Size-fractionated primary production studies in the vicinity of the Subtropical Front and an adjacent warm-core eddy south of Africa in austral winter

Results are presented of size-fractionated primary production studies condu cted in the vicinity of the Subtropical Front (STF), an adjacent warm-core eddy, and in Sub-antarctic waters during the third South African Antarctic Marine Ecosystem Study (SAAMES III) in austral winter (June/July) 1993. Thr oughout the investigation, total chlorophyll (Chl a) biomass and production were dominated by small nano- and picophytoplankton. No distinct patterns in total Chi a were evident. At stations (n = 7) occupied in the vicinity o f the STF, total integrated biomass values ranged from 31 to 53 mg Chi a m( -2). In the vicinity of the eddy, integrated biomass at the eddy edge (n = 3) ranged from 24 to 54 mg Chi a m(-2) and from 32 to 43 mg Chi a m(-2) in the eddy (n = 2). At the station occupied in the Sub-antarctic waters, tota l integrated biomass was 43 mg ChI a m(-2). Total daily integrated producti on was highest at stations occupied in the vicinity of the STF and at the e ddy edge. Here, total integrated production ranged from 150 to 423 mg C m(- 2) day(-1) and from 244 to 326 mg C m(-2) day(-1), respectively. In the edd y centre, total integrated production varied between 134 and 156 mg C m(-2) day(-1). At the station occupied in the Sub-antarctic waters, the lowest i ntegrated production (141 mg C m(-2) day(-1)) during the entire survey was recorded. Availability of macronutrients did not appear to limit total prod uction. However, the low silicate concentrations during the survey may acco unt for the predominance of small nano- and picophytoplankton. Differences in production rates between the eddy edge and eddy core were related to wat er column stability. In contrast, at stations occupied in the vicinity of t he STF, the control of phytoplankton production appears to be related to se veral processes, including water column stability and, possibly, iron avail ability.