The distribution of chlorophyll a in the tropical eastern Indian Ocean in austral summer
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The distribution of chlorophyll a in the tropical eastern Indian Ocean in austral summer HONG Lisha1,2 , WANG Chunsheng1,2∗ , ZHOU Yadong1,2 , CHEN Mianrun3 , LIU Hongbin3 , LIN Zhongyang1,4 , SONG Xunshu1,5 1 2
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The Second Institute of Oceanography, State Oceanic Administration, Hangzhou 310012, China Laboratory of Marine Ecosystem and Biogeochemistry, State Oceanic Administration, Hangzhou 310012, China Division of Life Science, Hong Kong University of Science and Technology, Hong Kong, China Key Laboratory of Submarine Geosciences, State Oceanic Administration, Hangzhou 310012, China State Key Laboratory of Satellite Ocean Environment Dynamics, Hangzhou 310012, China
Received 6 September 2011; accepted 24 June 2012 ©The Chinese Society of Oceanography and Springer-Verlag Berlin Heidelberg 2012
Abstract 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 PO3− 4 , 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. Key words: eastern Indian Ocean, chlorophyll a, size fraction, Indonesian Throughflow
1 Introduction
oceanography. As the physiological characteristics of different sizes of phytoplankton and their ecological functions are different, it is essential to understand the composition and consequences of variability in the size of phytoplankton. For example, large-sized phytoplankton are easily grazed by mesozoooplankton and the fixed carbon is ultimately exported to great depths, while carbon produced by small-sized phytoplankton is generally recycled in the euphotic zone. Therefore, it is essential to understand the material and energy transfer in the planktonic food web in marine ecosystems from the perspective of size structure of phytoplankton for the study of marine primary biomass (Fu et al., 2
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