Abstract
We measured the δ 13 C values of dominant primary consumers and their potential food sources in a groundwater‐fed lowland river. The δ 13 C of most consumers, such as Gammarus and Simulium , reflected that of the dominant forms of photosynthetic production, whereas the cased larvae of two caddis flies ( Agapetus and Silo ) were consistently 13 C depleted (mean δ 13 C: ‐41.2% and ‐40.4%, respectively) throughout the year. The river water was supersaturated (approximately 50 times atmospheric) with methane, reflecting both supersaturation in the groundwater and local production in fine sediments. We measured appreciable rates of methane oxidation, relative to water only controls, in the biofilms on gravel, on the caddis fly cases, and on the bottom of larger rocks. In addition, there was a marked difference in the ratio of methane‐oxidizing potential to chlorophyll across those substrata. This ratio was below detection in the biofilm (i.e., no methane oxidation) on the tops of rocks, greater on the bottom of rocks, and maximal for the gravels and the caddis cases. If the caddis larvae acquire most of their carbon by grazing the tops of such rocks (where they are normally found), then they must acquire their depleted δ 13 C values by occasionally grazing biofilm where the ratio of methane oxidation to chlorophyll was much greater, and the most likely candidate is from their own or conspecific cases. Grazing methane‐oxidizing bacteria could provide the caddis larvae with up to 30% of their carbon, which could represent a true subsidy from an ancient groundwater source.