| The present study analyzed seasonal and annual variations of Chlorophyll-a (Chl-a) and relative oceanic environments for long periods in the South China Sea (SCS), using remote sensing data of Chl-a concentration, sea surface wind, sea height anomaly, sea surface temperature and in situ historical cruises observation data of Chl-a concentration. The spatial and temporal variations of ocean primary productivity (OPP) in different sea areas of the SCS were estimated by Vertically Generalized Production Model (VGPM) by analyzing remote sensing data. And the paper also gave a preliminary study on the response of OPP to the monsoon variations, upwelling and other oceanographic environmental factors in the SCS.The monthly Climatology's averaged variations and spatial distribution of Chl-a concentration and relative ocean environments showed clear features. The profile of Chl-a concentration stood at the maximum during the winter monsoon (the northeasterly winds), especially in Dec, off the northwest coast of Luzon, where was joined with the coastal sea areas with higher Chl-a concentration to extend over the northern SCS, indicating a phytoplankton biomass increase. In the entire SCS, phytoplankton blooms were distributed broadly, especially in coastal sea water during this season. From spring, the northeasterly winds changed into the southwesterly winds, when Chl-a concentration was the lowest in the whole year. From Jun./Jul., high phytoplankton biomass appeared off the east coast of Vietnam and began to extend as a jet into the open sea. The jet had a higher Chl-a concentration on the coastal side and a lower Chl-a concentration on the seaward side. In Aug., the region of high phytoplankton biomass extended northeast, appearing obviously in the western SCS. It was time that the southeasterly winds prevailed in the SCS, which were stronger in the southwestern SCS than that in the northeastern SCS. From monthly climatology's spatial distribution of SST, we found a band of low SST along the region off the east coast of Vietnam in this period. The cold water region started at the coast and extended east toward the open sea. The phenomenon was not obvious from Sep., and then it transited into the conditions of winter. From in situ historical cruises observation data of Chl-a concentration, the results showed the seasonal variations of Chl-a concentration in Nansha Islands sea area were obvious from 1984 to 2004, which occurred higher in summer and winter, lower in spring and fall. The vertical profile of Chl-a concentration was high at the depth of 50-75m. In the cross section of the Luzon Strait, Chl-a concentration was higher at the depth of 25-75m in Sep. 2005 than in Sep. 2004 and Sep. 2006. In the winter of Nov.-Dec. 2006, about the vertical profile of Chl-a concentration, there was a peak at the depth of 0-25m and a small peak at the depth of about 75m in the region of Pear River Estuary; while Chl-a concentration was high at depth of about 50m and 75m, respectively in the regions off the northwest coast of Luzon and off the east coast of Vietnam.The spatial and temporal variations of OPP in the SCS from 1998 to 2005 was estimated by VGPM by analyzing SeaWiFS-derived Chl-a, Photosynthetically Active Radiation (PAR), NOAA/AVHRR-derived sea surface temperature (SST) and other oceanographic data. The results revealed evident seasonal variations of OPP, Chl-a, SST and sea surface wind in the whole sea and every typical sea area including the region of Basin, the Coast of Guangdong, the gulf of Tonkin, the southeast of Vietnam, the northwest of Luzon. It showed a strong peak in the whole sea in winter, the lowest in May and a little peak in Aug. Spatially, the OPP in shore areas was higher than that in the central sea basin obviously, and the OPP of every typical sea area varied according to the different geographic and environmental elements. The OPP was floating little in the region of Basin, always high in the Coast of Guangdong and the gulf of Tonkin, and high both in winter and summer in the southeast of Vietnam. Meanwhile, annual variation of OPP was not apparent, but it was lowest in 1998 than other years, especially in the southeast of Vietnam, where was hardly seen the higher OPP northeastward.The present study also gave a preliminary study on the response of OPP to the monsoon variations, upwelling and other oceanographic environmental factors in the SCS. In winter, strong mixing by strong winds, the cyclonic circulation and southwestward currents during the winter monsoon all have impacts on the distribution of phytoplankton biomass. The upwelling off the northwest coast of Luzon supports nutrients and phytoplankton biomass moving up from deep sea waters. The would-be downwind coastal jet in the surface layer is offset by the winter circulation gyre; the subsurface currents are stronger than the surface currents. It indicates that the region off the northwest coast of Luzon with high phytoplankton biomass is joined with the coastal sea areas with higher OPP, a southwestward spreading shape, to extend over the northern SCS, even into the entire SCS. In summer, when a large anticyclonic circulation appears during the southwesterly monsoon, the OPP spread from coastal sea areas to the open sea, in addition of the water with lots of nutrients discharged into the sea from coast and river mouth. At this time, southwesterly monsoon winds parallel the coastline, offshore Ekman transport induces a strong upwelling that supporting phytoplankton growth in a jet shape off the east coast of Vietnam in the western SCS. There also suggest being a strong offshore-directed current in the western SCS during the southwesterly monsoon. This upwelling delivered nutrients from bottom waters to the surface and from coastal waters to the open sea, inducing an increase in OPP in this sea area. But in other sea areas, such as off the northwest coast of Luzon and the gulf of Tonkin, the summer winds are not stronger than the winter winds, and the SST is higher, so this limited the vertical mixing and the increase of phytoplankton biomass. Onset of the summer monsoon in the SCS, phytoplankton biomass reached the lowest at the time of monsoon winds direction changing, and then it rose up. The vertical mixing and horizontal diffusion of nutrient transport was weak in the sea water, which had influence on the phytoplankton biomass increase, for the oceanographic and atmospheric factors were changing obvious onset of the summer monsoon in the SCS.The annual variations of primary productivity were not obvious, just a trend of little rising. Especially in 1998, the OPP was lower than other years for El Nino. In this year, the water temperature was abnormal high and the summer monsoon winds were weak, which had influenced on the development of circulation and upwelling. The phytoplankton biomass reduced and the OPP was not accumulated adequately, for the vertical mixing and horizontal diffusion of nutrient transport was not enough to develop.
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