Linking larval chironomids to methane:: seasonal variation of the microbial methane cycle and chironomid δ13C

Methane-derived carbon has been shown to be an important carbon source for macroinvertebrates in several studies of lake ecosystems using stable isotopes. Furthermore, season and lake morphology appear to influence the importance of methane as a carbon source. However, rarely have the dynamics of the methane cycle been measured concurrently with the isotope signatures of chironomid larvae. We examined the methane dynamics in 2 lakes with contrasting mixing regimes (polymictic and dimictic), while monitoring corresponding changes in chironomid larval delta C-13 throughout an annual cycle. Both methane turnover rates and abundance of methane-oxidising bacteria were higher in the dimictic lake, where correspondingly lower mean larval delta C-13 values of -44.2 to -61.7 parts per thousand. were recorded. In contrast, potential methane production and oxidation rates, as well as cell numbers of methane-oxidising bacteria were always lower in the polymictic lake; corresponding larval delta C-13 values ranged from -32.3 to -29.6 parts per thousand.. Furthermore, seasonal variation in larval delta C-13 was more pronounced in the dimictic lake (-50.1 +/- 5.9 parts per thousand) compared to the polymictic lake (-31.1 +/- 1.2 parts per thousand), reflecting the amplitude of turnover rates. This suggests strongly that lake characteristics have an influence on methane turnover rates and, in conjunction with season, affect the subsequent incorporation of methane-derived carbon into freshwater food webs via macroinvertebrates.