The stable carbon isotopic composition of Daphnia ephippia in small, temperate lakes reflects in-lake methane availability

Daphnia can ingest methane-oxidizing bacteria and incorporate methanogenic carbon into their biomass, leading to low stable carbon isotope ratios (expressed as delta C-13 values) of their tissue. Therefore, delta C-13 analysis of Daphnia resting eggs (ephippia) in lake sediment records can potentially be used to reconstruct past in-lake availability of methane (CH4). However, detailed multilake studies demonstrating that delta C-13 values of recently deposited Daphnia ephippia (delta C-13(ephippia)) are systematically related to in-lake CH4 concentrations ([CH4](aq)) are still missing. We measured delta C-13(ephippia) from surface sediments of 15 small lakes in Europe, and compared these values with late-summer [CH4](aq). delta C-13(ephippia) ranged from -51.6 parts per thousand to -25.9 parts per thousand, and was strongly correlated with [CH4](aq) in the surface water and above the sediment (r -0.73 and -0.77, respectively), whereas a negative rather than the expected positive correlation was found with delta C-13 values of carbon dioxide (CO2) (r -0.54), and no correlation was observed with [CO2](aq). At eight sites, offsets between delta(13) C-CO2 and delta C-13(ephippia) exceeded offsets between delta C-13(CO2) and delta C-13(algae) reported in literature. delta C-13(ephippia) was positively correlated with delta C-13 values of sedimentary organic matter (r 0.54), but up to 20.7 parts per thousand lower in all except one of the lakes (average -6.1 parts per thousand). We conclude that incorporation of methanogenic carbon prior to ephippia formation must have been widespread by Daphnia in our study lakes, especially those with high [CH4](aq). Our results suggest a systematic relationship between delta C-13(ephippia) values and [CH4](aq) in small temperate lakes, and that delta C-13(ephippia) analysis on sediment records may provide insights into past changes in in-lake [CH4](aq).