Population structure of two Antarctic ice-associated copepods, Drescheriella glacialis and Paralabidocera antarctica, in winter sea ice

The spatial distribution and population structure of two dominant ice-assoc iated copepods, Drescheriella glacialis and Paralabidocera antarctica, were studied during winter at nine locations in east Antarctic fast ice. These species accounted for at least 90% of the total metazoan abundance at each location. Abundances were high, reaching 175 individuals l(-1) (190,000 m(- 2)) for D. glacialis and 660 l(-1) (901,000 m(-2)) for P. antarctica. These abundances were probably partly supported by the high biomass of ice-algae (Pearson correlation coefficient, r = 0.75), as indicated by chlorophyll-a concentrations (1.7-10.1 mug l(-1)). The population structures of each spe cies suggested very different life-history strategies. All developmental st ages of D. glacialis were isolated from the ice cores, including females wi th egg sacs, supporting the hypothesis that this species reproduces in the sea ice during winter. This strategy might assist D. glacialis in leading a continually colonising existence, whereby it responds opportunistically to the availability of favourable habitat patches. The populations of P. anta rctica were composed primarily of nauplii (> 99%), consistent with past obs ervations of a synchronised life cycle for this species. The strong couplin g of the developmental cycle of P. antarctica to the growth and decay of se a ice suggests that local extinctions might occur in areas where ice break- out is unpredictable.