Seasonal fluctuation of export and recycled production in different subareas of the Southern Ocean
Goeyens, L.; Dehairs, F.A. (1993). Seasonal fluctuation of export and recycled production in different subareas of the Southern Ocean, in: Caschetto, S. (Ed.) Belgian scientific research programme on Antarctica: scientific results of phase II (10/1988-05/1992): 1. Plankton ecology and marine biogeochemistry. pp. II/08/1-79 In: Caschetto, S. (Ed.) (1993). Belgian scientific research programme on Antarctica: scientific results of phase II (10/1988-05/1992): 1. Plankton ecology and marine biogeochemistry. Belgian scientific research programme on Antarctica. Belgian Science Policy Office: Brussel. 295 pp., more In: Belgian scientific research programme on Antarctica. Belgian Science Policy Office: Brussel. , more |
| Abstract | As the role of the Southern Ocean in the global biogeochemical carbon cycle is nowadays a leading concern in oceanographic research, this study emphasizes the specific effects of nitrogenous nutrients on the origin, development and fate of primary production. The inherent consequences of nitrogen utilization by phytoplankton for the channelling of organic nitrogen towards in-situ regeneration or towards sedimentation are investigated. Nitrate depletion, the indication of the over-all new production during the ongoing season, ranged from near zero values to very high values (over 1000 mmol N m-2). As a general trend nitrate depletions were lowest in the closed pack ice zones, while strongly enhanced nitrate removal was observed in the marginal ice zone of the Scotia-Weddell Confluence area and in the continental shelf zone near Amery Ice Shelf in Prydz Bay. For the former area the average nitrate depletion amounted to 345 mmol N m-2, for the latter one it amounted to 570 mmol N m-2. The onset of the new growth season was characterized by elevated nitrate uptake rates. lasting for relatively short periods in the early season only. Suhsequently, ammonium was regenerated mainly due to grazing hut also due to bacterial degradation and started to accumulate in the upper layer of the water column. Especially in the marginal ice zone and in the vicinity of the ice edge, elevated ammonium concentrations of 2 µmol N I-1 or more were measured and this resulted in considerahle ammonium availahilities of >5%. The appearance of regenerated nitrogenous nutrients triggered a switch-over from predominantly new production towards increased importance of regenerated production. The fraction of nitrate based primary production decreased but slightly during the growth season in areas which were largely covered by remnants of winter ice during austral summer; nitrate always contributed for ≥50% to primary production (average f-ratio ≥0.5). On the contrary , a drastic shift towards regenerated production was observed in the seasonally ice free zones in the vicinity of the ice edge and on the continental shelf. The f-ratios decreased to 0.30 in the Scotia- Weddell Confluence area and to 0.20 in the Prydz Bay shelf zone. These changes were mainly driven by the preference of phytoplankton for regenerated nitrogen, with the phytoplankton's community structure affecting the uptake regime to a minor degree. It was observed that diatoms remained the dominant phytoplankton species in Prydz Bay even when f-ratios went down to 0.2. In open ocean areas, belonging to the Circumpolar Current system, such as the Scotia Sea and the offshelf region of Prydz Bay, nitrate and ammonium take remained essentially of equal importance during the progressing season and only a reduced shift towards increased ammonium uptake was seen.Export of primary production to subsurface waters (100 to 500 m depth region) was shown to be mimicked by the build-up of suspended barite stocks. This build-up was out of phase with the surface water processes but depended closely on the evolution of f-ratios in surface waters. Areas characterized by significant export towards mesopelagic depths were the open ocean systems of the Scotia Sea and the Circumpolar Current in the Indian Ocean's sector. These systems showed intermediate productivities. Excluding the impact by krill grazing, smallest export to subsurface layers, was observed in the marginal ice zone of the Scotia-Weddell Confluence and over the continental shelf in Prydz Bay. The latter regions demonstrated highest productivity as well as highest grazing pressure. The results obtained during different Antarctic cruises provides clear evidence for a distinction between different Antarctic ecosystems. Intensive new production, characteristic for fertile zones bordering the retreating ice edge was mainly conveyed towards the regenerating microbial network. This was mirrorred by enhanced ammonium availability and poor subsurface barite accumulation. On the other hand, the moderate to low primary production of open sea and close pack ice zones was available for export and only small amounts of the organic matter were remineralized in the upper layer. Additionally, we propose a scaling function, based on the correlation between barite concentrations in the subsurface Ba-maximum layer and oxygen concentrations in the O2-minimum layer observed for the Indian Ocean's sector. The equation provides a tool to estimate export of organic matter at any other Southern Ocean site starting with information on net barite accumulations over the season. |
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