Effects of a rapidly receding ice edge on the abundance, age structure andfeeding of three dominant calanoid copepods in the Weddell Sea, Antarctica

Open-water, marginal-ice and in-ice zones were sampled in the Weddell Sea d uring November and December, 1993 in an effort to examine the influence of the early spring bloom on the diet and population structure of the three bi omass dominant copepods: Metridia gerlachei, Calanus propinquus, and Calano ides acutus. The abundance of all three species in the upper 200 m was high est at stations in the open water, but individually, each species displayed a unique trend. M. gerlachei, which showed the least variability, was sign ificantly more abundant in open water than in the marginal-ice zone. The ab undance of Calanus propinquus was higher in open water than in the marginal -ice zone or in the ice. Calanoides acutus displayed the highest variabilit y, with significant differences between all three ice-cover zones. Diet ana lysis revealed no significant differences in the number of food items withi n each ice-cover zone and diatoms were the most numerous item identified in the guts of all three species. However, M. gerlachei and Calanus propinquu s also contained metazoan material, while Calanoides acutus did not. There were dramatic differences in the age composition of the species between the zones. Early copepodite stages of all three species predominated at the ic e edge and in open water. Numbers of M. gerlachei adult females were roughl y equivalent in all three zones while Calanoides acutus and Calanus propinq uus adult females composed a higher fraction of the total population within the ice. These results compare well with life-history data compiled by oth er authors and reinforce the importance of the ice edge to bloom-dependent Antarctic zooplankton.