COMPARISON OF SUMMER AND WINTER INORGANIC CARBON, OXYGEN AND NUTRIENTCONCENTRATIONS IN ANTARCTIC SEA-ICE BRINE

During summer (January 1991) and winter (April 1992) cruises to the so uthern Weddell Sea (Antarctica), brine samples were collected from fir st year sea ice and analysed for salinity, temperature, dissolved oxyg en and major nutrient concentrations. Additionally, the carbonate syst em was determined from measurements of pH and total alkalinity. During winter, brine chemical composition was largely determined by seawater concentration in the course of freezing. Brine temperatures ranged fr om -1.9 to -6.7 degrees C. Precipitation of calcium carbonate was not observed at the corresponding salinity range of 34 to 108. Removal of carbon from the total inorganic carbon pool (up to 500 mu mol C-t kg(- 1)) was related to reduced nutrient concentrations, indicating the pre sence of photosynthetically active ice algal assemblages in the winter sea ice. However, nutrient and inorganic carbon concentrations did ge nerally not reach growth limiting levels for phytoplankton. The combin ed effect of photosynthesis and physical concentration resulted in O-2 concentrations of up to 650 mu mol kg(-1). During summer, brine salin ities ranged from 21 to 41 with most values > 28, showing that the net effect of freezing and melting on brine chemical composition was gene rally slight. Opposite to the winter situation, brine chemical composi tion was strongly influenced by biological activity. Photosynthetic ca rbon assimilation resulted in a C-t depletion of up to 1200 mu mol kg( -1) which was associated with CO2 (ag) exhaustion and O-2 concentratio ns as high as 933 mu mol kg(-1). The concurrent depletion of major nut rients generally corresponded to uptake ratios predicted from phytopla nkton biochemical composition. Primary productivity in summer sea ice is apparently sustained until inorganic resources are fully exhausted, resulting in brine chemical compositions that differ profoundly from those of surface waters. This may have important implications for path ways of ice algal carbon acquisition, carbon isotope fractionation as well as for species distribution in the open water phytoplankton.