Chironomid incorporation of methane‐derived carbon in plankton‐ and macrophyte‐dominated habitats in a large shallow lake

While 13C‐depleted carbon derived from biogenic methane can substantially contribute to the benthic secondary production in deep stratified lakes, its role in shallow lakes is less clear. We investigated the dynamics of δ13C and δ15N in the larvae of Chironomus plumosus throughout an annual cycle in two ecologically distinct basins (open‐water plankton‐dominated and sheltered macrophyte‐covered) of a large (270 km2), shallow, polymictic and eutrophic lake (Võrtsjärv, Estonia, North Europe). The larval stable isotopic compositions were linked to the presence of methane‐oxidising bacteria (MOB) in larval guts and sediments. Molecular detection of MOB revealed their presence in various sediment types, but stable isotope (SI) analysis revealed clear differences in the feeding of chironomid larvae between the plankton‐ and macrophyte‐dominated habitats. In the plankton‐dominated habitat, the mean δ13C values of larvae remained relatively constant (−38.3‰ to −35.5‰) and corresponded closely to the sediment δ13C values. Mean δ13C values of chironomid larvae were generally lower in macrophyte‐dominated habitats (−43.4‰ to −33.0‰), and both seasonal and individual variation in larval δ13C values were more pronounced. MOB presence in larval guts proved a dietary contribution from biogenic methane in macrophyte‐dominated habitats. Both the SI and molecular results indicated that MOB could help support larvae even during the cold temperature‐limited and ice‐covered periods. Our study indicates that methane‐derived carbon makes a low but steady contribution to the larval chironomids throughout an annual cycle in large shallow Võrtsjärv. However, this contribution can be substantially higher in the lake habitats with abundant macrophytes. The study provides further evidence that a carbon flow pathway from biogenic methane can contribute to the benthic food web under variable habitat conditions in a shallow polymictic lake.