A 3-D mesoscale map of primary production at the Antarctic Polar Front: results of a diagnostic model

A diagnostic model is established to estimate synoptically the mesoscale distribution of primary production at the Antarctic Polar Front (APF). The model domain is a 3-D box, centred at roughly 50°S and 10°E, of about 1° latitude and 2° longitude horizontal extent, and of 300 m depth. The box was surveyed in high resolution during austral summer 1995/1996 with a towed undulating vehicle and by complementary ship-based measurements. Measurements of global solar radiation, of the underwater light field, and of the chlorophyll concentration from the survey are used as input variables for the model. The model is based on photosynthesis-light relationships, with parameters taken from in vitro incubations performed during the survey. The model results show mesoscale patches of elevated primary production along a meander of the APF, and lowest production in a cold cyclonic eddy south of the front. Production is confined to a shallower depth range in the front than outside, due to self-shading effects from generally higher mixed-layer chlorophyll concentrations. Self-shading effects account for variations of the percent light depths, and of the saturation light depth, by a factor of 2 within the survey area. Primary production at the surface varies horizontally between 7 and 56 mg C m−3 d−1, with a mean of 26 mg C m−3 d−1, and vertically integrated production ranges from 295 to 975 mg C m−2 d−1, with an areal mean of 585 mg C m−3 d−1. Changes by a factor of 2 in integrated production occur on horizontal scales as small as 10 km. Production rates also differ significantly between days as a result of changes in global solar radiation.