Browning and blueing – what is the fate of polar coasts?

Abstract

Covered by extensive ice sheets that often terminate as ice shelves or tidewater glaciers, polar coasts are also seasonally surrounded by sea ice. However, this “whitescape” is shrinking rapidly; the years 2012 and 2022 saw sea-ice minima for the Arctic and the Antarctic, respectively. The loss of ice is transforming these coastal areas, especially in the Arctic and West Antarctica. Meltwater transports particles, called glacial flour, from land to ocean, and when intensive enough this discharge creates “brown zones” across fjords in summer. Turbid glacial plumes hinder light penetration and darken coastal waters, thereby limiting photosynthesis, while turbulent mixing induces aggregation of particles and the resulting formation of so-called marine snow (clumps of largely organic matter that coalesce and sink through the water column) (Sci Total Environ 2021; doi.org/10.1016/j.scitotenv.2021.146491). When these particles reach the seabed, they cover benthic organisms such as this macroalga (Himantothallus grandifolius), photographed in Mackellar Inlet (62°S, Antarctic Peninsula) – a glaciomarine fjord that has not “browned” yet. Furthermore, warming intensifies iceberg calving and thus likely increases the risk of ice-scouring to benthic macroalgae in the shallows. However, glacial retreat also exposes new shallow-water habitats; by allowing phytoplankton blooms and seaweed expansion in these newly ice-free areas, this exposure process (“blueing”) increases carbon accumulation in marine systems (“blue carbon”), particularly in fjords, due to their high storage-to-sequestration efficiency (Sci Nat 2021; doi.org/10.1007/s00114-021-01748-8). In polar coastal zones, many climate-change–related feedbacks are anticipated. It is therefore crucial to assess the carbon sinks emerging due to marine ice losses; though little known, these sinks provide important negative feedbacks to climate change that could be recognized as a nature-based solution. To what extent will blueing balance the effects of advancing brown zones?