Evidence for the assimilation of ancient glacier organic carbon in a proglacial stream food web

We used natural abundance C-13, N-15, and C-14 to compare trophic linkages between potential carbon sources (leaf litter, epilithic biofilm, and particulate organic matter) and consumers (aquatic macroinvertebrates and fish) in a nonglacial stream and two reaches of the heavily glaciated Herbert River. We tested the hypothesis that proglacial stream food webs are sustained by organic carbon released from glacial ecosystems. Carbon sources and consumers in the nonglacial stream had carbon isotope values that ranged from -30 parts per thousand to -25 parts per thousand for C-13 and from -14 parts per thousand to 53 parts per thousand for C-14 reflecting a food web sustained mainly on contemporary primary production. In contrast, biofilm in the two glacial stream sites was highly C-14-depleted (-215 parts per thousand to 175 parts per thousand) relative to the nonglacial stream consistent with the assimilation of ancient glacier organic carbon. IsoSource modeling showed that in upper Herbert River, macroinvertebrates (C-14=-171 parts per thousand to 22 parts per thousand) and juvenile salmonids (C-14=-102 parts per thousand to 17 parts per thousand) reflected a feeding history of both biofilm (approximate to 56%) and leaf litter (approximate to 40%). We estimate that in upper Herbert River on average 36% of the carbon incorporated into consumer biomass is derived from the glacier ecosystem. Thus, C-14-depleted glacial organic carbon was likely transferred to higher trophic levels through a feeding history of bacterial uptake of dissolved organic carbon and subsequent consumption of C-14-depleted biofilm by invertebrates and ultimately fish. Our findings show that the metazoan food web is sustained in part by glacial organic carbon such that future changes in glacial runoff could influence the stability and trophic structure of proglacial aquatic ecosystems.