Stable isotope analysis of the origins of zooplankton carbon in lakes of differing trophic state.

Carbon stable isotope analysis was carried out on zooplankton from 24 United Kingdom lakes to examine the hypothesis that zooplankton dependence on allochthonous sources of organic carbon declines with increasing lake trophy. Stable isotope analysis was also carried out on particulate and dissolved organic matter (POM and DOM) and, in 11 of the lakes, of phytoplankton isolates. In 21 of the 24 lakes, the zooplankton were depleted in C relative to bulk POM, consistent with previous reports. δC for POM showed relatively little variation between lakes compared to high variation in values for DOM and phytoplankton. δC values for phytoplankton and POM converged with increasing lake trophy, consistent with the expected greater contribution of autochthonous production to the total organic matter pool in eutrophic lakes. The difference between δC for zooplankton and that for POM was also greatest in oligotrophic lakes and reduced in mesotrophic lakes, in accordance with the hypothesis that increasing lake trophic state leads to greater dependence of zooplankton on phytoplankton production. However, the difference increased again in hypertrophic lakes, where higher δC values for POM may have been due to greater inputs of C-enriched organic matter from the littoral zone. The very wide variation in phytoplankton δC between lakes of all trophic categories made it difficult to detect robust patterns in the variation in δC for zooplankton.