Trickle or treat: The dynamics of nutrient export from polar glaciers

Abstract

Cold‐based polar glacier watersheds contain well‐defined supraglacial, ice‐marginal, and proglacial elements that differ in their degree of hydrologic connectivity, sources of water (e.g., snow, ice, and/or sediment pore water), meltwater residence times, allochthonous and autochthonous nutrient, and sediment loads. We investigated 11 distinct hydrological units along the supraglacial, ice marginal, and proglacial flow paths that drain Joyce Glacier in the McMurdo Dry Valleys of Antarctica. We found that these units play unique and important roles as sources and/or sinks for dissolved inorganic nitrogen and dissolved inorganic phosphorus and for specific fractions of dissolved organic matter (DOM) as waters are routed from the glacier into nutrient‐poor downstream ecosystems. Changes in nutrient export from the glacial system as a whole were observed as the routing and residence times of meltwater changed throughout the melt season. The concentrations of major ions in the proglacial stream were inversely proportional to discharge, such that there was a relatively constant “trickle” of these solutes into downstream ecosystems. In contrast, NO3− concentrations generally increased with discharge, resulting in delivery of episodic pulses of dissolved inorganic nitrogen‐rich water (“treats”) into those same ecosystems during high discharge events. DOM concentrations or fluorescence did not correlate with discharge rate, but high variability in DOM concentrations or fluorescence suggests that DOM may be exported downstream as episodic treats, but with spatial and/or temporal patterns that remain poorly understood. The strong, nutrient‐specific responses to changes in hydrology suggest that polar glacier drainage systems may export meltwater with nutrient compositions that vary within and between melt seasons and watersheds. Because nutrient dynamics identified in this study differ between glacier watersheds with broadly similar hydrology, climate, and geology, we emphasize the need to develop conceptual models of nutrient export that thoroughly integrate the biogeochemical and hydrological processes that control the sources, fate, and export of nutrients from each system.

Patterns in Microbial Assemblages Exported From the Meltwater of Arctic and Sub-Arctic Glaciers

“…From our geochemical data, we calculated two indices that have been used previously in interpreting patterns in weathering and hydrology (e.g., Dubnick et al, 2017b). The sulfate mass fraction (SMF) is defined as the concentration of sulfate (SO 4 2− ), divided by the sum of sulfate and bicarbonate (HCO 3 −; Brown et al, 1996;Tranter et al, 2002).…”

“…This ratio is a crude approximation of the degree of carbonate dissolution versus silicate dissolution (Wadham et al, 2010). Higher DiMo values are likely to be more associated with carbonate weathering and a channelized drainage system, whereas lower numbers may be proportionally high in contributions from the distributed drainage system (Wadham et al, 2010;Dubnick et al, 2017b). All values were converted to µeq L −1 before calculation.…”

“…While recent progress has been made in determining factors that control the magnitude of these biogeochemical fluxes, important clues into solute generation and the operation of the subglacial drainage system may be uncovered through the investigation of more qualitative characteristics of these exports. For example, past work has successfully shown that chemical signatures of dissolved organic matter Lawson et al, 2014;Dubnick et al, 2017b) and elemental isotopes Hatton et al, 2019b) are related to the hydrological and lithological characteristics of the glacial environment.…”

Biogeochemical Connectivity Between Freshwater Ecosystems beneath the West Antarctic Ice Sheet and the Sub‐Ice Marine Environment

“…Riverine fluxes have long been recognized as an important source of carbon and nutrients to the oceans (Meybeck, 1982), with high fluxes of dissolved organic carbon (DOC) occurring at low latitudes (0-30°; 62% of total known global inputs) and northern high latitudes (60-90°; 19%) (Dai et al, 2012). Polar ice sheets and glaciers, which store~70% of the Earth's fresh water have only recently been identified as important repositories of organic matter (Hood et al, 2015;Lawson et al, 2014;Santibáñez et al, 2018) and biologically important nutrients (Bhatia et al, 2013;Dubnick et al, 2017;Hawkings et al, 2016;Wadham et al, 2016) that can be exported to marine environments. The Greenland Ice Sheet is thought to dominate the fluxes from large ice sheets due to its high rate of glacier mass turnover via surface (supraglacial) melt and iceberg calving (Hood et al, 2015); however, the absence of data from the Antarctic ice sheet (AIS) has prevented meaningful comparison between the worlds' major ice sheets.…”

Evaluation of lacustrine groundwater discharge, hydrologic partitioning, and nutrient budgets in a proglacial lake in the Qinghai–Tibet Plateau: using <sup>222</sup>Rn and stable isotopes

“…Porewater sampled in the aquifer immediately behind the lake shore can well represent groundwater discharging into the lake, as suggested previously (Lewandowski et al, 2015;Rosenberry et al, 2015;Schafran and Driscoll, 1993). LGD has been widely considered to occur within the first few metres of the lake shore (Schafran and Driscoll, 1993;Rosenberry et al, 2015;Lee X. Luo et al: Groundwater derived nutrient loadings in proglacial lake et al, 1980) and groundwater is considered to predominately discharge into the epilimnion since deep groundwater flow is highly limited by the Precambrian bedrock (Einarsdottir et al, 2017). Due to a negligible hydrological connection between the epilimnion and hypolimnion, the 222 Rn mass balance model is established to quantify LGD to the epilimnion from the lake shore.…”