Old Carbon Impacts on Arctic Stream Food Web

Arctic Grayling sampled as part of the study.

Permafrost stores large amounts of organic carbon (C) that has been frozen for thousands of years. Permafrost thaw can release C that can be transported by groundwater flow from soils to streams and lakes. Little is known about the linkages between permafrost C and food webs in aquatic ecosystems. This publication explores the contribution of old C from permafrost soils to stream organisms in the Arctic. In this study, C enters the food web through lower trophic levels, like photosynthetic algae and microbial communities. Invertebrates then consume algae and microbes, and in turn, fish feed on stream invertebrates. Findings document the assimilation of old C through food webs as the age of C in Arctic Grayling and Dolly Varden ranged from 3,195 years before present to modern (post-1950s). Analyses suggest that as the contribution of old C to the food web increases, fish growth and nutritional status decline. As permafrost continues to thaw, we expect the relative contributions from old C to stream ecosystems to increase with negative implications for fish populations in headwater streams.

Permafrost Hydrology Drives the Assimilation of Old Carbon by Stream Food Webs in the Arctic

Abstract

Permafrost thaw in the Arctic is mobilizing old carbon (C) from soils to aquatic ecosystems and the atmosphere. Little is known, however, about the assimilation of old C by aquatic food webs in Arctic watersheds. Here, we used C isotopes (δ¹³C, Δ¹⁴C) to quantify C assimilation by biota across 12 streams in arctic Alaska. Streams spanned watersheds with varying permafrost hydrology, from ice-poor bedrock to ice-rich loess (that is, yedoma). We measured isotopic content of (1) C sources including dissolved organic C (DOC), dissolved inorganic C (DIC), and soil C, and (2) stream biota, including benthic biofilm and macroinvertebrates, and resident fish species (Arctic Grayling: Thymallus arcticus and Dolly Varden: Salvelinus malma). Findings document the assimilation of old C by stream biota, with depleted Δ¹⁴C values observed at multiple trophic levels, including benthic biofilm (¹⁴C ages = 5255 to 265 years before present (y BP)), macroinvertebrates (4490 y BP to modern), and fish (3195 y BP to modern). Mixing model results indicate that DOC and DIC contribute to benthic biofilm composition, with relative contributions differing across streams draining ice-poor and ice-rich terrain. DOC originates primarily from old terrestrial C sources, including deep peat horizons (39–47%; 530 y BP) and near-surface permafrost (12–19%; 5490 y BP). DOC also accounts for approximately half of fish isotopic composition. Analyses suggest that as the contribution of old C to fish increases, fish growth and nutritional status decline. We anticipate increases in old DOC delivery to streams under projected warming, which may further alter food web function in Arctic watersheds.

O’Donnell, J. A., M. P. Carey, J. C. Koch, X. Xu, B. A. Poulin, J. Walker, and C. E. Zimmerman. 2019. Permafrost hydrology drives the assimilation of old carbon by stream food webs in the Arctic. Ecosystems.