Contrasting regimes of production and potential for carbon export in the Sub-Antarctic and Polar Frontal Zones south of Tasmania

We report on mid-summer gross primary production and new production in the Sub-Antarctic Zone (SAZ) and the Polar Frontal Zone (PFZ) south of Tasmania during the SAZ-Sense expedition (January¨CFebruary 2007). The aim of our study was to assess how well documented regional variability in surface Chl-a biomass translates into variability of primary production and potential export in the area. The selected sites for process studies contrasted in terms of euphotic and mixed layer depths, macro nutrient concentrations and primary production with short term (days) temporal variability of production. Daily euphotic layer integrated gross primary production (from short term 13C incubations) was higher in the SAZ than in the PFZ by about an order of magnitude. Within the SAZ highest production was reached south-west of Tasmania, contrasting with surface ocean Chl-a biomass which was highest in the eastern SAZ. In most cases regenerated production (from 15N-ammonium uptake experiments) was significantly larger than new production (from 15N-nitrate uptake experiments) with f-ratios mostly ¡Ü0.3. Mixed layer and euphotic layer depths, relative availability of nitrate and ammonium, level of Fe sufficiency, grazing pressure, and opposing effect of Fe and ammonium on nitrate uptake, appear to control these regional differences in SAZ production. Overall low new production values reflected a low relative potential for carbon export in the area, confirming the low export ratios reported by others. These conditions prevail throughout the study area but, unexpectedly, are most marked in the eastern SAZ where Fe was reported not to be limiting. For the eastern SAZ we speculate that the availability of ammonium sustained by grazing pressure inhibited nitrate uptake and primary production by counteracting the effects of Fe sufficiency and mixed layer shallowness, which are factors potentially conducive to enhanced production. This lower production combined with decreased f-ratios led to the observed poor potential for carbon export and poor carbon sequestration in the SAZ-East.