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

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Read, J. F. , Pollard, R. T. and Bathmann, U. (2002): Physical and biological patchiness of an upper ocean transect from South Africa to the ice edge near the Greenwich Meridian , Deep-Sea Research II, 49 (18), pp. 3713-3733 . doi: 10.1016/S0967-0645(02)00108-X
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An undulating CTD (SeaSoar) was towed from the Subtropical Front at 421S, 121E to the ice edge at 57.51S, 21W.The instrument measured temperature, salinity, pressure, and light in the top 400m of the water column. Fluorescenceand particle counts also were recorded. Fluorescence was calibrated to chlorophyll a, and particle counts wereconverted to biovolume to provide estimates of phyto- and zooplankton, respectively. The size of particles countedcovered the range 0.258.0 mm.Three major fronts, Subtropical, Subantarctic, and Polar were intersected and identified by their physicalcharacteristics. The Polar Front showed surface and subsurface signatures separated by nearly 300 km. Considerablesmall-scale structure was observed between the fronts, in particular a large (150-km diameter), warm-core eddy wasfound in the northern part of the Subantarctic Zone.Both fronts and eddy were associated with significant biological signals. Phyto- and zooplankton correlated well inthe frontal regions and south of the Polar Front. The Polar Frontal Zone was associated with elevated concentrationsof biomass, whereas concentrations were low at the Subantarctic Front and south of 551S. In contrast, the warm-coreeddy in the Subantarctic Zone contained the highest zooplankton biovolume of the section while chlorophyllconcentration 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 ofbiomass 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 structurecorrelated significantly with the distribution of plankton, suggesting strong physical controls on both smallzooplankton and phytoplankton.

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