Community structure and trophic ecology of zooplankton in the scotia sea marginal ice zone in winter (1988)

Abstract

Zooplankton community structure and trophic ecology were investigated in the marginal ice zone of the Southern Scotia Sea during the austral winter of 1988 as part of the AMERIEZ program. In the study area, near the South Orkneys (34–49°W, 57–61.5°S), water emerging from the Weddell Sea mixed with Scotia Sea water to form a complex field of mesoscale eddies and meanders. Three primary zooplankton communities were identified: a shallow cold water assemblage typical of Weddell Sea water; a shallow/upper mesopelagic assemblage of subantarctic species introduced into the southern Scotia Sea with warm core eddies from the Polar Front; and a deep (>400 m) mesopelagic community with circumtantartic species. Zooplankton numerical dominants were cyclopoid copepods (mostly Oithona spp.); biomass dominants were four calanoid copepods, Calanoides acutus, Rhincalanus gigas, Calanus propinquus and Metridia gerlachei. Calanoides acutus and R. gigas were undergoing winter diapause in the mesopelagic zone (>400 m) whereas M. gerlachei and C. propinquus were distributed throughout the water column. In the subantarctic eddies M. gerlachei was replaced by M. lucens as a dominant and R. gigas was more abundant than in cold water to the south. Diet analysis of 35 dominant species revealed five feeding guilds among the zooplankton. One consisted primarily of small herbivorous copepods. Two closely related guilds consisting of copepods, krill and salps were omnivorous, feeding on phytoplankton, protozoans, metazoans and crustacean debris (molts). Two guilds were constituted by predatory copepods and chaetognaths. A sixth group, all copepods, which included important numerical and biomass dominants of the region, was trophically inactive. A comparison of gut fullness between winter 1988 and fall 1986 revealed that all but two of 19 small particle grazing species had significantly less food in their guts in winter. The exceptions were the biomass dominants C. propinquus and M. gerlachei, which were actively feeding in winter. Phytoplankton biomass was low in winter, averaging two-thirds that in fall. Several major trophic pathways from phytoplankton through apex predators (birds and mammals) are proposed for winter in the southern Scotia Sea with zooplankton playing a central role in each.