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Productivity and carbon transformations on the Greenland Ice Sheet

Final Report Summary - PROGRIS (Productivity and carbon transformations on the Greenland ice sheet)

Glaciers and ice sheets cover round 16 000 000 km2, or approximately 10 % of the Earth's land surface (Benn and Evans 1998), and so represent a globally important, yet very little studied, ecosystem. The biogeochemical cycling of carbon and other elements in the glacial ecosystem is assumed to be largely influenced by microbial communities which inhabit glacier surfaces, beds and even glacial ice cavities (Priscu and Christner 2004, Hodson et al. 2008). Glacier surfaces, also known as supraglacial environments, have been shown to support abundant and diverse microbial communities that mainly consist of heterotrophic and photoautotrophic bacteria (Mueller et al. 2001, Säwström et al. 2002, Christner et al. 2003, Stibal et al. 2006). Thus, they have the potential to play a significant role in local and regional carbon budgets by means of primary and secondary production and respiration.

The Greenland ice sheet is the second largest body of ice on Earth (1 700 000 km2, i.e. > 10 % of the world's ice surface) and its ablation zone is up to approximately 100 km wide (Zwally et al. 2005). This allows a setting of a gradient along which carbon concentration and transformation rates can be measured. A relationship between the distance from a source of microbes and C and the C transformation rates may be inferred on the basis of the field analyses, and used in global models. The underlying hypothesis of the proposed research was that the productivity of the supraglacial ecosystem is constrained primarily by the source of microbial cells, and secondarily by rock-derived nutrients. We expected that the principal source of microbes and rock-derived nutrients is wind-borne debris originating from soils and sediments in deglaciated areas close to the glacier margin. Thus, the main hypothesis tested in this study was that there is a relationship between the distance of a site on the glacier surface from the glacier margin / terminus and the abundance and activity of the local microbial community.

Objectives

In order to test the hypothesis above, the main objectives of this study were:

(a) to quantify the spatial distribution and coverage of surface debris (cryoconite) in the ablation zone of the Greenland ice sheet along a gradient from the glacier edge to the equilibrium line round 60 - 80 km inland;
(b) to determine the quantity and quality of C and principal nutrients (N, P) in the surface waters and the debris along the gradient;
(c) to determine the abundance of microbes associated with the surface waters and sediment along the gradient;
(d) to measure primary and secondary production and respiration of the supraglacial microbial community along the gradient;
(e) to derive a relationship between the position along the gradient (i.e. distance from the margin / terminus) and the quantity and quality of C and its transformation rates.

These objectives have been successfully achieved during the course of the project and a total of 4 published articles have been acknowledged to this grant so far.

Major outcomes:

(1) Microbial activity and carbon fixation along a transect of the Greenland ice sheet

We have presented data on microbial abundance and productivity, collected along a transect across the ablation zone of the Greenland ice sheet in the summers of 2009 and 2010. We analysed the relationships between the physical, chemical and biological variables using multivariate statistical analysis. Concentrations of debris-bound nutrients increased with distance from the ice sheet margin, as did both cell numbers and activity rates before reaching a peak (photosynthesis) or a plateau (respiration, abundance) between 10 and 20 km from the margin. The results of productivity measurements suggest an overall net autotrophy on the Greenland ice sheet and support the proposed role of ice sheet ecosystems in carbon cycling as regional sinks of CO2 and places of production of organic matter that can be a potential source of nutrients for downstream ecosystems. Principal component analysis based on chemical and biological data revealed three clusters of sites, corresponding to three 'glacier ecological zones', confirmed by a redundancy analysis (RDA) using physical data as predictors. RDA using data from the largest 'bare ice zone' showed that glacier surface slope, a proxy for melt water flow, accounted for most of the variation in the data. Variation in the chemical data was fully explainable by the determined physical variables.

- Stibal, M., Lis, G.P. Lawson, E., Mak, K.M. Wadham, J.L. and Anesio, A.M. 2010. Concentration and quality of organic matter across the ablation zone of the Greenland Ice Sheet. Annals of Glaciology 51(56): 1-8.
- Stibal, M.; Telling. J.; Cook, J.; Mak, K.M.; Hodson, A. and Anesio, A.M. 2012. Environmental controls on microbial abundance and activity on the Greenland Ice Sheet: a multivariate analysis approach. Microbial Ecology 63:74-84.

(2) Nitrogen fixation along a transect of the Greenland ice sheet

Nitrogen inputs and microbial nitrogen cycling were investigated along the 79 km transect into the Greenland ice sheet during the main ablation season in summer 2010. The depletion of dissolved nitrate and production of ammonium (relative to icemelt) in cryoconite holes on Leverett Glacier, within 7.5 km of the ice sheet margin, suggested microbial uptake and ammonification respectively. Nitrogen fixation occurred at both in a debris-rich 100m marginal zone and up to 5.7 km upslope on Leverett Glacier. No positive nitrogen fixation was detected > 5.7 km into the ablation zone of the ice sheet. Estimates of nitrogen fluxes onto the transect suggest that nitrogen fixation is likely of minor importance to the overall nitrogen budget of Leverett Glacier and of negligible importance to the nitrogen budget on the main ice sheet itself. However, nitrogen fixation is potentially important as a source of nitrogen to microbial communities in the debris rich marginal zone close to the terminus of the glacier, where nitrogen fixation may aid the colonisation of subglacial and moraine-derived debris.

- Telling, J., Stibal, M., Anesio, A. M., Tranter, M., Nias, I., Cook, J., Bellas, C., Lis, G., Wadham, J. L., Sole, A., Nienow, P., and Hodson, A. 2012. Microbial nitrogen cycling on the Greenland ice sheet, Biogeosciences, 9: 2431-2442.

(3) Impact on albedo

During the project we also showed unequivocally that heavily pigmented and actively photosynthesising microalgae and cyanobacteria are present on the bare ice of the Greenland ice sheet. The reasons for the darkening of parts of the Greenland ice sheet surface during the summer months leading to reduced albedo and increased melting has previously been explained by inputs from dust particles. Using imaging microspectrophotometry, we demonstrate that intact algal cells and filaments absorb light with characteristic spectral profiles across ultraviolet and visible wavelengths, whereas inorganic dust particles typical for these areas display little absorption. Our results indicate that the pigmented microbial community growing directly on the bare ice, through their photophysiology, have an important role in changing albedo, and subsequently may impact melt rates on the ice sheet. This paper is likely to have a very high impact on this topic since it is providing unequivocal evidence that microbial growth on the surface of the Greenland Ice Sheet has a direct impact on ice albedo.

- Yallop, M.L. Anesio, A.M. Perkins, R.G. Cook, J., Telling, J., Fagan, D., MacFarlane, J., Stibal, M., Barker, G., Bellas. C., Hodson, A., Tranter, M., Wadham, J. and Roberts, N.W. 2012. Photophysiology and albedo-changing potential of the ice algae community on the surface of the Greenland ice sheet. ISME Journal 6: 2302-2313.

The results obtained in this study have provided us with the first credible and globally applicable estimate of the supraglacial productivity and its constraints of the Greenland ice sheet.

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