Comparison of the exploitation of methane-derived carbon by tubicolous and non-tubicolous chironomid larvae in a temperate eutrophic lake

Abstract

Carbon and nitrogen stable isotope ratios (δ13C and δ15N) in three sympatric species of larval chironomids were analyzed in a temperate eutrophic shallow lake in Japan. Markedly lower δ13C values were reported in Chironomus plumosus (−51.2 ‰) and Tanypus sp. (−43.5 ‰) than those in photoautotrophic carbon sources [particulate organic matter (POM) and sediment]. There were positive correlations between δ13C and δ15N in the two chironomid species. These results indicated that they assimilated carbon derived from biogenic methane by exploiting methane-oxidizing bacteria (MOB). In contrast, Propsilocerus akamusi exhibited similar δ13C values to those of POM or sediment. A δ13C-based isotope mixing model was used to estimate the dietary contributions of MOB to each chironomid species. The mean contributions ranged from 11 to 15 % in C. plumosus, 13 to 19 % in Tanypus sp., but only up to 5 % in P. akamusi. In an aquarium, we observed that individuals of C. plumosus and Tanypus sp., which exhibited low δ13C values, built U-shaped larval tubes in the sediment, and an oxidized layer developed around these tubes. Propsilocerus akamusi did not exhibit this behavior. These results suggest that tube building may provide larval chironomids with greater access to methane-derived carbon through increased opportunities to feed on MOB.