Small-scale variations in phytoplankton standing stock and productivity across the oceanic fronts in the Southern Ocean

Observations on phytoplankton standing stock and productivity were carried out across the Antarctic Polar Front (PF) and the Subtropical Convergence (STC) employing a small-scale sampling scheme (1 sample per 1 or 2 miles) through 20 or 40 miles transects. Maximum of chlorophyll standing stock (0.185μg chl. a・l^) was found at the midst of physical and chemical changes in the PF transect, which was 2-fold higher than those in the Antarctic side and 3-fold higher than those in the Subantarctic side. Community forming the maximum standing stock consisted of a large number of the Antarctic species, Nitzschia spp. (72% of them was Flagilariopsis spp. in old nomenclature). On the other hand, the maximum standing stock (0.309μg chl. a・l^) in the STC transect was found at just north of the physical gradients at the front, and was associated with a warm saline water mass carrying nutrients of relatively low concentrations. Although the productivity also was maximum at the maximum standing stock, there was no indication of increase in the photosynthetic rates. Species appeared in the community of maximum standing stock in the STC transect included warm water forms such as Ceratium arietinum var. glacilentum, C. massiliense, Oxytoxum scolopax, Podolampas spinifera and Prorocentrum rostratum and a neritic form, Protoperidinium pallidum. Discussions were made for the formation of the maximum phytoplankton standing stock at the PF and the STC