Relationship between ?13C of chironomid remains and methane flux in Swedish lakes

Abstract

1.?Methanogenic carbon can be incorporated by methane-oxidising bacteria, leading to a 13C-depleted stable carbon isotopic composition (?13C) of chironomids that feed on these microorganisms. This has been shown for the chironomid tribe Chironomini, but very little information is available about the ?13C of other abundant chironomid groups and the relationship between chironomid ?13C 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 ?13C 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 ?13C values (?31.3 to ?27.0‰), most likely reflecting ?13C of algae and other plant-derived organic matter. Remains of Chironomini and Tanypodinae had lower ?13C 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 ?13C of Chironomini (r = ?0.90, P = 0.006). Methane release from the sediments was also negatively correlated with ?13C 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 ?13C 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 ?13C composition and the importance of allochthonous organic matter input between the studied lakes may also have influenced Chironomini ?13C. More detailed studies with a higher number of analysed samples and detailed measurement of ?13C of different ecosystem components (e.g. methane, dissolved inorganic carbon) will be necessary to further resolve the relative contribution of different carbon sources to ?13C of chironomid remains.

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