Abstract
It is widely recognized that the mesoscale eddies play an important part in the biogeochemical cycle in ocean ecosystem, especially in the oligotrophic tropical zones. So here a heterogeneous cyclonic eddy in its flourishing stage was detected using remote sensing and in situ biogeochemical observation in the western South China Sea (SCS) in early September, 2007. The high-performance liquid chromatography method was used to identify the photosynthetic pigments. And the CHEMical TAXonomy (CHEMTAX) was applied to calculate the contribution of nine phytoplankton groups to the total chlorophyll a (TChl a) biomass. The deep chlorophyll a maximum layer (DCML) was raised to form a dome structure in the eddy center while there was no distinct enhancement for TChl a biomass. The integrated TChl a concentration in the upper 100 m water column was also constant from the eddy center to the surrounding water outside the eddy. However the TChl a biomass in the surface layer (at 5 m) in the eddy center was promoted 2.6-fold compared to the biomass outside the eddy (p < 0.001). Thus, the slight enhancement of TChl a biomass of euphotic zone integration within the eddy was mainly from the phytoplankton in the upper mixed zone rather than the DCML. The phytoplankton community was primarily contributed by diatoms, prasinophytes, and Synechococcus at the DCML within the eddy, while less was contributed by haptophytes_8 and Prochlorococcus. The TChl a biomass for most of the phytoplankton groups increased at the surface layer in the eddy center under the effect of nutrient pumping. The doming isopycnal within the eddy supplied nutrients gently into the upper mixing layer, and there was remarkable enhancement in phytoplankton biomass at the surface layer with 10.5% TChl a biomass of water column in eddy center and 3.7% at reference stations. So the slight increasing in the water column integrated phytoplankton biomass might be attributed to the stimulated phytoplankton biomass at the surface layer.
Highlights
Most of the tropical and subtropical zones in the open ocean are permanently stratified
There was no notable signature of cooler water at the surface layer (5 m), but more distinct water masses could be distinguished based on the salinity distribution
The isopycnal sigma-t = 24 kg m-3 almost overlapped with the baseline of euphotic zone in C2, which was uplifted to about 50~80 m, in comparison with the 93.2±18.6 m in the surrounding water
Summary
Most of the tropical and subtropical zones in the open ocean are permanently stratified. Nutrients that are essential for the growth of marine phytoplankton are abundant below the thermocline, but normally they have to rely on diffusion to reach the sun-lit euphotic zone to be utilized by phytoplankton [1,2]. One of the common mesoscale events in the tropical and subtropical marine ecosystems, provide a means by which nutrients below the thermocline can be relatively quickly delivered into the upper euphotic zone [2]. The responses of Chl a biomass to the mesoscale cyclonic eddy were concluded as four mainly processes, including eddy advection, doming DCML to the near surface layer, promoting phytoplankton biomass supported by the nutrient pumping and convergence of the biomass under the currents [6,7]. The combined action of different processes made it be necessary to study the biological response to the mesoscale eddy in the view of 3D structure
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