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To study the effect of hydrographic factors on the spatial distributions of chlorophyll a (Chl a),an investigation was carried out in the tropical eastern Indian Ocean (80°-100°E along 7°S,and 7°-18°S along 80°E) in December 2010.The fluorescent method was used to obtain total Chl a and size-fractioned Chl a at the 26 stations.The results show that surface Chl a concentration averaged at (0.168±0.095) mg/m3 s.d.(range: 0.034-0.475 mg/m3),concentrations appeared to be higher in the west for longitudinal variations,and higher in the north for latitudinal variations.Furthermore,the surface Chl a concentration was lower (0.034-0.066 mg/m3) in the region to the south of 16°S.There was a strong subsurface Chl a maximum layer at all stations and the depth of the Chl a maximum increased towards to the east and south along with the respective nitracline.The spatial variation of Chl a was significant: correlation and regression analysis suggests that it was primarily affected by PO43-,N(NO3-N+NO2-N) and temperature.Size-fractionated Chl a concentration clearly showed that the study area was a typical oligotrophic open ocean,in which picophytoplankton dominated,accounting for approximately 67.8% of total Chl a,followed by nanophytoplankton (24.5%) and microphytoplankton (7.6%).The two larger fractions were sensitive to the limitation of P,while picophytoplankton was primarily affected by temperature.