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Jenine McCutcheon, Department of Earth and Environmental Sciences
Introduction
Melting of the Greenland Ice Sheet (GrIS) is a leading cause of land-ice mass loss and cryosphere-attributed sea level rise. Over the last 25 years, there has been about a 40 per cent increase in surface melting and runoff from the (GRIS) as a north−south oriented band of low-albedo ice, known as the Dark Zone, has developed along the western margin of the ice sheet.
Ice sheet mass loss is predominantly determined by the incoming shortwave radiation flux modulated by surface albedo, which is a measure of the amount of solar radiation reflected by a surface. Albedo depends on the physical structure of surface ice and the presence of light absorbing particulates, such as pigmented glacier algae.
While glacier algal blooms can cover almost 80 per cent of the ice surface in this region, blooms exhibit a high degree of interannual variability in intensity and spatial extent that is yet to be understood. This study aimed to better quantify the parameters that control glacier algal growth and constrain the impact of these blooms on ice sheet albedo, melting, and contributions to sea-level rise.
Methodology
Surface ice geochemistry and microbiology was studied at five sites across the ablation zone in southwest GrIS (Figure 1). Surface snow and ice samples were collected from clean ice, high algal biomass ice, snow, dispersed cryoconite ice, cryoconite holes, floating biofilm and supraglacial stream water habitats. To identify potential nutrient limitations on glacier algal growth, a series of soluble nutrient addition incubation experiments were completed. The composition, health and productivity of glacier algae assemblages were monitored using rapid light response curves performed with pulse amplitude modulation fluorometery.
Figure. 1: Sample collection locations and habitats on the Greenland Ice Sheet.
(a) Sample collection sites 1–5 across the ablation zone in southwest Greenland Ice Sheet and rock sample collection site 6 Russell Glacier terminus;
(b) Photographs of surface ice habitats
Outcomes
Nutrient incubation experiments showed a significant response to phosphorus addition. Specifically, the maximum rates of electron transport (a proxy for photosynthesis rate) were significantly higher in the treatment containing phosphorus than each of the control, ammonium, and nitrate treatments. These results indicate that phosphorus was the limiting nutrient for glacier algae. The delayed response to phosphorus addition suggests that phosphate storage is sufficient to sustain glacier algal productivity for about five days (Figure 2).
Figure 2: Glacier algal photophysiological response to nutrient addition.
(a) Relative electron transport rates (rETR) measured during rapid light curves following 120 h incubation;
(b) Maximum quantum efficiency in the dark-adapted state (Fv/Fm) and;
(c) Maximum electron transport rate (rETRmax) after 24 and 120 h incubation. All plots show mean values ± their standard errors, n = 4.
Sequential phosphorus extractions revealed that organic phosphorus accounted for up to 86 per cent of the solid-phase phosphorus in high algal biomass ice, while exchangeable and mineral phosphorus comprised the remainder. Molar concentrations of total organic carbon, total nitrogen, and organic phosphorus all showed a positive correlation with particulate mineral phosphorus concentrations. The solid-phase nutrient ratios, which reflect the nutrient pools in the algal dominated biomass, indicated phosphorus as the limiting nutrient. We found that as the concentration of solid-phase mineral phosphorus increases, the Carbon:Nitrogen:Phosphorus ratio decreased and approached the Redfield ratio.
The measured mineral phosphorus is likely sourced from trace hydroxylapatite identified in Dark Zone surface ice dust. The hydroxylapatite presents an important link between mineral dust and glacier algal blooms because it contains bio-essential phosphorus.
Algal, bacterial, and fungal community compositions clustered according to sampling sites, but exhibited spatial variability across the Dark Zone. Collectively, the microbial community was intermixed with mineral dust and occurred as disseminated particulates in high algal biomass ice and aggregated cryoconite granules in dispersed cryoconite ice and cryoconite hole material (Figure 3).
Figure 3: Scanning electron micrographs of cell-mineral associations in Dark Zone surface habitats.
Scanning electron microscopy micrographs showing (a) bacteria, (b) fungi, and (c) glacier algae that comprise the Dark Zone microbial community. Microbes in (c) high algal biomass ice are more disseminated than those in (d) cryoconite granules. In all surface ice habitats, exopolymer enables microbial cells to (e) adhere to mineral surfaces, and (f) trap and bind mineral grains (indicated by the arrows). In (c) A refers to algal cells and MD to mineral dust.
We found that mineral dust facilitates glacier algal bloom development by supplying the needed phosphorus to the supraglacial algal communities. We documented that high algal biomass ice contained over 30 times more particulate mass per volume of ice than clean ice, with mineral dust accounting for about 94 per cent of these particulates. In spite of its dominance by mass, mineral dust is not the primary cause of ice surface darkening in the GrIS Dark Zone as radiative transfer modelling indicates that the dust has a negligible effect on albedo reduction compared to pigmented glacier algae. Accordingly, the spatial extent and melt rate of phosphorus-bearing ice may in part constrain the spatial distribution of the algal blooms producing the darkening observed on the landscape-scale.
Conclusions
Here we demonstrate, through nutrient addition experiments and spatially resolved mineralogical and geochemical data, that phosphorus is a limiting nutrient for glacier algae in the Dark Zone of the GrIS. We identify phosphorus-bearing minerals (hydroxylapatite) as the likely phosphorus nutrient source fueling glacier algal blooms and demonstrate that mineral nutrient availability is a second-order control on albedo by modulating glacier algal bloom development.
The complexity of this rapidly changing Arctic system makes it difficult to anticipate future changes to ice sheet albedo, melt rates, and contributions to sea level rise. Our data provide a quantitative link between mineral-derived nutrients and glacier algae blooms, and demonstrate that mineral dust is an essential nutrient source for glacier algae. This biogeochemical process must therefore be incorporated into predictive models thereby improving our understanding of how glacier algal blooms will contribute to ice sheet melting in the future.
McCutcheon, J., Lutz, S., Williamson, C. et al. (2021). Mineral phosphorus drives glacier algal blooms on the Greenland Ice Sheet. Nature Communications, 12, 570. doi.org/10.1038/s41467-020-20627-w
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