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The fate of ice biota released via meltwater into pools of seawater trapped between melting ice floes (crack pools) was followed in late January in the southern Weddell Sea. Low salinity crack pools shared the following features: nitrate exhaustion, high pH and POC/PON ratios, high bacterial biomass composed of large cells, and a dense algal assemblage dominated to over 90% by only two diatom species. It is suggested that this “climax stage” evolved from a nutrient rich, moderate biomass situation prevailing in high salinity crack pools, and is representative of summer succession of sea ice biota. “Overflow” production following nitrate exhaustion by the algae resulted in internal (lipid) and external (presumably mucus) carbon pools. The latter must fuel bacterial biomass build-up, as algal mortality appeared to be low. The large algal and bacterial stocks point to low grazing pressure exerted by phagotrophic protists, presumably due to poor food quality (e.g. high C/N ratios) and/or excessive mucus production. It is concluded that environmental selection of the abundant ice algal species occurs under conditions prevailing in the disintegrating ice cover during summer, which differ drastically from those generally referred to as characteristic of the sea ice habitat at large (a combination of low temperature, low light and high salinity).

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 accu~nulation of algal biomass in excess of what is accounted for by the apparent D1C and nutrient depletion. As long as nitrate is present in non-limiting quant~ties, 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 f~xation is maintained, but is d~rected, 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 rnetazooplankton (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.