Three series of experimental results were obtained in situ during the U.S. Russian Ice Station Weddell 1 Expedition 1992 in the western Weddell Sea. Changes in salinity, silicate, and chlorophyll a concentrations were examined over sampling scales of hours, days, and months as microalgal populations grew in young sea ice. Sea ice growth rates were 0.38 cm h-1 for ice up to 9 cm (May 19--20), 0.13 cm h-1 for ice growth to 28 cm over the next 8 days, and 0.03 cm h-1 during 81 days of observations on ice 42--97 cm thick (March 18--June 7). It was shown that at the initial stage of ice formation, salt and nutrient accumulation occurred, then ice desalination intensified with increasing ice thickness. Brine within the ice showed a 12-hour period of oscillatory motion during the first day of ice growth and a 1.5- to 2-hour oscillation in the skeletal layer of 28-cm ice. Small numbers of diatoms were entrapped from seawater during the initial ice formation. Their reproduction (in terms of chlorophyll a concentration) markedly increased after the third day of ice formation. The highest concentrations of chlorophyll a (100 to 1000 times higher than the underlying seawater) were recorded within the bottom, brown-colored layer of all young ice cores studied during the 81-day experiment. The species composition of ice algal populations (99 species) was more diverse and rich than observed for phytoplankton (18 species) in surface seawater. The temporal and spatial distribution of all parameters studied were controlled by meteorological factors and brine drainage mechanisms. ¿ American Geophysical Union 1995