Seasonality of cladoceran and bryozoan resting stage δ13C values and implications for their use as palaeolimnological indicators of lacustrine carbon cycle dynamics

The stable carbon isotope composition, expressed as delta C-13 values, of chitinous resting stages of planktivorous invertebrates can provide information on past changes in carbon cycling in lakes. For example, the delta C-13 values of cladoceran ephippia and bryozoan statoblasts have been used to estimate the past contribution of methane-derived carbon to lake food webs and variations in the delta C-13 value of planktonic algae. Limited information, however, is available concerning seasonal variations in delta C-13 values of these organisms and their resting stages. We measured the seasonal variation in delta C-13 values of Daphnia (Branchiopoda: Cladocera: Daphniidae) and their floating ephippia over a 2-year period in small, dimictic Lake Gerzensee, Switzerland. Floating ephippia of Ceriodaphnia (Branchiopoda: Cladocera: Daphniidae) and statoblasts of Plumatella (Phylactolaemata: Plumatellida: Plumatellidae) were analysed during parts of this period. Furthermore, delta C-13 values of remains from all three organism groups were analysed in a 62-cm-long sediment core. Throughout the year, Daphnia delta C-13 values tracked the delta C-13 values of particulate organic matter (POM), but were more negative than POM, indicating that Daphnia also utilize a relatively C-13-depleted carbon source. Daphnia ephippia delta C-13 values did not show any pronounced seasonal variation, suggesting that they are produced batch-wise in autumn and/or spring and float for several months. In contrast, delta C-13 values of Ceriodaphnia ephippia and Plumatella statoblasts followed variations in delta C-13(POM) values, Ceriodaphnia values being the most negative of the resting stages. Average cladoceran ephippia delta C-13 values in the flotsam agreed well with ephippia values from Gerzensee surface sediments. In contrast, average Plumatella statoblast delta C-13 values from the flotsam were 4aEuro degrees more negative than in the surface sediments. In the sediment core, delta C-13 values of the two cladocerans remained low (mean -39.0 and -41.9aEuro degrees) throughout the record. In contrast, Plumatella had distinctly less negative delta C-13 values (mean -32.0aEuro degrees). Our results indicate that in Gerzensee, Daphnia and Ceriodaphnia strongly relied on a C-13-depleted food source throughout the past 150 years, most likely methane-oxidising bacteria, whereas this food source was not a major contribution to the diet of bryozoans.