Seasonal variability in phytoplankton stable carbon isotope ratios and bacterial carbon sources in a shallow Dutch lake

Ecosystem metabolism of lakes strongly depends on the relative importance of local vs. allochthonous carbon sources and on microbial food-web functioning and structure. Over the year ecosystem metabolism varies as a result of seasonal changes in environmental parameters such as nutrient levels, light, temperature, and variability in the food web. This is reflected in isotopic compositions of phytoplankton and bacteria. Here, we present the results of a 17-month study on carbon dynamics in two basins of Lake Naarden, The Netherlands. One basin was restored after anthropogenic eutrophication, whereas the other basin remained eutrophic. We analyzed natural stable carbon isotope abundances (C-13) of dissolved inorganic carbon, dissolved organic carbon and macrophytes, and combined these data with compound-specific C-13 analyses of phospholipid-derived fatty acids, produced by phytoplankton and bacteria. Isotopic fractionation (epsilon) between phytoplankton biomass and CO2(aq) was similar for diatoms and other eukaryotic phytoplankton, and differences between sampling sites were small. Highest epsilon values were observed in winter with values of 23.5 +/- 0.6 parts per thousand for eukaryotic phytoplankton and 13.6 +/- 0.3 parts per thousand for cyanobacteria. Lowest epsilon values were observed in summer: 10.5 +/- 0.3 parts per thousand for eukaryotic phytoplankton and 2.7 +/- 0.1 parts per thousand for cyanobacteria. The annual range in C-13(bact) was between 6.9 parts per thousand and 8.2 parts per thousand for the restored and eutrophic basin, respectively, while this range was between 11.6 parts per thousand and 13.1 parts per thousand for phytoplankton in the restored and eutrophic basin, respectively. Correlations between C-13(phyto) and C-13(bact) were strong at both sites. During summer and fall, bacterial biomass derives mainly from locally produced organic matter, with minor allochthonous contributions. Conversely, during winter, bacterial dependence on allochthonous carbon was 39-77% at the restored site, and 17-46% at the eutrophic site.