Abstract
Abstract. Climate change is affecting the rate of carbon cycling, particularly in the Arctic. Permafrost degradation through deeper thaw and physical disturbances results in the release of carbon dioxide and methane to the atmosphere and to an increase in lateral dissolved organic matter (DOM) fluxes. Whereas riverine DOM fluxes of the large Arctic rivers are well assessed, knowledge is limited with regard to small catchments that cover more than 40 % of the Arctic drainage basin. Here, we use absorption measurements to characterize changes in DOM quantity and quality in a low Arctic (Herschel Island, Yukon, Canada) and a high Arctic (Cape Bounty, Melville Island, Nunavut, Canada) setting with regard to geographical differences, impacts of permafrost degradation, and rainfall events. We find that DOM quantity and quality is controlled by differences in vegetation cover and soil organic carbon content (SOCC). The low Arctic site has higher SOCC and greater abundance of plant material resulting in higher chromophoric dissolved organic matter (cDOM) and dissolved organic carbon (DOC) than in the high Arctic. DOC concentration and cDOM in surface waters at both sites show strong linear relationships similar to the one for the great Arctic rivers. We used the optical characteristics of DOM such as cDOM absorption, specific ultraviolet absorbance (SUVA), ultraviolet (UV) spectral slopes (S275–295), and slope ratio (SR) for assessing quality changes downstream, at base flow and storm flow conditions, and in relation to permafrost disturbance. DOM in streams at both sites demonstrated optical signatures indicative of photodegradation downstream processes, even over short distances of 2000 m. Flow pathways and the connected hydrological residence time control DOM quality. Deeper flow pathways allow the export of permafrost-derived DOM (i.e. from deeper in the active layer), whereas shallow pathways with shorter residence times lead to the export of fresh surface- and near-surface-derived DOM. Compared to the large Arctic rivers, DOM quality exported from the small catchments studied here is much fresher and therefore prone to degradation. Assessing optical properties of DOM and linking them to catchment properties will be a useful tool for understanding changing DOM fluxes and quality at a pan-Arctic scale.
Highlights
Climate change has important impacts on carbon cycling, in the Arctic
The chromophoric dissolved organic matter (cDOM) absorption is significantly higher (p < 0.05) in samples from Herschel Island compared with Cape Bounty across the entire spectrum (Fig. 2a) with acDOM350 of 14.5± 5.1 and 5.5±4.9 m−1 respectively (Fig. 2b)
Comparing the streams on Herschel Island (Fig. 3b), the highest dissolved organic carbon (DOC) and acDOM350 values are found in the headwaters of Ice Creek West
Summary
Climate change has important impacts on carbon cycling, in the Arctic. Approximately 1300 Gt of organic carbon is stored in permafrost soils in the Northern Hemisphere (Hugelius et al, 2014), which is 40 % more than currently circulating in the atmosphere. Associated with warming is the development of surface (physical) disturbances such as active layer detachments or retrogressive thaw slumps (Lacelle et al, 2010; Lamoureux and Lafrenière, 2009; Lewkowicz, 2007; Ramage et al, 2018) and thermal perturbation of the subsurface (Lafrenière and Lamoureux, 2013). As these processes influence freshwater systems, they have impacts on the biological production and the biogeochemistry of the Arctic Ocean. “Small” in this context refers to smaller than the large Arctic rivers, as the actual size distribution of these watersheds remains unknown
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