Latitudinal metazoan plankton zones in the antarctic circumpolar current along 6°W during austral spring 1992

Abstract

During the ANT X/6 cruise of R.V. Polarstern as a part of Southern Ocean JGOFS, the mesozooplankton and smaller metazoans were sampled from five depth layers between 0 and 500 m, and daily egg production was measured in copepods. The latitudinal and vertical abundance, biomass and species distribution were recorded twice along the 6°W meridian between the Weddell Gyre and the Polar Frontal Zone in October and November 1992. Carbon weight-length relationships are presented for the dominant copepod species Calanoides acutus, Rhincalanus gigas, Metridia gerlachei and Oithona similis. Total biomass measured by weighing filters of two size fractions and calculated from specific abundance and length estimations both averaged to about 5 g ashfree dry weight (AFDW) per m 2 in the Polar Frontal region (PFr), somewhat lower in October than in November, and 2 g AFDW m −2 in the Antarctic Zone (AZ) between the PFr and the Weddell Gyre. Antarctic calanoid copepods as a group dominated biomass in both regions, but the cyclopoid copepod Oithona similis had the highest numerical abundance and in general also the highest biomass of all species. Mean copepodid abundance below 100 m was not different in the PFr and the AZ, but the abundance in the upper 100 m was much higher in the PFr. Daily egg production of Calanoides acutus was highest in the PFr. The community composition in the PFr and the AZ indicated that accumulation in the Antarctic convergence or a difference in timing of the spring rise to the surface was not the main cause of the latitudinal spring peak in the PFr. Probably the physical conditions are most favourable here for zooplankton to sustain populations. This seems most advantageous for species with a long reproductive period, allowing them to produce several generations per year. Life cycle strategies of Antarctic zooplankton species only can be compared in the framework of their specific conditions for growth and persistence in different latitudinal zones, and the distribution and transport patterns of their populations.