Characteristics Of Chlorophyll A, Primary Productivity And Photosynthetic Fluorescence In The East Tropical Indian Ocean During The Boreal Spring Inter-mosoon

To better understand phytoplankton standing stock and their roles in biogeochemical cycle in the east tropical Indian Ocean, four transects of 37 stations were investigated from April 26 th to May 14 th 2012. We studied the spatial distribution of Chlorophyll a and primary production, and explored the coupling between phytoplankton communities and their habitat. Phyto-PAM was applied to assess photosynthetic activities of phytoplankton communities in east tropical Indian Ocean. Conbining with remote sensing data, this paper analysed the temporal and spatial variation of surface chlorophyll a in the east tropical Indian Ocean.The results are summarized as follows:(1). Vertically, stratification was prevalent in the water column.The temperature was higher while the salinity was lower in upper mixing layer during the investigation, and prominent thermo-haloclines were formed at 80~100 m. Influenced by Sunda Strait along Transect A, salinity increased from the surface of Stiation 01 to the open sea horizontally.(2). Surface chlorophyll a concentration was lower than 0.10 mg/m3 and a very obvious subsurface chlorophyll a maximum(0.16~0.40 mg/m3) appeared around the thermo-haloclines.(3). Primary production rate in the east tropical Indian Ocean ranged between 83~403 mg C ? m-2? d-1.Transect A nearby Sunda straithad the lowest primary production rate, and the highest primary production occurred across and along the equator, followed by Transect D paralleled to Summtra Island.(4). Correlation analysis suggested that chlorophyll a concentration was significantly positively related with salinity, water depth, NO2-, NO3-, PO43-, while it has a significant negative correlation with temperature. With no exogenous source, the N/P was lower than 10 and PO43- concentration less than 0.4μmol/L, which indicatedpotential DIN limitation and absolute limitation of phosphorus in this region. Thermo-haloclines were pervasive in the east tropical Indian Ocean, and it prevented deeper water with rich nutrient from replenishing the upper mixing layer.Consequently, the sea surface chlorophyll a was rather lower and subsurface chlorophyll a maximum appeared near the thermo-haloclines.(5). During the boreal spring inter-monsoon, Fv/Fm had a range of 0.04~0.75 and revealed significant vertical difference with no station difference in east tropical Indian Ocean. Fv/Fm reached to its maximum around thermocline, and it was rather lower at the surface and beneath the thermocline. Phytoplankton at the depth of 0 m, 20 m were acclimated to high irradiance and had lower light utilization efficiencies(α=0.05). Photosynthesis characteristics of phytoplankton communities can reflect their photosynthetic physiology and acclimation/adaption to ambient environment and help explain the mechanisms of chlorophyll a vertical distribution patterns.(6). Satellite derived sea surface chlorophyll a data were in conformity with our field survey data. In a normal year, low chlorophyll a is perennial in the east equatorial Indian Ocean and seasonal variations of chlorophyll a occur around Sunda Strait and in south region of Java Island. Climate changes, such as ENSO and Indian Ocean Dipole, can profoundly affect the distribution patterns of primary production as well as local fisheries in the east tropical Indian Ocean.