Glacier ecosystem response to episodic nitrogen enrichment in Svalbard, European High Arctic

Abstract

We describe the climatology, hydrology and biogeochemistry of an extreme nitrogen deposition event that occurred in the highly glacierised environment of the European High Arctic during June 1999. Meteorological analysis, three-dimensional air mass trajectories and a 3D transport model show that blocking high pressures over Scandinavia and the rapid advection of western European pollution toward Svalbard were sufficient to cause the most concentrated (1.15 ppm NO 3 -N and 1.20 ppm NH 4 -N), high magnitude (total 26 mm and up to 2.4 mm h -1 at 30 m above sea level) nitrogen deposition event on record in this sensitive, high Arctic environment (78.91°N, 11.93°E). Since the event occurred when much of the catchment remained frozen or under snow cover, microbial utilisation of nitrogen within snowpacks and perennially unfrozen subglacial sediments, rather than soils, were mostly responsible for reducing N export. The rainfall event occurred long before the annual subglacial outburst flood and so prolonged (ca. 10 day) water storage at the glacier bed further enhanced the microbial assimilation. When the subglacial outburst eventually occurred, high runoff and concentrations of NO 3 -(but not NH 4 ? ) returned in the downstream rivers. Assimilation accounted for between 53 and 72% of the total inorganic nitrogen deposited during the event, but the annual NO 3 -and NH 4? runoff yields were still enhanced by up to 5 and 40 times respectively. Episodic atmospheric inputs of reactive nitrogen can therefore directly influence the biogeochemical functioning of High Arctic catchments, even when microbial activity takes place beneath a glacier at a time when terrestrial soil ecosystems remain frozen and unresponsive.

Microbial nitrogen cycling on the Greenland Ice Sheet

“…In contrast to the transect of this study, annual nitrogen inputs on high Arctic Svalbard valley glaciers are likely dominated by precipitation rather than icemelt (Hodson et al, 2005). Deposition of NO − 3 on Svalbard glaciers has increased 65 % since preindustrial times due to anthropogenic deposition (largely from fossil fuel burning; Kekonen et al, 2005), substantially increasing the total nitrogen deposition on the glaciers (Hodson et al, 2005(Hodson et al, , 2010c. While NO − 3 deposition from precipitation on the GrIS has increased even more over the same period (Olivier et al, 2006), the impact as a percentage of total nitrogen inputs is likely to be ameliorated in part by the dominance of icemelt as a nitrogen input (Fig.…”

“…Studies on Svalbard valley glaciers have demonstrated that the uptake of NH + 4 (aq) onto cryoconite is focused within the early melt season, consistent with uptake from snowmelt (Wynn et al, 2007;Hodson et al, 2010c;Telling et al, 2011). Adsorption of early season snowmelt NH + would allow the NH + 4 (aq) to be utilised by microbes in the bare ice zone later into the melt season after the snow pack has migrated upslope, delaying microbial nitrogen limitation and subsequent nitrogen fixation to later in the melt season (Telling et al, 2011).…”

“…Mass balance considerations suggest that nitrification may be important in glacial catchments in the Arctic (Hodson et al, 2005;Wynn et al, 2007), the Alps (Tockner et al, 2002), the Rockies (Baron et al, 1995;Campbell et al, 2000), and the maritime Antarctic (Hodson, 2006;). Significant ammonia retention has been demonstrated on the catchment scale of two Arctic valley glaciers, which may be due to microbial uptake (Hodson et al, 2008(Hodson et al, , 2010c. Microcosm experiments have indicated active nitrification and nitrate reduction, but not nitrogen fixation, beneath a Canadian glacier (Boyd et al, 2011).…”

Nitrate dry deposition in Svalbard

“…Interestingly, the modelled dry deposition flux shows a strong influence of the variability in measured atmospheric concentrations for both HNO 3 and p-NO 3 . This indicates an episodic behaviour of dry deposition, a pattern that has recently been shown to be important for total nitrate deposition in precipitation (Ku¨hnel et al, 2011), with suggested importance for glacial ecosystems (Hodson et al, 2005;Hodson et al, 2009;Roberts et al, 2010). Back trajectories were run for the 18 days with highest atmospheric concentrations (sum of HNO 3 and p-NO 3 ) to see the influence of direct transport upon these periods (not shown).…”

Subglacial and Proglacial Ecosystem Responses to Climate Change

“…Not much is known about generation and assimilation of N 2 O in subglacial and proglacial environments. Studies of Midtre Lovénbreen, Svalbard, have provided indirect evidence of subglacial nitrification, denitrification and microbial assimilation of ammonia (Wynn et al, 2006(Wynn et al, , 2007Hodson et al, 2010), indicating active microbial influence on nitrogen cycling. …”