Shifting DOC concentration and quality in the freshwater lakes of the Kangerlussuaq region: An experimental assessment of possible mechanisms

Abstract

Since 2003, concentrations of dissolved organic carbon (DOC) in lakes in the Kangerlussuaq region declined by 14-55 percent, with these decreasing DOC concentrations potentially altering lake ecology and reflecting changes in regional carbon (C) cycling. To evaluate possible mechanisms responsible for this shift, we performed experiments to test the effects of dust addition, bacterial activity, or photodegradation on DOC concentration and two DOC quality metrics: the specific ultraviolet absorbance (SUVA 254 ) and the chromophoric DOC spectral slope coefficient (S 275-295 ). Lake-water DOC concentrations did not decline in any treatments, but there were changes in DOC quality. Dust addition increased SUVA 254 and decreased the magnitude of S 275-295 in one lake, the impacts of bacterial activity were variable, and sunlight exposure elicited a decline in SUVA 254 and an increase in the magnitude of S 275-295 in all lakes. These results suggest that DOC pools in the study lakes are photoreactive, even though the lakes are characterized by long residence times, but that declining DOC concentration did not result from this mechanism. While the tested mechanisms did not explain the decline in DOC concentration observed in recent years, they did yield new information about how dust, bacterial activity, or light can influence DOC quality in the lakes of the Kangerlussuaq region.ARTICLE HISTORY

Chemical weathering in a moraine at the ice sheet margin at Kangerlussuaq, western Greenland

“…DOC contents in the moraine pond waters are similar to the range found in supraglacial streams and glacially fed lakes near the study area Burpee, Anderson, and Saros 2018) and in other ice lakes in the Kangerslussuaq area (Anderson and Stedmon 2007;Saros et al 2015;Northington and Saros 2016;Osburn et al 2017;Fowler, Saros, and Osburn 2018). Apparently, they are high enough to sustain aerobic microbiological (heterotrophic) activities .…”

Annual and seasonal variability in high latitude dust deposition, West Greenland

“…Rates of dust deposition measured over short time periods are highly variable and inevitably depend on whether or not the measuring period coincides with dust emission events. The individual daily trap rates of dust deposition for this study (0.01–0.98 g m −2 d −1 ) are comparable with previous short‐term measurements in the Kangerlussuaq area by Fowler et al (2018; 0.93 g m ‐2 d ‐1 ) and our site averages (0.06–0.16 g m −2 d −1 ) are similar to those from high latitude locations in southern South America (0.014–0.158 g m −2 d −1 ; Cosentino et al, 2021). Previous studies have suggested year‐round dust emissions in Kangerlussuaq with peaks in spring and autumn (Bullard & Mockford, 2018) but also with dust storms in the winter (Hobbs, 1931).…”

“…20 km west of the ice‐sheet margin over four consecutive 2 week periods in summer (2001) and calculated an average annual deposition rate of 2 to 8 g m −2 yr −1 . Fowler et al (2018) sampled dust deposition over 14 days in July 2016 at a location c . 35 km west of the ice‐sheet margin.…”

Differences in the Quantity and Quality of Organic Matter Exported From Greenlandic Glacial and Deglaciated Watersheds

“…The hydroclimatic gradient impacts the quantity and quality of DOM in lakes (Osburn et al, 2017) and streams (Kellerman et al, 2019), which is also modified by other factors such as altitude and catchment relief (Anderson et al, 2018). Organic matter cycling in Greenlandic lakes has changed over the past several decades because of warming (Fowler et al, 2018) and possibly from changes in DOM and nutrient inputs to lakes from melting permafrost (Anderson et al, 2017). These changes should also impact DOM exports to fjords and the coastal ocean; however, the magnitude of current fluxes, and therefore the magnitude of expected changes due to future warming, is currently unknown.…”

“…Molecular characterization and 14 C ages of DOM in meltwater rivers draining the GrIS suggest that DOM is relatively labile and predominantly derived from algal or microbial activity on the surface of the ice sheet (supraglacial), with smaller contributions from englacial and subglacial OM sources (Bhatia et al, 2013; Lawson et al, 2014; Musilova et al, 2017; Stibal et al, 2010). In contrast, OM sources in deglaciated watersheds in Greenland may predominantly consist of SOM, vascular plant material, and OM derived from lake and stream ecosystems (Anderson et al, 2018; Fowler et al, 2018), similar to those in major Arctic rivers (Hernes et al, 2013; Holmes et al, 2008; Lafrenière & Sharp, 2004; Mann et al, 2016; Wickland et al, 2012). Retreat of the GrIS coinciding with increases in the extent of deglaciated watersheds would therefore represent a shift in the source of terrestrial DOM to the coastal ocean, as has been observed in other deglaciating regions.…”