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 Unite d Kingdom lakes to examine the hypothesis that zooplankton dependence on al lochthonous 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 isol ates. In 21 of the 24 lakes, the zooplankton were depleted in C-13 relative to bulk POM, consistent with previous reports. delta(13)C for POM showed r elatively little variation between lakes compared to high variation in valu es for DOM and phytoplankton. delta(13)C values for phytoplankton and POM c onverged with increasing lake trophy, consistent with the expected greater contribution of autochthonous production to the total organic matter pool i n eutrophic lakes. The difference between delta(13)C for zooplankton and th at for POM was also greatest in oligotrophic lakes and reduced in mesotroph ic lakes, in accordance with the hypothesis that increasing lake trophic st ate leads to greater dependence of zooplankton on phytoplankton production. However, the difference increased again in hypertrophic lakes, where highe r delta(13)C values for POM may have been due to greater inputs of C-13-enr iched organic matter from the littoral zone. The very wide variation in phy toplankton delta(13)C between lakes of all trophic categories made it diffi cult to detect robust patterns in the variation in delta(13)C for zooplankt on.