Geographically widespread 13 C‐depletion of grazing caddis larvae: A third way of fuelling stream food webs?

Stream ecosystems are supported by both green (i.e. based on grazing) and brown (i.e. detritus) food webs, whereas methane‐derived carbon is not considered generally to be important; here, we add circumstantial evidence for this potential third way . Grazing cased‐caddis (Trichoptera) larvae in the family Glossosomatidae can be very abundant in springs and headwaters and frequently have much lower stable carbon isotope ratios (i.e. they are depleted in the heavier 13 C stable isotope) than the biofilm (epilithon) on the upper surfaces of the stones on which they live, and which is their presumed diet. Evidence for similar isotopic depletion in other lotic invertebrates is currently limited, however; even for glossosomatids it has been observed so far only in some streams draining the southern English cretaceous chalk and in a few headwaters in northern California. If this phenomenon proves to be more widespread, among streams or taxa, it could imply a more general underpinning of stream food webs by isotopically light carbon derived from methane and accessed via consumers feeding on methanotrophic bacteria. Here, we sampled 58 stream sites to examine whether caddis larvae are also 13 C‐depleted in streams draining other geologies. We focused mainly on carboniferous limestone and sandstone, as well as on further chalk streams representative of most of the British chalk aquifer: together, these new sites covered an area of almost 90,000 km 2 , around three times greater than that surveyed previously. At all 58 sites methane gas was supersaturated relative to the atmospheric equilibrium, and at 49 of them larvae were conspicuously 13 C‐depleted (from −17.5‰ to −3.6‰) relative to the bulk epilithon (components of which we know can oxidise methane). Although still most pronounced on chalk, this phenomenon was geographically and geologically much more widespread than shown previously and suggests methane‐derived carbon could indeed play a prominent role in stream food webs (i.e. the third way ).