Seasonal variations in distribution patterns of sympagic meiofauna in Arctic pack ice

During two expeditions of the R. V. Polarstern'' to the Arctic Ocean, pack ice and under-ice water samples were collected during two different seasons: late summer ( September 2002) and late winter (March/April 2003). Physical and biological properties of the ice were investigated to explain seasonal differences in species composition, abundance and distribution patterns of sympagic meiofauna ( in this case: heterotrophs > 20 mu m). In winter, the ice near the surface was characterized by extreme physical conditions ( minimum ice temperature: - 22 degrees C, maximum brine salinity: 223, brine volume: <= 5%) and more moderate conditions in summer ( minimum ice temperature: - 5.6 degrees C, maximum brine salinity: 94, most brine volumes: >= 5%). Conditions in the lowermost part of the ice did not differ to a high degree between summer and winter. Chlorophyll a concentrations (chl a) showed significant differences between summer and winter: during winter, concentrations were mostly < 1.0 mu g chl a l(-1), while chl a concentrations of up to 67.4 mu mol l(-1) were measured during summer. The median of depth-integrated chl a concentration in summer was significantly higher than in winter. Integrated abundances of sympagic meiofauna were within the same range for both seasons and varied between 0.6 and 34.1 x 10(3) organisms m(-2) in summer and between 3.7 and 24.8 x 10(3) organisms m(-2) in winter. With regard to species composition, a comparison between the two seasons showed distinct differences: while copepods (42.7%) and rotifers (33.4%) were the most abundant sea-ice meiofaunal taxa during summer, copepod nauplii dominated the community, comprising 92.9% of the fauna, in winter. Low species abundances were found in the under-ice water, indicating that overwintering of the other sympagic organisms did not take place there, either. Therefore, their survival strategy over the polar winter remains unclear.