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 δ¹³C values) of their tissue. Therefore, δ¹³C 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 δ¹³C values of recently deposited Daphnia ephippia (δ¹³C_ephippia) are systematically related to in-lake CH₄ concentrations ([CH₄]_aq) are still missing. We measured δ¹³C_ephippia from surface sediments of 15 small lakes in Europe, and compared these values with late-summer [CH₄]_aq. δ¹³C_ephippia ranged from -51.6‰ to -25.9‰, 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 δ¹³C values of carbon dioxide (CO₂) (r -0.54), and no correlation was observed with [CO₂]_aq. At eight sites, offsets between δ¹³ CCO2 and δ¹³C_ephippia exceeded offsets between δ¹³ CCO2 and δ¹³C_algae reported in literature. δ¹³C_ephippia was positively correlated with δ¹³C values of sedimentary organic matter (r 0.54), but up to 20.7‰ lower in all except one of the lakes (average -6.1‰). 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 δ¹³C_ephippia values and [CH₄]_aq in small temperate lakes, and that δ¹³C_ephippia analysis on sediment records may provide insights into past changes in in-lake [CH₄]_aq.