The distribution of chlorophyll a in the tropical eastern Indian Ocean in austral summer

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 (80A degrees-100A degrees E along 7A degrees S, and 7A degrees-18A degrees S along 80A degrees 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/m(3) s.d. (range: 0.034-0.475 mg/m(3)), 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/m(3)) in the region to the south of 16A degrees 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 PO (4) (3-) , 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.