Remote sensing reveals Antarctic green snow algae as important terrestrial carbon sink

Antarctica's terrestrial ecosystems are limited, with ice-free ground comprising only 0.18% of the continental area. Even in the Antarctic Peninsula, the most vegetated region, only 1.34% of exposed ground is vegetated. However, algal blooms frequently appear in coastal snowfields as green and red patches. These blooms, first described in the 1950s and 1960s, host diverse algal species and play key roles in nutrient and carbon cycling. Given their potential to cover hundreds of square meters, snow algae are significant photosynthetic primary producers. Warming in the Antarctic Peninsula (exceeding 1.5 °C over pre-industrial temperatures) is influencing vegetation, and understanding snow algae's role and response to warming is crucial. Satellite remote sensing offers a powerful tool for mapping and monitoring Antarctica's terrestrial biosphere, but current methods often exclude snow algae due to their spectral profiles. This study uses multi-year data from the Sentinel 2 satellite constellation, combined with in situ measurements, to estimate the distribution, size, and biomass of snow algal blooms across the Antarctic Peninsula.

Previous research has highlighted the importance of snow algae in various ecosystems, including their diverse species composition and roles in nutrient and carbon cycling. Studies in the Northern Hemisphere have used airborne hyperspectral imaging and predictive models to quantify snow algae biomass and bioalbedo. Satellite observations have also investigated snow and ice algae on larger scales, showing their influence on darkening and melt of the Greenland ice sheet. However, limitations in spectral and spatial resolution of freely available imagery and challenges like light scattering and cloud cover have hindered large-scale studies of snow algae in Antarctica. Existing remote sensing estimates of vegetation biomass are often biased towards plants on exposed ground and exclude snow algae.

This research utilized multiple years of data from the European Space Agency's Sentinel 2 satellites to estimate the distribution, size, and biomass of snow algal blooms across the Antarctic Peninsula. Two field campaigns were conducted at Ryder Bay, Adelaide Island (67°S), in 2017/18 and the Fildes Peninsula, King George Island (62°S), in 2018/19. Field spectroscopy was used to measure hemispherical directional reflectance factors (HDRFs) for green snow algae. A scaled integral approach, adapted from Painter et al. (2001), was used to quantify snow algae within Sentinel 2 imagery. This approach relates spectral reflectance profiles to chlorophyll absorption and is less sensitive to bidirectional reflectance distribution function (BRDF) effects. Field measurements included spectral reflectance factors, cell concentration, dry biomass, gas exchange, and nutrient status. A linear regression model was developed to relate chlorophyll absorbance within Sentinel 2 bands to the concentration of green snow algae cells. To reduce noise, filters were applied based on size, average biomass and other criteria derived from field spectrometer data. Pixel cell concentrations were converted to cells m⁻² using average field observations of layer thickness and snow density. Snow algal dry biomass was estimated using the average measured mass of a green algae cell. Geospatial analysis was conducted using QGIS 3.6.2-Noosa and ArcMap 10.5.1 with datasets including the REMA DEM, RACMO 2m Annual Temperature Model, and the Mapping Application for Penguin Populations and Projected Dynamics penguin colony database. Net carbon exchange rate (NCER) was measured using an ADC Scientific Ltd LCPro-SD infrared gas analyser using a modified soil chamber.

The study identified 1679 individual blooms of green snow algae, covering 1.9 km² and totaling 1.3 × 10³ tonnes (dry mass). Bloom area ranged from 300 m² to 145,000 m². Pixel cell concentrations varied between 1.9 × 10⁴ cells ml⁻¹ and 1.7 × 10⁵ cells ml⁻¹. These results compared well with in situ measurements. Green snow algae biomass estimates from Sentinel 2 imagery ranged from 5 to 5791 g dry mass m⁻² (averaging 58 g dry mass m⁻²), similar to in situ measurements (averaging 30 g dry mass m²). Based on average %C content, total annual dry biomass is equivalent to 479 tonnes of carbon within a growth season. Assuming a 122-day season and 17 h of photosynthesis, snow algae would need an average NCER of -0.064 µmols CO₂ m⁻²s⁻¹ to build up the observed biomass. Rates of ecosystem respiration (ER) indicated that snowpack heterotrophs were also active. NCER was negative across a range of sunlight conditions, suggesting positive net ecosystem production. Geospatial analysis showed that blooms were predominantly in coastal snowfields on the western side of the Peninsula, occurring over a latitudinal range of 62.3°S–68.1°S. This range corresponds with a region experiencing average summer air temperatures >0°C. Most blooms were in low-lying snowfields, with larger blooms closer to penguin colonies. The average area and mean cell concentration were significantly larger at blooms <1 km from a penguin colony, highlighting the influence of marine fertilisation. Spatial trends showed both average area and maximum bloom elevation increasing towards the north of the Peninsula. 62% of blooms were on small islands with no high ground for range expansion; these may be lost with warming, but the increase in bloom area on larger landmasses is predicted to outweigh this loss. In situ measurements showed elevated nitrate and phosphate concentrations, enriched δ¹⁵N in algae from sites near marine fauna, implying nutrient inputs from higher trophic levels.

The findings show that green snow algae represent a significant terrestrial carbon sink in the Antarctic Peninsula. The strong dependence on positive summer temperatures and nutrient supply from marine fauna influences the distribution of these blooms. The model predicts a net increase in snow algae biomass with warming due to the expansion of suitable habitats at lower latitudes, despite potential losses in small islands. This increase is primarily due to larger blooms forming near penguin colonies on bigger land masses. However, losses from small islands may reduce terrestrial biodiversity. Further research is needed to fully understand the response of snow algae communities to warming temperatures, changes in precipitation, and potential disruptions in marine food webs.

This study provides the first large-scale estimate of green snow algae biomass and distribution in the Antarctic Peninsula, revealing its significant contribution as a terrestrial carbon sink. A warming climate is projected to cause a net increase in snow algae biomass. However, potential biodiversity loss on small islands warrants further investigation. Future research should incorporate red snow algae blooms, improve understanding of algal species' dispersal, life cycles, and plasticity, and comprehensively characterize carbon fluxes from Antarctic terrestrial vegetation.

The study focused primarily on green snow algae, excluding red and orange blooms which may significantly increase total biomass estimates. The remote sensing method might overestimate or underestimate bloom areas due to sub-pixel blooms or snow cover. Uncertainty remains concerning nutrient resupply mechanisms throughout the melt season and the dispersal mechanisms, life cycles, and plasticity of snow algal species. The snapshot nature of the satellite imagery used limits the assessment of interannual variability.