South China Sea locates at the Western Pacific Oceanic Basin and is bounded by China to the north, by the Philippines to the east, by Vietnam to the west and, by Malaysia to the south. The meteorological conditions of the area are dominated by the temperature variations between the periods of Asian Winter Monsoon and East Indian Ocean Summer Monsoon. Further, substantial amount of river water entering the South China Sea (through the Pearl River (Zhujiang) and the general surface runoffs) generates significant impacts on the oceanographic conditions.

In South China Sea, there is a broad continental shelf extending more than 100 km south of the coast of China (Fig 1) at 18°N to 23°N. The shelf water in the northern South China Sea comprises of a mixture of water masses and is dominant by the runoff of the Pearl River. Moreover, the Subtropical Lower Water and Northern Intermediate Water from the Pacific Ocean through the Luzon Strait, as well as the warm South China Sea Current (Guan, 1981; Guo, et al, 1988), also affect significantly the oceanographic situations there.
 

I.  Meteorology and Hydrography of the South China Sea

As noted, seasonal variability in this region is governed by the winter and summer monsoons. During the winter months, ie from November to February, the prevailing winds are from the NE side. They bring cold, dry, continental air from the Northern Asia to the South China coast. During the summer months, ie June to August, the prevailing winds are from the SE and SW sides of the sea. They bring moisty oceanic air from the western equatorial Pacific and the eastern Indian Oceans to the region. Hence, the properties of the shelf waters are influenced by seasonal variations at the NE and SW monsoon periods. The seasonal variation in oxygen content is particularly noticeable for the oceanographic stations in the shelf area (Chan, 1970). Through the analysis of oxygen saturation in surface waters, the seasonal variation of D.O. in the shelf water was found extremely apparent (Wang and Kester, 1988).

The impacts of monsoon-related seasonal variations are summarized in Fig 2. The first arrival time for the two monsoons varies from year to year. However, the summer monsoon is well-established during June, July and August. The winter NE monsoon results in stronger winds than the summer monsoon, as indicated by the number of days per month of which the average wind speed is greater than Force 5 (Fig 2b). The prevailing winds have a direct effect on the surface water currents of the shelf region. The current speeds are about 0.6 knots to the SW during the winter monsoon. They change to 0.2 to 0.4 knots to the NE during the summer monsoon (Fig 2c).

The reversal of surface currents also affects the water masses seasonally. During the winter, the SW current carries a relatively low salinity water mass from the East China Sea through the Taiwan Straits to the South China Sea. In contrast, the winter NE winds carries parts of the Kuroshio Current into the South China Sea through the Luzon Strits (Williamson, 1970). The Kuroshio water mass is warm (26° to 29°C) and of high salinity (34.4 to 35.0 ppt). In summer, the SW monsoon results in the large increase in rainfall and river discharges (Fig 2d). it causes the reduction of salinity in the coastal waters and the production of seasonal pycnoclines. Annual data from a mid-shelf station off Hong Kong (21°.5'N, 115°E) revealed that from December to March, the water column was vertically mixed at the depth of 88 m. A well-developed thermocline was however existed at the depth of 15-30m from May to September.

The SW winds blowing along the SW-to-NE part of continental shelf may induce upwelling during the summer, bring nutrients to the eutrotic zone on the outer portion of the shelf and, enhance primary production of the waters (Wang and Kester, 1988). The seasonal stratification also stimulate the seasonal changes in primary production and nutrient cycling. The phosphate to phytoplankton ratio (log (phyt.) / PO₄-P) is in the following descending order : spring > autumn> winter > summer (Wang and Lin, 1988).
 

II.  Hydrography of Pearl River Estuary

The oceanographic conditions of the South China Sea are greatly influenced by the presence of freshwater inputs from the Pearl River. The Pearl River Estuary (also known as Lindingyang) is located at 113°33'E - 114°09'E and 22°12'N -  22°45'N (Fig 1). Fuman, which is at the top of the estuary, is 4 km in width. The maximum width of the Pearl River Estuary is 65 km and the distance from Fuman to the river mouth is 50 km. The surface area of the Pearl River Estuary totals 2,110 km². The average depth of the river estuary is about 9 m (Wang and Peng, 1992).

A major portion of the river water enters the South China Sea via the eastern side of the estuary. In contrast, the river flow is relatively slow at the eastern band and so sedimentation near Macau and Zhuhai is extremely obvious. Generally, the freshwater mass from Pearl River meets the oceanic currents from South China Sea at the central harbour of Hong Kong. However, the exact interface of freshwater and oceanic current still depends on the seasonal variations in rainfall and freshwater discharge from the Pearl River. The flow speeds and flow directions of the water mass near Hong Kong is summarised in Fig 3 and Fig 4.

The temperature variations during wet season at the cross section between Hong Kong and Zhuhai are shown in Fig 5. As revealed, the spatial temperature profile is mainly affected by the spatial distributions of freshwater and intruded seawater in the river mouth. However, the seasonal temperature profile is generally influenced by seasonal variations in atmospheric temperature (Fig 6). The highest water temperature occurs in the months of summer of each year.

The salinity profile of the Pearl River Estuary is influenced by the intrusion of seawater from South China Sea. While most of the freshwater appears at the surface layers because of salinity gradient, it is noted that the salinity in the eastern band of the estuary is greater than that in the western band (Fig 7 and Fig 8).
 

III.  Nutrients Availability in the Pearl River Estuary

A better understanding of the nutrient budget in the Pearl River Estuary may help to elucidate the red tide formation mechanisms in the South China Sea. While the mean flow of the Pearl River from Fuman to Hanman is 5,663 m/sec, the waters of the River contain large amounts of suspended solids and organic particles. The amount of suspended matters in the estuary averaged 19.2 mg/L. The suspended matters discharged into the South China Sea could be as large as 3.43 x 10 t/yr, with inputs from the rainy months total 1.65 x 10 t (Wang and Peng, 1992). About 79% of suspended matter is deposited at Lndiyang and only 20.8% of suspended matter is dispersed into the South China Sea. Particulate organic carbon (POC) and nitrogen (PON) ranged from 3.5 to 5.76 x 10 t/yr and 0.31 to 5.10 x 10 t/yr respectively. The effluxes of POC and PON to the sea were 1.93 x 10 t/yr and 9.95 x 10 t/yr respectively as recorded by researchers at the early nineties (Wang and Peng, 1992).

Dissolved phosphate in the Pearl River Estuary is generally high in autumn and winter, but low in spring and summer (Wang and Tan, 1989). In the surface water, variations in dissolved phosphate provide a buffer-effect (below 1 umol / L) to salinity. The concentration of PO₄-P is relatively less in the bottom waters. During the flood periods (June and August), the depth-average concentration of PO₄-P was particularly high. This suggests that although some phosphates might have been consumed by phytoplankton, discharges from the upper river, which is the prime source of total phosphate for the estuary, might compensate the biological consumption. The concentration of dissolved silicate was also low in winter and spring, high in summer and autumn. It is noteworthy that the variation is similar to that of phytoplankton biomass.

With contrast, the concentrations of silicate in the surface waters are higher than those in bottom. Their concentrations in the flood periods are greater than those in the dry season. Dissolved inorganic nitrogen (DIN) is low in winter and spring but high in summer and autumn. Their variations areclosely related to those of phytoplankton productivity. While NH₄-N might be chemically converted to NO₃-N and NO₂-N, there was a high concentration of nitrate in the surface water but a lower one in the bottom waters. The patterns of nitrite and ammonia were different from that of nitrate, which has a higher concentrations in the bottom water. Nitrate accounted for over 93% of the total dissolved inorganic nitrogen. It reveals that inorganic nitrogen in the surface and bottom waters had not reached the thermodynamically equilibrium status.
 

VI.  Eutrophication and Red Tide

Hence, the Pearl River Estuary is strongly influenced by the complicated processes of physical, chemical and biological interactions (Wang and Tan, 1989). Near the river mouth, the influence from phosphate on phyplankton is clearly less because of the buffering effects. Because of the hydrodynamic effects, silicate concentrations are constrained by physical processes in the surface waters. The biological and chemical effects of silicates, however, are more obvious in the bottom layer. DIN is suffered by a greater influence from chemical process because of mixture between salt - fresh water in the estuary. The N, P, and Si carried by the river runoff can have greater impacts on the biogeochemical balance of nutrients cycle only at the river mouth.

In the waters of the Pearl River Estuary, the atomic ratio of N/P was 15:1, which is close to the N/P ratio for the body of diatoms and some dinoflagellates. Hence, it is not surprised to see frequent eutrophication and red tide in the Pearl River Estuary (Hodgkiss and Ho, 1997). The ratios of C:N:P in the waters are 79:15:1, indicating that the water has been enriched with organic matters of high N and P contents (Wang and Peng, 1992). Results of a Nutrient Quality Index (NQI) analysis, by the use of COD, DIN, PO₄-P and Ch1.a as variables, showed that water quality of the Pearl River Estuary could be classified into three levels : the top and the west of the estuary are at a 'eutrophic level'; the rest of the estuary is either oligotrophic or mesotrophic (Peng and Wang, 1991). Increased discharges of domestic sewage and industrial wastes from several cities in the watershed during the eighties and nineties have resulted in increased red tide, which is closely related to increase in organic matter particularly the N and P inputs (Wang and Ho, 1997).