Physical and biological patchiness of an upper ocean transect from South Africa to the ice edge near the Greenwich Meridian

An undulating CTD (SeaSoar) was towed from the Subtropical Front at 42°S, 12°E to the ice edge at 57.5°S, 2°W. The instrument measured temperature, salinity, pressure, and light in the top 400 m of the water column. Fluorescence and particle counts also were recorded. Fluorescence was calibrated to chlorophylla, and particle counts were converted to biovolume to provide estimates of phyto- and zooplankton, respectively. The size of particles counted covered the range 0.25–8.0 mm. Three major fronts, Subtropical, Subantarctic, and Polar were intersected and identified by their physical characteristics. The Polar Front showed surface and subsurface signatures separated by nearly 300 km. Considerable small-scale structure was observed between the fronts, in particular a large (150-km diameter), warm-core eddy was found in the northern part of the Subantarctic Zone. Both fronts and eddy were associated with significant biological signals. Phyto- and zooplankton correlated well in the frontal regions and south of the Polar Front. The Polar Frontal Zone was associated with elevated concentrations of biomass, whereas concentrations were low at the Subantarctic Front and south of 55°S. In contrast, the warm-core eddy in the Subantarctic Zone contained the highest zooplankton biovolume of the section while chlorophyll concentration was only moderate. We hypothesise that such a distribution results from grazing. The overall distribution of phyto- and zooplankton changed across the Subantarctic Front. The vertical extent of biomass deepened from north to south across the front. Zooplankton tended to be concentrated in relatively narrow, deep-extending bands to the north but were more widely spread to the south. The small-scale hydrographic structure correlated significantly with the distribution of plankton, suggesting strong physical controls on both small zooplankton and phytoplankton.