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Glaciers as a source of ancient and labile organic matter to the marine environment

Abstract

Riverine organic matter supports of the order of one-fifth of estuarine metabolism1. Coastal ecosystems are therefore sensitive to alteration of both the quantity and lability of terrigenous dissolved organic matter (DOM) delivered by rivers. The lability of DOM is thought to vary with age, with younger, relatively unaltered organic matter being more easily metabolized by aquatic heterotrophs than older, heavily modified material2,3,4. This view is developed exclusively from work in watersheds where terrestrial plant and soil sources dominate streamwater DOM. Here we characterize streamwater DOM from 11 coastal watersheds on the Gulf of Alaska that vary widely in glacier coverage (0–64 per cent). In contrast to non-glacial rivers, we find that the bioavailability of DOM to marine microorganisms is significantly correlated with increasing 14C age. Moreover, the most heavily glaciated watersheds are the source of the oldest (4 kyr 14C age) and most labile (66 per cent bioavailable) DOM. These glacial watersheds have extreme runoff rates, in part because they are subject to some of the highest rates of glacier volume loss on Earth5. We estimate the cumulative flux of dissolved organic carbon derived from glaciers contributing runoff to the Gulf of Alaska at 0.13 ± 0.01 Tg yr-1 (1 Tg = 1012 g), of which 0.10 Tg is highly labile. This indicates that glacial runoff is a quantitatively important source of labile reduced carbon to marine ecosystems. Moreover, because glaciers and ice sheets represent the second largest reservoir of water in the global hydrologic system, our findings indicate that climatically driven changes in glacier volume could alter the age, quantity and reactivity of DOM entering coastal oceans.

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Figure 1: Map of the Gulf of Alaska drainage area.
Figure 2: Relationship between Δ 14 C-DOC and bioavailable DOC for the 11 rivers sampled.

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Acknowledgements

E. Neal and K. Smikrud contributed to the GOA glacier water flux estimates and K. Smikrud also provided Fig. 1. A. Arendt provided data on glacial wastage. We thank X. Xu at the KCCAMS facility for 14C analyses, and R. Dyda and B. Holmes at UC Davis for assistance with lignin and 13C-DOC analyses, respectively. Funding came from the US National Science Foundation (EAR-0838587), a University of Alaska Seed Grant (to E.H.) and the USDA Forest Service, Pacific Northwest Research Station. The use of trade or firm names in this publication does not imply endorsement by the US Department of Agriculture.

Author Contributions E.H., J.F., R.G.M.S. and D.S. designed the research. J.F. performed the fieldwork and J.F., R.G.M.S. and E.H. performed laboratory analyses. P.J.H., R.E. and D.D. supported analyses and aided data interpretation. E.H. wrote the paper with substantial contributions from J.F., R.G.M.S. and P.J.H. All authors discussed the results and commented on the manuscript.

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Hood, E., Fellman, J., Spencer, R. et al. Glaciers as a source of ancient and labile organic matter to the marine environment. Nature 462, 1044–1047 (2009). https://doi.org/10.1038/nature08580

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