MESOSCALE DISTRIBUTION AND POPULATION-DYNAMICS OF RHINCALANUS-GIGAS AND CALANUS-SIMILLIMUS IN THE ANTARCTIC POLAR OPEN-OCEAN AND POLAR FRONTAL ZONE DURING SUMMER

Two dominant Antarctic copepod species, Rhincalanus gigas and Calanus simillimus, were sampled at stations along a 420 km transect running f rom Maurice Ewing Bank (50 degrees S, 42 degrees W) towards South Geor gia (54 degrees S, 37.5 degrees W) during early January 1994 and again 1 mo later. The transect spanned the Polar Frontal Zone (PFZ), crossi ng the Polar Front (PF) into Antarctic surface waters. In January, dia tom blooms were present immediately south of the PF and at open ocean stations further south where unusually high total pigment levels up to 800 mg m(-2) were recorded. In early January, north of the PF, R. gig as had not yet commenced spawning although recruitment had begun to th e south. Early stages of C. simillimus copepodites were, however, pres ent at all stations sampled. South of the PF both gonad maturity and l ipid levels in R. gigas showed a clear positive relationship with the elevated pigment concentrations. C. simillimus females, on the other h and, were ripe at the majority of stations sampled regardless of pigme nt levels. A month later postbloom conditions were present over the gr eater part of the transect and the summer generation of both species w as present at all stations. A proportion of the summer generation of C . simillimus had already reached adulthood and their gonads were matur ing. The populations of both species were older in PFZ waters, and mea n population age showed a strong correlation with water temperature. P rosome length of stages CIV, CV and CVIo R. gigas was negatively corre lated with sea surface temperature. This was not the case for C. simil limus females, suggesting that they had been recruited earlier in the season under different environmental conditions. Lipid analysis indica ted that R. gigas predominantly stored wax esters, in contrast to C. s imillimus where the dominant lipid class was triacylgycerol. These obs ervations coupled with literature data on feeding and seasonal vertica l distribution seem to indicate fundamently different life history pat terns.