[A-OS16] Physical, biogeochemical, and ecological processes and variability in the Indian Ocean

[AOS16-01] Different environmental factors control phytoplankton populations in the eastern Indian Ocean

*Mitsuhide Sato^1,2, Fuminori Hashihama³, Hongbin Liu² (1.Graduate School of Integrated Science and Technology, Nagasaki University, 2.Division of Ocean Science, Hong Kong University of Science and Technology, 3.Department of Ocean Sciences, Tokyo University of Marine Science and Technology)

Keywords:flow cytometry, mixotrophy, surface mixed layer, nutrients, eastern Indian Ocean

Pico- and nanophytoplankton communities in the eastern Indian Ocean were analyzed during the fall–winter inter-monsoon season to clarify the environmental factors that control the horizontal and vertical distributions of phytoplankton. The study area included the Bay of Bengal, the South Indian Subtropical Gyre, south of the Indonesian Islands, and west of Australia. Prochlorococcus, Synechococcus, and eukaryotic phytoplankton were enumerated by onboard flow cytometry, and their cell concentrations at the subsurface peak were estimated by fitting a curve function to their discrete vertical profile data. The mean Synechococcus abundance within the surface mixed layer was higher in the Bay of Bengal than in other provinces and showed a significant positive correlation with temperature and nitrate + nitrite (N+N) concentration. Similarly, the cell concentration of eukaryotic phytoplankton in the surface mixed layer correlated with temperature but did not decrease with decreasing N+N availability. Instead, the proportion of potentially phagotrophic eukaryotic phytoplankton, as assessed using a fluorescent probe in combination with flow cytometry, increased with decreasing N+N concentrations in the surface mixed layer. This suggests that nitrogen uptake from particles may have alleviated their nitrogen limitation. The cell size of eukaryotes on the eastern edge of the eastern Indian Ocean was larger than that in the other ocean provinces, suggesting the input of different plankton assemblages from the surrounding waters. Inter-provincial variations in cell concentrations at the subsurface peak were smaller than those within the surface mixed layer. The cell concentration of Synechococcus at the peak was positively correlated with temperature, whereas that of eukaryotes was positively correlated with light intensity at that depth. This suggests a possible light limitation on them. In contrast to these two populations, the spatial variation in the cell concentration of Prochlorococcus was relatively small, and a subsurface peak was absent at some stations.