Interannual variability and species turnover of crustacean zooplankton in Shield lakes

We estimated apparent species turnover rates and richness of the zooplankto n annually over a 12-year period in eight lakes in south-central Ontario. A lthough species richness varied little among years (CV = 13%), apparent spe cies turnover rates averaged 16%/year. This apparent turnover varied among years and was influenced by census interval, the number of censuses, the oc currence of rare species, and lake pH. However, Monte Carlo simulations ind icated that turnover attributable to sampling error was high. That is, desp ite high apparent turnover rates, we cannot be certain whether interannual changes in community composition result from immigration and extinction of species because sampling error could largely account for all apparent turno ver. Regardless of the source of apparent turnover (sampling or immigration s and extinctions), high turnover rates imply that zooplankton biodiversity can be underestimated in short-term studies because we detect a different assemblage of species every year. Only one third of the total species pool for each lake was detected every year. Annual data underestimated long-term species pools by 33-50%.