Biogeochemistry of Antarctic sea ice: A case study on platelet ice layers at Drescher Inlet, Weddell Sea

Abstract

Interstitial water samples collected from sea-ice platelet layers at a coastal site in the eastern Weddell Sea, Antarctica, were analyzed for total alkalinity, pH, major nutrient, oxygen, dissolved inorganic carbon (DIC) concentrations and biomass. Based on the data obtained here and in previous studies on sea-ice ecosystems, a conceptual description of biogeochemical processes during maturation of algal blooms in semi-enclosed sea-ice habitats is presented. The concept invokes an initial phase of nitrate-based 'new' production during which nutrient replenishment from the surrounding seawater surpasses algal demand, leading to rapid accumulation of algal biomass in excess of what is accounted for by the apparent DIC and nutrient depletion. As long as nitrate is present in non-limiting quantities, primary production proceeds in close agreement with Redfield ratios. During later stages of the bloom, algal nutrient demand exceeds replenishment, and nitrate is completely consumed, where the photosynthetic quotient indicates utilization of ammonium. Following nitrate exhaustion, photosynthetic carbon fixation is maintained, but is directed towards production of carbon-rich metabolites and excretion of dissolved organic matter, leading to further DIC drawdown and alteration of algal biochemical composition from the nitrate-replete state. During both phases, cell mortality and lysis in conjunction with inefficient feeding of metazooplankton (if present) lead to liberation and subsequent accumulation of dissolved matter including major nutrients. The co-occurrence of phosphate accumulation, strong oxygen supersaturation and depleted DIC concentration thus suggest that heterotrophic oxidation of organic matter may not represent the major pathway by which nutrients are regenerated in sea-ice ecosystems.