ECOLOGY OF METRIDIA-GERLACHEI GIESBRECHT IN THE WESTERN BRANSFIELD STRAIT, ANTARCTICA

The population dynamics, distribution, abundance and feeding behavior of Metridia gerlachei Giesbrecht were studied in the western Bransfield Strait in the period from mid-December 1986 through late March 1987. The greatest abundance of all copepodite stages was found in the Gerlache Strait and in the Bransfield Current, approaching an average abundance of 500 individuals (ind.) m-3 in the upper 200 m. However, much of the population occurred below 200 m, where it sometimes approached 1000 ind. m-3 at certain depths. Feeding rates on phytoplankton, determined from analysis of gut pigments and gut evacuation rate, suggest that daily rations may average between 25 and 50% body carbon day-1 throughout the spring and summer. A strong diel cycle was observed in grazing rate. Cluster analysis of cephalothorax length-frequency distributions, based on measurements of approximately 10,000 individual CIV and CV copepodids, shows that the largest individuals consistently occurred in phytoplankton-rich near-shore regions, whereas the smallest individuals occurred in phytoplankton-poor offshore areas. Shipboard experiments demonstrated that Metridia gerlachei CVI females feed on the eggs of another abundant Antarctic copepod, Calanoides acutus. Spawning appears to take place before December; as many as two or three generations may be produced during the summer before the last spawning period in January.