The role of methane-derived carbon as an energy subsidy to benthic invertebrates in streams

Abstract

Recent evidence suggests that methane-derived carbon can provide an energy source to freshwater food webs. Cased caddis (Trichoptera) larvae, such as Agapetus fuscipes and Silo nigricornis, show consistently depleted carbon stable isotope ratios, suggesting a reliance on methane-derived carbon rather than the organic matter fixed by plants either in the stream or imported from the land. These two invertebrate species can be very abundant and thus there is considerable potential for trophic transfer of methane-derived carbon further up the food web. Hitherto, the evidence for this link between cased caddis and methane has been restricted to streams and rivers on permeable chalk geology across southern Britain. This thesis examines the geographical distribution of this phenomenon and specifically whether it occurs elsewhere in catchments on different geology. It also examines the potential magnitude and importance of this methane-derived carbon source to these cased caddis populations. Twenty-nine sites on varying geology across Britain were sampled from April to November 2011. The results suggest that the use of methane-derived carbon by cased caddis and other primary consumers is more widespread than first thought. To assess the proportion of methane-derived carbon contributing to cased caddis larvae, secondary production in the focal caddis taxa (Agapetus fuscipes and Silo nigricornis) was measured regularly, using the size-frequency method, at eight permanent sites selected from the various geologies. This, combined with stable carbon isotope measurements, suggests that methane-derived carbon may form a considerable subsidy in these freshwater systems and indeed may be widespread across the UK.