Differential impact of thermal and physical permafrost disturbances on High Arctic dissolved and particulate fluvial fluxes

Abstract

Climate warming and changing precipitation patterns have thermally (active layer deepening) and physically (permafrost-thaw related mass movements) disturbed permafrost-underlain watersheds across much of the Arctic, increasing the transfer of dissolved and particulate material from terrestrial to aquatic ecosystems. We examined the multiyear (2006-2017) impact of thermal and physical permafrost disturbances on all of the major components of fluvial flux. Thermal disturbances increased the flux of dissolved organic carbon (DOC), but localized physical disturbances decreased multiyear DOC flux. Physical disturbances increased major ion and suspended sediment flux, which remained elevated a decade after disturbance, and changed carbon export from a DOC to a particulate organic carbon (POC) dominated system. As the magnitude and frequency of physical permafrost disturbance intensifies in response to Arctic climate change, disturbances will become an increasingly important mechanism to deliver POC from terrestrial to aquatic ecosystems. Although nival runoff remained the primary hydrological driver, the importance of pluvial runoff as driver of fluvial flux increased following both thermal and physical permafrost disturbance. We conclude the transition from a nival-dominated fluvial regime to a regime where rainfall runoff is proportionately more important will be a likely tipping point to accelerated High Arctic change. Fluvial fluxes of dissolved and particulate material from Arctic watersheds are strongly controlled by the presence of permafrost. Arctic fluvial systems have adapted to strong seasonality in flow generation and relatively low inputs of permafrost-derived terrestrial material (e.g., sediment, carbon), with downstream aquatic ecosystems also adapted to this type of low energy regime 1-2. However, Arctic fluvial systems are changing due to permafrost disturbance caused by climate warming and shifting precipitation patterns 3-6. Coupled climate-terrestrial models consistently predict widespread permafrost thaw and disturbance over the next century 7. Multiple pan-Arctic studies show that observed climate change is already transferring significant quantities of terrestrial material to downstream aquatic ecosystems 8-10 as a physical response to increases in the magnitude, frequency, and type of permafrost disturbance 11-14. Several types of widely observed permafrost disturbance can be broadly grouped into two main disturbance types: thermal and physical 15. Thermal disturbance refers to climatic conditions that alter the extent of soil thaw and/or active layer depth (e.g., above average summer air temperatures), usually during one or multiple thaw seasons. Thermal disturbances may have little geomorphic expression on the land surface, but a measurable impact on surface water quality as sub-surface drainage connectivity changes with increased thaw and active layer depths 16-17. Physical disturbance refers to permafrost-thaw related mass movements that displace or rearrange the physical properties of the ...

Emerging dominance of summer rainfall in driving High Arctic terrestrial-aquatic connectivity

“…Combined, our results show lower intensity rainfall events are important mechanisms for supplying Arctic rivers with dissolved and particulate terrestrial material 10 , but the low intensity rainfall events are unable to overcome energy thresholds needed to transport the freshly-mobilized particulate material downstream to watershed outlets. This results in the temporary storage of particulate material within steam channels, where it can be processed and become available for microbial use during low energy hydrological periods.…”

“…Despite the increased ux of organic and inorganic material from terrestrial surfaces to stream networks 10 , localized ALDs did not increase the downstream, multiyear watershed-scale ux of particulate material due to temporary channel storage and uvial energy limitations within the mainstems 9 . Nevertheless, at all watershed scales the ALDs led to a shift in the primary form of C export from a DOC-to a POC-dominated ux, with the magnitude and persistence of impact increasing with the areal extent of watershed disturbance (Fig.…”

“…Given that the potential for export of terrigenous material is higher during late summer (Fig. 3a), the increased proportion of annual available stream power expended during pluvial events disproportionately increased the erosion and delivery of terrestrial material into the downstream aquatic ecosystem [9][10] . Optical DOM indices measured at the CBAWO support increased levels of terrigenous DOM export during pluvial responses (2012-2017; Fig.…”

“…To fully couple the POC continuum from disturbed sources to downstream at the catchment outlet, rainfall events had to result in stream power > 30 W in the mainstems. Pluvial responses with less energy (stream power < 30 W) rapidly mobilize and exhaust temporary in-channel particulate stores 22 , but are an important mechanism resupplying channel stores with newly mobilized/eroded terrestrial material 10 . This suggests that, at the watershed-scale, the effect of physical permafrost disturbance on particulate material export is smaller than the shift toward a pluvially-dominated hydrological regime in this environment.…”

“…biological productivity are at their annual maximums [18][19][20] . The projected increase in the magnitude and frequency of summer rainfall events [7][8] is likely to have substantial impacts on the terrestrial-aquatic ux of organic and inorganic materials in Arctic watersheds 5,[10][11] . However, few hydrological and biogeochemical records capture the full thaw season due to the logistical constraints imposed by working in remote locations.…”