Macronutrient biogeochemistry in Antarctic land-fast sea ice: Insights from a circumpolar data compilation

Antarctic sea ice plays an important role in Southern Ocean biogeochemistry and mediating Earth's climatesystem. Yet our understanding of biogeochemical cycling in sea ice is limited by the availability of relevant dataover sufficient temporal and spatial scales. Here we present a new publicly available compilation of macronutrientconcentration data from Antarctic land-fast sea ice, covering the full seasonal cycle using datasets fromaround Antarctica, as well as a smaller dataset of macronutrient concentrations in adjacent seawater. We show astrong seasonal cycle whereby nutrient concentrations are high during autumn and winter, due to supply fromunderlying surface waters, and then are utilised in spring and summer by mixed ice algal communities consistingof diatoms and non-siliceous species. Our data indicate some degree of nutrient limitation of ice algal primaryproduction, with silicon limitation likely being most prevalent, although uncertainties remain around the affinitiesof sea-ice algae for each nutrient. Remineralisation of organic matter and nutrient recycling drive substantialaccumulations of inorganic nitrogen, phosphate and to a lesser extent silicic acid in some ice cores toconcentrations far in excess of those in surface waters. Nutrient supply to fast ice is enhanced by brine convection,platelet ice accumulation and incorporation into the ice matrix, and under-ice tidal currents, whilstnutrient adsorption to sea-ice surfaces, formation of biofilms, and abiotic mineral precipitation and dissolutioncan also influence fast-ice nutrient cycling. Concentrations of nitrate, ammonium and silicic acid were generallyhigher in fast ice than reported for Antarctic pack ice, and this may support the typically observed higher algalbiomass in fast-ice environments.