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Keywords:

  • chironomids;
  • lake sediment;
  • methane;
  • methane-oxidising bacteria;
  • stable carbon isotopes

Summary

1. Methanogenic carbon can be incorporated by methane-oxidising bacteria, leading to a 13C-depleted stable carbon isotopic composition (δ13C) of chironomids that feed on these microorganisms. This has been shown for the chironomid tribe Chironomini, but very little information is available about the δ13C of other abundant chironomid groups and the relationship between chironomid δ13C and methane production in lakes.

2. Methane flux was measured at the water surface of seven lakes in Sweden. Furthermore, fluxes from the sediments to the water column were measured in transects in two of the lakes. Methane fluxes were then compared with δ13C of chitinous chironomid remains isolated from the lake surface sediments. Several different chironomid groups were examined (Chironomini, Orthocladiinae, Tanypodinae and Tanytarsini).

3. Remains of Orthocladiinae in the seven study lakes had the highest δ13C values (−31.3 to −27.0‰), most likely reflecting δ13C of algae and other plant-derived organic matter. Remains of Chironomini and Tanypodinae had lower δ13C values (−33.2 to −27.6‰ and −33.6 to −28.0‰, respectively). A significant negative correlation was observed between methane fluxes at the lake surface and δ13C of Chironomini (r = −0.90, P = 0.006). Methane release from the sediments was also negatively correlated with δ13C of Chironomini (r = −0.67, P = 0.025) in the transect samples obtained from two of the lakes. The remains of other chironomid taxa were only weakly or not correlated with methane fluxes measured in our study lakes (P > 0.05).

4. Selective incorporation of methane-derived carbon can explain the observed correlations between methane fluxes and δ13C values of Chironomini. Remains of this group might therefore have the potential to provide information about past changes in methane availability in lakes using sediment records. However, differences in productivity, algal δ13C composition and the importance of allochthonous organic matter input between the studied lakes may also have influenced Chironomini δ13C. More detailed studies with a higher number of analysed samples and detailed measurement of δ13C of different ecosystem components (e.g. methane, dissolved inorganic carbon) will be necessary to further resolve the relative contribution of different carbon sources to δ13C of chironomid remains.