In the Amundsen Sea, significant global warming accelerates ice melt, and is consequently altering many ocean properties such as sea ice concentration, surface freshening, water column stratification, and underwater light properties. To examine the influence of light, which is one of the fundamental factors for phytoplankton growth, incubation experiments and field surveys were performed during the austral summer of 2016. In the incubation experiments, phytoplankton abundance and carbon biomass significantly increased with increasing light levels, probably indicating light limitation. Growth rates of the small pennates (mean 0.42 d-1) increased most rapidly with an increase in light, followed by those of Phaeocystis antarctica (0.31 d-1), and the large diatoms (0.16 d-1). A short-term study during the field survey showed that phytoplankton distribution in the surface layer was likely controlled by different responses to light and the sinking rate of each species. These results suggest that the approach adopted by previous studies of explaining phytoplankton ecology as a characteristic of two major taxa, namely diatoms and P. antarctica, in the coastal Antarctic waters might cause errors owing to oversimplification and misunderstanding, since diatoms comprise several species that have different ecophysiological characteristics.
Keywords: Amundsen Sea; Diatoms; Growth rate; Light limitation; Phaeocystis antarctica; Phytoplankton.
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