SEASONAL-VARIATION OF PHYTOPLANKTON COMMUNITY STRUCTURE AND NITROGEN UPTAKE REGIME IN THE INDIAN SECTOR OF THE SOUTHERN-OCEAN

This study investigates the dynamics of phytoplankton communities and nitrogen uptake in the Indian sector of the Southern Ocean during spri ng and summer. The study area is oligotrophic (Chl a stocks < 50 mg m( -2)); nevertheless, a large spatial variation of phytoplankton biomass and community structure was observed. During both seasons the phytopl ankton community in the seasonal ice zone showed higher biomasses and was mainly composed of large diatom cells. However, in the permanently open ocean zone the community had low biomass and was chiefly compose d of nano- and picoflagellates. In the polar front zone, although biom ass was higher, the community structure was similar to the open ocean zone. The results suggest that the variation in phytoplankton communit y structure on a larger scale resonates with gradients in water column stability and nutrient distribution. However, significant changes in biomass and nutrient stocks but little change in community structure w ere observed. Absolute nitrogen uptake rates were generally low, but t heir seasonal variations were highly significant. During spring the co mmunities displayed high specific nitrate uptake (mean rate = 0.0048 h (-1)), and diatoms (in the seasonal ice zone) as well as nano- and pic oflagellates (in the permanently open ocean zone and polar front zone) were mainly based on new production (mean f-ratio = 0.69). The transi tion to summer was accompanied by a significant reduction in nitrate u ptake rate (0.0048 h(-1) --> 0.0011 h(-1)) and a shift from predominan tly new to regenerated production (f-ratio 0.69 --> 0.39). Ammonium pl ayed a major role in the seasonal dynamics of phytoplankton nutrition. The results emphasize that, despite a large contrast in community str ucture, the seasonal dynamics of the nitrogen uptake regime and phytop lankton community structure in all three subsystems were similar. Addi tionally, this study supports our previous conclusion that the seasona l shift in nitrogen uptake regime can occur with, as well as without, marked changes in community structure.