Seasonal variations in abundance, diel vertical migration, and population structure of Metridia gerlachei at Port Foster, Deception Island, Antarctica

During five cruises in a 1.5-yr period between February 1999 and November 2000, we studied the abundance, vertical distribution, and population structure of the calanoid copepod Metridia gerlachei (Giesbrecht) at Port Foster, a 160 m deep embayment of Deception Island, Antarctica. M. gerlachei was found at Port Foster year round with abundance peaking in early winter 2000, averaging about 200 copepods m−3 over the entire water column, and up to 800 copepods m−3 at mid-water depths. Diel vertical migration of the M. gerlachei population was observed during some parts of the year, while there was no evidence for seasonal vertical migration. Based on analysis of developmental stage-frequency analysis and abundance data, we speculate that M. gerlachei had three generations per year, began to spawn early in the spring, and had the strongest reproductive effort late in the fall. Using daily carbon ration estimates, M. gerlachei accounted for <1% of the daily removal of phytoplankton standing stock during most of the year, with the exception of early winter when M. gerlachei was most abundant and removed up to 68–205% of phytoplankton standing stock per day. During the early winter, when carnivorous ctenophore Callianira spp. anomalously made up 50% of the macrozooplankton community, which was typically dominated by krill, we hypothesize that M. gerlachei had a large role in the Port Foster carbon cycle as both grazer and prey.