Seasonal pigment patterns of surface phytoplankton in the subtropical southern hemisphere

Pigment indices were used to characterise the seasonal succession of phytoplankton, and associated changes in chlorophyll a and accessory pigments, in subtropical waters of the three ocean basins in the southern hemisphere. Diagnostic indices revealed the dominance of small flagellates and elevated biomass during winter–spring in the Pacific, mixed flagellate–prokaryote communities and intermediate biomass during early summer in the Atlantic, and prokaryote dominance with low biomass in mid-summer in the Indian Ocean. Photo-pigment indices indicated only a small variation in the chlorophyll a proportion of total pigments across the ocean basins, but the accessory pigments varied considerably. Under low temperature and irradiance conditions, the photosynthetic carotenoids were prominent, but as temperatures and irradiance increased and nutrients declined, there was a significant increase in the proportion of photoprotective carotenoids. At high temperatures and irradiances, the photoprotective carotenoids were the largest component of the pigment pool, exceeding the proportion of chlorophyll a. These variations in phytoplankton composition, and their photoacclimation status, could be explained according to environmental changes and have implications for satellite estimations of biomass and primary production.