Spatial and seasonal variability of the carbon isotopic signature of Daphnia and their ephippia in four French lakes: Implications for the study of carbon transfers in lake food webs

Abstract

Abstract Carbon isotope analysis (δ13C) of Daphnia resting‐eggs (ephippia) in sedimentary records can be used to reconstruct past carbon transfers in pelagic food webs in lakes. However, there may be seasonal variability of cladoceran δ13C and ephippia production that could affect their use as palaeoecological indicators of pelagic carbon transfers. This is particularly likely in stratified lakes where availability of different pelagic carbon sources is seasonal. In addition, there are currently no studies on spatial variability of ephippia δ13C in deep areas of lakes and its implications for sampling strategies in paleolimnological studies. Four French lakes were sampled for over a year to evaluate seasonal variation of the carbon sources consumed by Daphnia using analyses of the Daphnia carbon isotope signature (δ13CDaphnia) and suspended organic matter signature (δ13Cseston). Daphnia ephippia were also collected from surface sediments at maximum depth in the four lakes. Ephippia signatures (δ13Cephippia) were compared to the δ13CDaphnia to evaluate ephippia production periods and the implications for the use of δ13Cephippia in trophic functioning studies. In addition, spatial variability of δ13CDaphnia among the four lakes and the variability of δ13Cephippia in the deep area of one lake were assessed to determine the relevance of a single sampling point in the context of paleolimnological studies. A similar pattern was observed in all four lakes: δ13CDaphnia was close to δ13Cseston during the summer stratification period, but δ13CDaphnia became much lower than δ13Cseston after the autumnal turnover. Ephippia seem to be produced either during the summer stratification period or after autumnal turnover depending on the lake. No spatial variability was observed among the sampling points in δ13CDaphnia in the four lakes, and δ13Cephippia seems to be spatially randomly distributed in the deep zone of the studied lake. In the four lakes, summer δ13CDaphnia values were never below −40‰, reflecting mainly the consumption of phytoplankton, which in turn uses varying amounts of CO2 from respiration (depending on the lake). After autumnal turnover, the δ13CDaphnia values reached values far below −40‰, reflecting the transfer of a non‐negligible part of C‐CH4 for three of the lakes. Seasonal stratification and turnover mechanisms seem to influence the availability of carbon sources in the pelagic compartment of the four lakes. This study shows that the timing of ephippia production affects the information provided by δ13Cephippia due to seasonal differences in the mechanisms (stratification, autumnal turnover) that determine which carbon sources are available in the pelagic compartment. Therefore, using δ13Cephippia to study past pelagic transfers of carbon in stratified lakes may entail uncertainty if used alone and requires multi‐proxy studies. Finally, δ13Cephippia appear to be homogeneously distributed in the deepest area of a medium‐sized, single basin. A single core retrieved from the deepest part of the lake should therefore provide a representative sample of the ephippia produced in a lake.