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 from Maurice Ewing Bank (50°S, 42°W) towards South Georgia (54°S, 37.5°W) during early January 1994 and again 1 mo later. The transect spanned the Polar Frontal Zone (PFZ), crossing the Polar Front (PF) into Antarctic surface waters. In January, diatom 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. gigas had not yet commenced spawning although recruitment had begun to the south. Early stages of C. simillimus copepodites were, however, present at all stations sampled. South of the PF both gonad maturity and lipid levels in R. gigas showed a clear positive relationship with the elevated pigment concentrations. C. simillimus females, on the other hand, were ripe at the majority of stations sampled regardless of pigment levels. A month later postbloom conditions were present over the greater part of the transect and the summer generation of both species was present at all stations. A proportion of the summer generation of C. simillimus had already reached adulthood and their gonads were maturing. The populations of both species were older in PFZ waters, and mean population age showed a strong correlation with water temperature. Prosome length of stages CIV, CV and CVI-female R. gigas was negatively correlated with sea surface temperature. This was not the case for C. simillimus females, suggesting that they had been recruited earlier in the season under different environmental conditions. Lipid analysis indicated that R. gigas predominantly stored wax esters, in contrast to C. simillimus where the dominant lipid class was triacylgycerol. These observations coupled with literature data on feeding and seasonal vertical distribution seem to indicate fundamently different life history patterns.

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