Carbon pathways to zooplankton: insights from the combined use of stable isotope and fatty acid biomarkers

1. Numerous studies have quantified the relative contribution of terrestrial- and phytoplankton-derived carbon sources to zooplankton secondary production in lakes. However, few investigated the pathways along which allochthonous and autochthonous carbon (C) was actually conveyed to consumers. 2. We suggest that the combined use of fatty acid and stable isotope biomarkers could solve this issue. We conducted a field study on two oligotrophic lakes, in which primary production increased significantly between 2002 and 2004. We used modelling to estimate the contribution of terrestrial- and phytoplankton-derived C to particulate organic C (POC) and zooplankton production from their d13C values in 2002 and 2004. 3. According to the isotope model, phytoplankton-derived C accounted for a major part of the POC pool in both lakes and supported more Daphnia sp. production in 2004 than in 2002. Fatty acid data revealed that increased contribution of algal-C to Daphnia production, although common between both lakes, was achieved through C pathways that were different. In one lake, Daphnia grazed more intensively on phytoplankton, whereas in the other there was greater grazing on bacteria. In the latter case, the increased primary production resulted in greater release of algal-derived dissolved organic C (DOC), which may have supported extra bacterial and eventually Daphnia, production. 4. This is the first study illustrating that the combination of fatty acid and stable isotope biomarkers could further our understanding of the factors controlling the relative magnitude of food webs pathways conveying organic matter to zooplankton