The seasonal cycle of the Lazarev Sea macrozooplankton community and a potential shift to top-down trophic control in winter

Abstract

Between 2004 and 2008, during the German Southern Ocean GLOBEC programme, four large scale bio-oceanographic surveys were conducted in the Lazarev Sea for the Lazarev Sea Krill Survey (LAKRIS). These surveys were completed in Autumn (April–May) 2004, Summer (December–January) 2005/06, Winter (July–August) 2006, and Summer (December–January) 2007/08. On each occasion macrozooplankton communities were sampled by RMT-8 in the upper 200 m of the water column. Chlorophyll a biomass averaged ∼1.5 mg m −3 (max=8.2 mg m −3) in Summer 05/06, 0.88 mg m −3 (max=2.77 mg m −3) in Summer 07/08, 0.24 mg m −3 (max=0.73 mg m −3) in Autumn 04, and 0.042 mg m −3 (max=0.1 mg m −3) in Winter 06. Macrozooplankton densities did not differ significantly between seasons and were 53, 68, 59, and 48 ind. 1000 m −3 in Summer 05/06, Summer 07/08, Autumn, and Winter, respectively. Total macrozooplankton biomass, however, increased significantly from summer (0.88 and 0.97 g dry weight 1000 m −3 in Summer 05/06 and Summer 07/08, respectively) to Autumn 04 (2.66 g dry weight 1000 m −3) and Winter 06 levels (1.75 g dry weight 1000 m −3). This biomass increase was due to both an increased occurrence of Euphausia superba and fish and a shift to a larger size structure in the latter group. Siphonophores (predominantly Diphyes antarctica), chaetognaths (predominantly Eukrohnia hamata and Sagitta gazellae) and euphausiids (predominantly Thysanoessa macrura and E. superba) contributed >80% to total densities in all four surveys. However, a strong and distinctive change in assemblage structure was observed between seasons. Key amongst these was a shift within the euphausiids from a dominance of T. macrura in summer to that of E. superba in autumn and winter; a winter decrease in E. hamata; an autumn and winter decrease in Tomopteris sp.; and a winter increase in the abundance of the grazers Clio pyramidata sulcata and Ihlea racovitzai, hyperiids, and the myctophid fish Electrona antarctica. Carnivorous macrozooplankton formed the major trophic group in all seasons, contributing 44–60% to total macrozooplankton abundance and 39–58% to total macrozooplankton biomass in all four surveys. The combined predation pressure of carnivores and potential increased winter carnivory by E. superba and other omnivorous species, was expected to be high. In view of low winter primary production, we suggest that the epipelagic food web of the Lazarev Sea shifted from being bottom-up controlled in summer to top-down controlled in winter.