. When sampling effort was increased, the proportion of turnover attributable to sampling error decreased. The result of high apparent turnover is that biodiversity assessments are dependent on the extent of the sampling programme. I assessed crustacean zooplankton richness in eight lakes at different temporal and spatial scales, using three methods of estimation: cumulative, asymptotic, and Chao’s index. Percent species detected increased with the number of spatial, intra-annual, or inter-annual samples taken. Single samples detected only 50\% of the annual species pool and 33\% of the total estimated species pool. This suggests that previous estimates of zooplankton richness, based on single samples in individual lakes, were seriously underestimated. Single-year richness estimates provided poor predictions of multiple-year richness. The relationship between richness and environmental variables was dependent on the method of estimation and the number of samples used. I conclude that richness should be treated as an ’index’ rather than an absolute and sampling efforts should be standardized.