The Gulf of Tonkin is a semi-closed gulf northwest of the South China Sea, experiencing reversal seasonal monsoon. Previous studies of
water conditions have been conducted in the western waters of the gulf, but very few studies of the Chlorophyll-a (Chl-a) distribution have
been carried out for the entire gulf. The present study investigates seasonal and spatial distributions of Chl-a and water conditions in the Gulf
of Tonkin by analyzing Sea-viewing Wide Field-of-View Scanner (SeaWiFS) derived Chlorophyll-a (Chl-a), in situ measurements, sea
surface temperatures (SST), and other oceanographic data obtained in 1999 and 2000. The results show seasonality of Chl-a and SST
variations in the Gulf of Tonkin, and reveal phytoplankton blooming events in the center part of the gulf during the northeast monsoon
season. In summer, Chl-a concentrations were relatively low (< 0.3 mg m 3
) and distributed uniformly throughout most of the area, with a
belt of higher Chl-a concentrations along the coast, particularly the coast of Qiongzhou Peninsula; in winter, Chl-a concentration increased
(0.5 mg m 3
) in the entire gulf, and phytoplankton blooms offshore-ward from the northeast coast to the center of the gulf, while Chl-a
concentrations reached high levels (0.8–1 mg m 3
) in the center of the blooms. One peak of Chl-a concentrations was observed during the
northeast monsoon season in the year. SST were high (27–29 jC) and distributed uniformly in summer, but lower with a large gradient from
northeast (17 jC) to southwest (25 jC) in winter, while strong northeast winds (8–10 m/s) were parallel to the east coast of the gulf.
Comparison of Chl-a values shows that SeaWiFS derived Chl-a concentrations match well with in situ measurements in most parts of the
gulf in May 1999, but SeaWiFS derived Chl-a are higher than in situ data in river mouth waters. The seasonal variation of Chl-a
concentrations and SST distribution were associated with the seasonally reversing monsoon; the winter phytoplankton blooms were related to
vertical mixing and upwelling nutrients drawn by the northeast wind.
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