The distribution of chlorophyll a in the Southwestern Indian Ocean in summer

An investigation of the distribution and size composition of chlorophyll a concentration was carried out in the Southwestern Indian Ocean (47--70 degree E, 25--40 degree S) in January 2011, and factors that might affect the distribution and size composition were discussed. Fluorometric method was used to obtain total and size-fractioned chlorophyll a concentrations. The survey was carried out during the austral summer, and there were 6 stations altogether, two of which located in the region of the south Indian Ocean Subtropical Gyre (IOSG) and the other four located in the subtropical convergence zone (SCZ). Water samples were collected from 13 layers in 0--200m column of each station and the vertical profiles of chlorophyll a, nutrients (NO super(-) sub(3), PO super(3) sub(4) super(-), and SiO super(2) sub(3) super(-)), temperature and salinity were obtained. The objective of this study was to investigate the spatial distribution and size fraction of chlorophyll a concentration and to relate them to environmental factors. The results showed that the surface chlorophyll a concentration was low and did not exceed 0.07mg/m super(3), and the depth of the subsurface chlorophyll a maximum was deep, at approximately 100m and sometimes reaching as deep as 150m in the region of IOSG; However, in the SCZ, the surface chlorophyll a concentration was higher (0.164--0.247 mg/m super(3)), and the subsurface chlorophyll a maximum layer occurred between 50--70m. In the region of IOSG, phytoplankton growth is limited by nutrients supply, while the frontal system formed by the Agulhas Return Current (ARC) and the Subtropical Convergence (STC) is a region of intense mesoscale activity presenting enhanced levels of biological production and chlorophyll a concentration. Based on the discussion of the factors affecting the distribution of phytoplankton, it appeared that NO super(-) sub(3) was the main factor that limited the growth of phytoplankton in the southwestern Indian Ocean, and PO super(3) sub(4) super(-) could be the main limiting factor in some areas. Size-fractioned chlorophyll a concentration clearly showed that the study area was a typical oligotrophic open ocean. The picophytoplankton (0.2--2 mu m) chlorophyll a always dominated, representing 71.1% of the total chlorophyll a concentration on average, while the average contribution of nanophytoplankton (2--20 mu m) and microphytoplankton (>20 mu m) chlorophyll a to total chlorophyll a concentration were 24.2% and