Nutrient Limitation Of Eutrophication In Guangzhou Sea Area

Guangzhou Sea area belonging to the junction area between fresh and salt water locates in the Pearl River Estuary. Kumen, Jiaomen, Hongqimen, Hengmen, which are four of the the 8 biggest river channels locate in the Pearl River. Guangzhou sea area covers 4600km², north to Huangbu port, south to the Heng Channels, and East to Guanghzou Sea-route. The current flows into the Guangzhou Sea area directly through the four river channels. Its population density, properous social and its economic development make it one of the most influenced sea area by human activity.In this paper based on the grade of seawater quality standard in China (GB 3097-1997), the eutrophication in Guangzhou sea area was assessed with a model of assessing eutrophication for coastal waters, which referred to nutrient-limited characters through assessing the contents of dissolved nitrogen and phosphate and N/P. There also showed correlation between Skeletonema costatum to phosphate and dissolved nitrogen.Water samples were collected from thirteen stations from from May to December of 2003 in Guangzhou sea area. Samples for phytoplankton were fixed with acid lugol solution and analyzed with the microscope in laboratory. Samples for nutrients were filtered with0.45um Millipore filter, then transported in ice to the laboratory within 2hr. The nutrients were analyzed by the UV3010 PC.The annual average of phosphate were 0.052 mg·L^-1, there was much higher. The contents of phosphate were 1.156 times the forth grade of seawater quality standard in China. The contents of phosphate decreased gradually from inner river to outer river, descent from nearshore to offshore. The contents of phosphate were higher in benthal than those in surface water.The annual average of dissolved nitrogen were 1.143 mg·L^-1, there was much higher. The contents of dissolved nitrogen were 2.286 times the forth grade of seawater quality standard in China. NO₃^--N was the main form of dissolved nitrogen in Guangzhou sea area. The contents of dissolved nitrogen were higher in surface water than those in benthal.The results showed that there were mostly stations at the stage of phosphorus-limited potential eutrophication or slightly-phosphorus-limited potential eutrophication in Guangzhou sea area. The concentrations of phosphate and DIN were increased year after year in recent 20 years in Guangzhou sea area.As it was at the stage of phosphorus-limited potential eutrophication in Guangzhou sea area, so the density of Skeletonema costatum was low. During the period studied, Skeletonema costatum was dominant throughout the in Guangzhou sea area. The density of Skeletonema costatum increased from the inner river to the outer river. The annual mean density of Skeletonema costatum were 2.9×10⁵ cells/l. One peak was observed in a typical annual dynamics monitoring. The density of Skeletonema costatum was higher in spring and winter, however lower in summer and autumn. The results showed that the density of Skeletonema costatum was negative correlation with phosphate(R=- 0.42).