An investigation of the distribution and size composition of chlorophyll a concentration (>20 μm, 2-20 μm, 0.2-2 μm) was carried out from April to May 2010 in the South Equatorial Current (SEC) region (10°59'-14°6'S,80°00'-113°3'E) of the Indian Ocean. The survey was carried out during the inter-monsoon, which is characterized by weak vertical mixing and low upwelling. The objective of this study was to investigate the spatial distribution and size fraction of chlorophyll a concentration and to relate this to environmental factors. 9 stations were surveyed and water samples were collected from 17 layers in 0-200 m water column of each station. A total of 9 profiles of size-fractionated chlorophyll a, nutrients (NO₃^-、PO₄^3- and SiO₃^2-), temperature and salinity were obtained. According to the location of the stations, the whole survey region was divided into two subregions, CIO (Central Indian Ocean) survey region and EIO (Eastern Indian Ocean) survey region. Results showed that the water column of the survey area is well stratified and the mixed layer was about 50 m throughout the region. The surface water was characterized by high temperature and low concentrations of nutrients. The eastern part of the EIO region was influenced by the intrusion of the Indonesian Throughflow and the South Java Current, which has high temperature and low salinity. Three characteristics of the chlorophyll a distribution were revealed: (1) Low chlorophyll a concentration of surface water was widespread in the region (0.122±0.052 mg/m³ on average), but spatial pattern was also observed. The chlorophyll a concentration in the CIO survey region was uniformly low; however, relatively higher chlorophyll a concentration and higher inter-station variation was observed in the EIO survey region. (2) A subsurface chlorophyll a maximum layer occurred between 60 and 80 m, which coincided with the nutricline. (3) The 0.2-2 μm chlorophyll a always dominated and represented 75% of the total chlorophyll a on average, while the average contribution of 2-20 μm and >20 μm chlorophyll a to total chlorophyll a concentration were 20% and 5%, respectively. It was suggested that the phytoplankton community in this area was predominated by picoplankton.
Factors affecting the distribution of phytoplankton were discussed.The low chlorophyll a concentration in surface water can be attributed to the low concentrations of nutrients in upper layers, which were less than or close to the half-saturation constants (K_s values) for nutrients uptake of phytoplankton in oligotrophic seas. Chlorophyll a concentration increased rapidly in response to the increasing nutrient concentrations in the nutricline. The dominance of picoplanton was also resulted from the low nutrients concentration because picoplankton were more competent over the netplankton and nanoplankton under oligotrophic condition. Our results indicated that hydrological structure could affect phytoplankton via exerting influence over nutrients distribution.
Based on the comparison between the present research and studies in other oceans, the SEC region was classified as one of the oligotrophic regions, in which the low level of nutrients, especially NO₃^-, played a major role in limiting the growth of phytoplankton.