The relative roles of anthropogenic nitrogen (N)
deposition and climate change in causing ecological change in remote Arctic
ecosystems, especially lakes, have been the subject of debate over the last
decade. Some palaeoecological studies have cited isotopic signals (
In recent years it has been demonstrated that anthropogenic nitrogen
deposition, primarily from fossil fuel combustion, has reached areas very
remote from the original sources, including high-latitude sites in the
Arctic. Evidence includes contemporary deposition monitoring (AMAP, 2006),
the snowpack record of the Greenland ice sheet (Hastings et al., 2009) and
palaeolimnological records in Arctic lakes (Holtgrieve et al., 2011).
However, contemporary deposition data are sparse in such remote areas due to
logistical and cost limitations. According to AMAP (2006), “more
observations for NO
Location of sampling regions and catchments within western Greenland.
The largest ice-free region of Greenland is found in the south-west, where a
great number of lakes have been the subject of several limnological and
palaeolimnological studies. The region between the edge of the ice sheet,
the key international airport hub at Kangerlussuaq and the coastal town of
Sisimiut was selected for an integrated study into the potential effects of
nitrogen deposition on Arctic lakes (Fig. 1) without the confounding
effects of climate warming, since there was no significant warming trend in
the region for most of the 20th century (Hanna et al., 2012). This
region contains lakes showing very different
Around half of the precipitation in western Greenland falls as snow (e.g. 45 % at Sisimiut and 52 % at Kangerlussuaq from 1994 to 1997; Yang et al., 1999, and 40 % at the ice sheet in 2011–2013; Johansson et al., 2015). Hence snowpack chemistry (if unchanged following deposition) can provide the data required for estimating annual deposition of pollutants in remote Arctic regions where regular deposition monitoring is not possible due to logistical and financial constraints. In high-snowfall regions with a fairly continuously accumulating snowpack, late-season snowpack may provide a good estimate of total deposition inputs over the snow season, which may cover more than 6 months at high-altitude or high-latitude sites (e.g. Rockies – Turk et al., 2001; Ingersoll et al., 2008; Williams et al., 2009). However, in western Greenland the inland areas experience very low precipitation inputs and sublimation of accumulated snowpack is also important (Johansson et al., 2015). Annual mean precipitation at Sisimiut from 2001 to 2012 was 631 mm, while at Kangerlussuaq it was 258 mm (Mernild et al., 2015). Much greater accumulation of snowpack also occurs in the coastal areas, so it is expected that there is a gradient of precipitation, snowpack accumulation and resultant deposition of pollutants from the interior ice sheet margin to the coast.
Most Arctic precipitation chemistry and acid deposition fluxes
(NO
The processing of NO re-emitted from the snowpack and transported away from the area, depending
on wind speed; redeposited by dry deposition; re-oxidised back to NO
Hence Erbland et al. (2015) define “NO
Photolysis is associated with large fractionation of N
(
Sampling catchment details (based on centroid of lakes).
Here we use the strong precipitation gradient across three regions in western
Greenland to test the hypothesis that the delivery of total inorganic
nitrogen (TIN: NO
As part of a wider study of the ecology and palaeolimnology of low Arctic
lakes, deposition study sites were based in three clusters of lake
catchments along an assumed deposition gradient from the ice sheet margin to
the coast (Fig. 1, Table 1), hereafter referred to as ice sheet, Kelly
Ville and coastal sites. Three lake catchments were chosen within each
region on the basis of previous studies and suitability for
(palaeo-)limnological studies reported elsewhere. Five replicated late-season snowpack samples were collected within the terrestrial part of each
catchment, with a further three replicates obtained from the snowpack on the
frozen lake surface. Hence, for the purposes of the present study which
considers spatial patterns, eight samples from each of three lake catchments are
considered to represent 24 replicated samples within each region. All
catchments are located within a narrow latitudinal band around 67
Snowpack depth and density were measured during repeat traverses of each catchment along a grid-based pattern to obtain a spatial coverage of 50–150 measurements per catchment. Depth was measured every 100 m with a graduated pole, while density was estimated by taking a snow core of known volume using a 37 mm internal diameter plastic pipe at every fifth measurement point and weighing in the field using a spring balance. Snowpack sampling locations were selected to obtain representative spatial coverage within each lake catchment, recognising the spatial variations in aspect, altitude and snowpack depth where snowpack coverage was unevenly distributed within catchments. Within each catchment, samples were obtained from upper, mid-level and lakeside elevations and different aspects, but logistical constraints limited sampling to just five locations. In addition, three lake snowpack samples on top of the lake ice were obtained from equally spaced locations along the longest axis of the frozen lake. Hence eight samples per catchment were collected, but comparisons of terrestrial snowpack and snow accumulated on lake ice were also possible.
Snowpack was sampled according to USGS ultra-clean protocols (Clow et al.,
2002; Ingersoll et al., 2008, 2009). In summary, all sampling equipment and
sample bags were triple rinsed with distilled deionised water (DDIW), with
field blanks obtained by rinsing off the sampling shovel and scoop into a
clean sample bag with a DDIW wash bottle in the field. Depth-integrated snow
samples were collected with a polycarbonate scoop and kept frozen in clean
polyethylene bags until they were processed in the laboratory. The whole
snowpack was sampled down to ground level and hence represents an integrated
sample incorporating the net effects of post-depositional processing over the
winter season. Fresh latex gloves for each sample were worn at all times
while sampling and processing in the laboratory. In the laboratory, snow
samples were allowed to thaw at room temperature overnight and then filtered
through 0.45
Sampling was carried out in the late winter period to capture as much of the accumulated snowpack as possible without the risk of substantial snowmelt occurring (see de Caritat et al., 2005); all total snowpack samples were collected between 22 March and 1 April 2011. Snowpack profile temperature and physical description were noted as per Ingersoll et al. (2009) for assessment of snowpack status and whether melt was in progress. In addition, to assist with bulk deposition estimates, ad hoc sampling of rainfall and fresh falling snow was carried out on numerous occasions during field campaigns in each region during 2011 and 2012.
The nitrogen (
Chloride (Cl
Nitrite (NO
Generalized linear mixed models (GLMMs) were used to investigate how snowpack
chemistry and isotope variables varied between regions (15 catchment
replicates plus nine lake snow replicates;
Snow depth measurements and snow water equivalents (SWE) by
catchment and region. Sample number (
Post hoc pairwise comparison of the GLMM-estimated regional means was performed using Tukey contrasts and the generalized linear hypothesis testing (GLHT) framework. For models of differences in snow sample type (terrestrial versus lake ice) post hoc comparisons were restricted to comparison of sample type within a region using appropriate contrast matrices.
All statistical analyses were performed using the R statistical language (version 3.3.2; R Core Team, 2016) with the lme4 package (version 1.1.12; Bates et al., 2015) for fitting GLMMs and the multcomp package (version 2.4.6, Hothorn et al., 2008) for GLHT post hoc comparisons.
Estimates of snowpack depth and snow water equivalents (SWEs) are presented in
Table 2. Note that snow water equivalent calculations were carried out for
the subset of points at which snow mass was measured in the snow tube and
then corrected for mean snow depth across each catchment based on the much
larger number of snow depth measurements. Snow density ranged from 0.20 to
0.34 g cm
Snow depth measurements confirm that there is a major difference in snowpack accumulation from 16 to 20 cm mean catchment snow depth (max. 103 cm; overall 35.7 mm SWE) close to the ice sheet, up to 50–74 cm mean catchment snow depth (max. 260 cm; overall 180.8 mm SWE) at the coast. Depth and SWE are slightly higher at the central Kelly Ville catchments relative to the ice sheet sites, but all are still much lower than the coastal sites. At the two inland regions, snow cover was much more patchy than at the coast, where continuous cover was found except on the steepest slopes (Fig. 2).
Details of snowpack samples for chemical and isotopic analysis are provided
in Table 3a (terrestrial) and 3b (lake ice snowpack). At the ice sheet,
sampled terrestrial snowpack varied from 24 to 103 cm depth, while sampled
lake ice snowpack reached only 23 cm at maximum depth, reflecting the
heterogeneous snow distribution around the catchment and suggesting wind
redistribution of the snow. Sampled terrestrial snowpack depth had a smaller
range at the Kelly Ville catchments, from 23 to 65 cm, and again the lake
ice snow depth was much shallower, reaching only 30 cm at maximum depth. At the
coastal sites, terrestrial snowpack depths from 28 to 240 cm were sampled,
while lake ice snow ranged from 19 to 60 cm depth. During the sampling
period, air temperatures were all well below 0
Comparison of aggregated snowpack chemistry (
Concentrations of major ions in western Greenland snowpack are very low
(
In order to investigate the influence of non-sea-salt atmospheric sources of
ions, the proportion of sea-salt contributions was subtracted using Cl
Comparison of ad hoc rain and fresh snow samples with mean snowpack
data (from Table 4; italics) (concentrations in
For nutrients, concentrations in seawater are assumed to be negligible; hence
snowpack concentrations are assumed to be due entirely to non-sea-salt
atmospheric inputs. Nitrate concentrations are very low at all sites but
significantly lower (
Exploratory data analysis of separate terrestrial snowpack (
The only sites at which melting snow was observed during sampling were at the
coast, raising the possibility that in coastal catchments, the snowpack on
lake ice could be the recipient of meltwater drainage from catchment slopes
whereby preferential elution from catchment snowpack could explain the
higher concentrations in the lake snow. Alternatively, losses of some ions
such as NO
For aggregated snowpack
In addition to the analysis of accumulated snowpack in the study regions, ad
hoc sampling of fresh snow and rainfall was carried out on numerous occasions
during late winter, summer and autumn field campaigns. Unfortunately, most of
the bulk deposition samples collected were subject to major contamination by
bird strikes by the northern wheatear (
Mean annual meteorological data and deposition estimates based on snowpack chemistry and mean precipitation (2001–2012) for Sisimiut (coast), Kangerlussuaq and SS903 at the ice sheet margin (Johansson et al., 2015; Mernild et al., 2015).
Fresh snow collected from coastal sites in 2011 had slightly higher nutrient
concentrations compared to catchment snowpack but had very low concentrations
of sea-salt-related ions, indicating that in the fresh falling snow the
influence of sea-salt inputs was minimal. Presumably marine aerosols
accumulate in the snowpack over winter, which may explain the higher
concentrations of NH
Logistical challenges prevented the routine monitoring of non-snowpack precipitation, and while around half of annual precipitation falls as snow in western Greenland, this does mean that annual deposition fluxes can only be estimated using best available data. In this region, we assume that snowpack concentrations of atmospherically derived ions are representative of total annual precipitation and hence can obtain a first approximation of deposition fluxes by using mean snowpack solute concentrations with measured annual precipitation data at Sisimiut and Kangerlussuaq (Mernild et al., 2015) and scaled for ice sheet data at SS903 from 2011 to 2012 (Johansson et al., 2015). Estimated deposition loads based on mean snowpack chemistry and mean 2001–2012 precipitation levels for Sisimiut (coast) and Kangerlussuaq (inland regions) are shown in Table 6.
While NO
The gradient in precipitation from the coast to the ice sheet has been
attributed by Mernild et al. (2015) to katabatic winds moving downslope from
the ice sheet interior, distance from oceanic moisture sources and orographic
enhancement by coastal mountains, all contributing to much greater
precipitation at the coast relative to areas further inland, towards the ice
sheet. There is a major difference in the chemistry of snowpack from inland
to the coast which is primarily driven by the greater influence of marine
inputs (sea spray and aerosols) at the coast, clearly shown by highly
elevated concentrations of Na
Snowpack solute concentrations in this study are comparable to values
recorded in studies on the Greenland ice sheet. Fischer et al. (1998a)
studied chemistry of recent firn along ice sheet transects and recorded a
range of 110–150 ng g
Our results are also within the range of other studies of Arctic
precipitation and ice cores. Kekonen et al. (2002) recorded peak
concentrations of 3–4
Hence the chemistry of western Greenland snowpack is comparable to the Greenland
ice sheet and other areas of the Arctic remote from pollution sources but
with lower acid anion concentrations than more polluted regions of the Arctic
such as parts of the Russian Federation and NW Europe (De Caritat et al.,
2005; Hole et al., 2006b). Snowpack concentrations in this part of the Arctic
are also generally lower than those recorded in remote alpine systems such as
the Rockies (e.g. 10–12
There are very few data for recent atmospheric deposition in Greenland, but there have been studies of snowpack and ice core records of pollutants on the Greenland ice sheet (e.g. Dye2, 200 km from Kangerlussuaq; Dye3, 380 km; Summit, 800 km; Burkhart et al., 2006). Therefore, despite the lack of contemporary deposition data for the region, there are numerous records of relative change in nitrogen deposition loads over the past 200 years or more which provide evidence of changes in long range sources as opposed to local emission sources and assist in the interpretation of the spatial deposition patterns observed in the current study.
Ice-core records from Greenland show that increases in NO
Fluxes of NO
Burkhart et al. (2004) reviewed several studies demonstrating that deposition
flux (but not concentration) is strongly dependent on snow accumulation
(Legrand and Kirchner, 1990), which is consistent with our results that show
a much higher deposition flux at the coast where the snowpack is much greater
than inland, even if only on a seasonal basis (unlike the ice sheet). Unlike
larger-scale studies of Arctic precipitation showing sulfur to be the main
acidifying substance (Hole et al., 2009), NO
Modelled wet deposition of nss-SO
Finally, while our deposition estimates are comparable to modelled values and
other Arctic studies in regions remote from pollution sources, the reliance
of our estimates on chemical data from snowpack and a small number of
rainfall samples means that our estimates should be viewed as approximate.
Comparison of rainfall chemistry with snowpack within each region suggests
that mean rainfall concentrations could be as low as 60 % or as high as
170 % of those in snowpack, with uncertainties therefore conservatively
in the range
Isotope delta values of snowpack NO
In the current study
There are no published studies on the triple isotope analysis of O in coastal
Greenland NO
The non-mass-dependent fraction of oxygen associated with tropospheric ozone
means that there is positive correlation between ozone concentration and
Hence there is a very strong gradient of declining snowpack differing isotopic composition of inputs due to differing sources of
snowpack NO a gradient in post-depositional processing and fractionation of
NO
There are very few studies in western Greenland to provide evidence for the likely source regions for anthropogenic N or other acid deposition precursors. Kahl et al. (1997) argue that trajectories to Summit on the central ice sheet are similar to Dye 3 on the ice sheet in southern Greenland (Davidson et al., 1993), and that in winter, 94 % belong to westerly transport patterns (in fact moving from south-western coastal zones north-east onto the ice sheet). Geng et al. (2014) assume the dominance of North American pollutant sources at Summit. For our sites in western Greenland it appears that similar long-range source areas would apply. Alternative approaches (lake sediment records of Pb isotopes) have indicated that European sources are also important contributors to pollution across the region (Bindler et al., 2001a, b), while the modelling study of Zatko et al. (2016) suggests that our study region is an area of wind convergence, with airflow mainly from the interior down to the coast. Hence there is no clear indication in the literature of the key local-source regions affecting our study areas, but some evidence that coastal and inland areas are likely to be exposed to similar long-range sources.
While it is possible there may be major differences in pollutant source
regions across Greenland, in particular from the coast to the interior, the
spatial scope of our study is very small relative to the size of the ice
sheet and the modelled gradients shown by Zatko et al. (2016). Hence, while
differential source regions cannot be ruled out, the study areas are very
close to each other relative to distances from source regions. A striking
result is the similarity of the coastal isotopic data (both
The linkages between the
Comparison of regional snowpack ion concentrations (aggregated
data –
Heaton et al. (2004) speculate that preferential deposition of enriched
NO
Comparison of lake ice and terrestrial snowpack concentrations
(
Another possibility is that there could be a greater proportional
contribution of dry deposition at the low precipitation inland sites
relative to the coastal sites. Studies of daily variations in surface snow
chemistry and isotopic composition at a coastal site in Svalbard indicated
that increasing NO
Higher levels of volatilisation of NO
The observed spatial isotopic gradient could potentially be the result of two
opposing processes which could act to produce the same gradient: higher
melting losses at the coast and higher sublimation losses inland. At the
coast, higher temperatures may result in greater melting and preferential
elution of the heavier isotope, leaving a more depleted snowpack. Inland,
lower temperatures reduce melting effects but lower cloud cover and
precipitation along with a much smaller snowpack cause greater relative
sublimation losses, leading to isotopic enrichment of the remaining snowpack.
Such a process could explain the much more depleted
Comparison of regional aggregated (
While the relative importance of these processes cannot be determined
conclusively from the current study, there are additional clues when
comparing the terrestrial snowpack with the lake ice snowpack. At the coast
and Kelly Ville, NO
Our data suggest that coastal snowpack more closely represents the source
isotopic composition, while increased post-depositional processing occurs,
moving inland as precipitation levels and snowpack accumulation rates
decrease. Periodic melting events indicated by ice layers in the coastal
snowpack may facilitate the downward transport of the relatively depleted
NO
Burkhart et al. (2004) observed that almost all NO
Since post-depositional processing occurs primarily in the photic zone of the
snowpack (e.g. modelled values from 6 to 51 cm in Greenland; Zatko et al.,
2016), a larger proportion of the snowpack at the inland sites must be
exposed to such processing during spring, while much deeper snowpack at the
coast will retain a greater proportion of unprocessed NO
The modelling study of Zatko et al. (2016) also indicates that up to
100 % of snowpack NO
If the much higher
Ice core data show seasonal variation in the recent isotopic signature, with
summer values higher than winter values, which was not apparent in pre-industrial
ice (Hastings et al., 2004, 2009). The
There are major differences in snowpack accumulation and SWE from inland to
the coast, reflecting the annual precipitation, which is twice as high at the
coast than inland. Late-season snowpack in western Greenland shows a strong
chemical gradient from the ice sheet margin to the coast. For inland
snowpack, chemistry is comparable to remote locations such as Summit on the
central ice sheet as well as other Arctic locations remote from industrial
sources. At the coast, sea-salt ions dominate the accumulated snowpack but
are much less important in fresh snow. While NO
A lack of summer rainfall chemistry data prevents accurate estimation of
annual deposition fluxes, but net deposition inputs to catchments may be
approximated by assuming (on the basis of a small number of ad hoc rainfall
samples) that snowpack chemistry is representative of annual mean
precipitation, since snow represents around half of annual precipitation. On
this assumption there is a strong deposition gradient from inland to the
coast, which is much more pronounced for NH
While chemistry and deposition show similarities to other studies of ice
sheet snowpack, stable isotope data show major differences. There is a
gradient of declining
Future changes in climate are likely to affect the gradients in snowpack
chemistry, stable isotopes and deposition observed in the current study,
given the importance of precipitation and other climatic factors in driving
spatial differences. In 2012 the coastal town of Sisimiut recorded its
highest annual precipitation since records began (1004 mm) and records from
this station show the strongest increasing precipitation trend across the
Greenland network of
All underlying site locations, chemistry and isotope data
are publicly available via the following link:
CJC, NJA, GS, VJ and JK designed the study. CJC, NJA, GS, VJ and EW carried out fieldwork and sample preparation. JK and AM carried out laboratory analyses. All authors contributed to writing and interpretation of the data.
The authors declare that they have no conflict of interest.
This project was funded by NERC (long range atmospheric nitrogen deposition as a driver of ecological change in Arctic lakes; grant NE/G020027/1 to UCL, NE/G019509/1 to UEA and NE/G019622/1 to Loughborough University). We thank Karen Schleiss of Kangerlussuaq and Morten Nielsen of DTU (Sisimiut) for ad hoc rainfall sampling. Figure 1 was produced by Wendy Phillips of GAES. The study would not have been possible without the dedication in the field from James Shilland, Simon Patrick, Ewan Shilland and Simon Turner of UCL. Edited by: Silvio Pantoja Reviewed by: two anonymous referees