The Distribution Feature Of Nutrients And The Study Of Their Influence On Ecosystem In The Northern Beibu Gulf

Beibu Gulf is a natural semi-enclosed bay of China which located in northwestern of the South China Sea. The Gulf which is in the Beibu Gulf economic cooperation zone is being significantly affected by urbanization, industrialization and agricultural activities in recent years and suffering increasing pressure on the water environment. In order to develop a more reasonable and scientific total amount of nutrient solution discharging into the sea control plan and marine environment quality evaluation system, a systematic study of the distribution, circulation and structural characteristics of nutrients in the Beibu Gulf are required as well as an objective assessment of the eutrophication status and the pressure of the Beibu Gulf.Two voyages were held in April and August,2011, and hydrological, chemical and biological factors were obtained in the northern Beibu Gulf. The findings suggest that nitrogen nutrients have a higher concentration in the entrance of Qiongzhou Strait and the waters near to Fangcheng Harbor, while in spring even exceeded the standard value of Chinese standard seawater quality in northwestern of Hainan Island. In spring, the average concentration of dissolved inorganic nitrogen (DIN), total dissolved nitrogen (DN) and total nitrogen (TN) is7.93μmol/L,21.9μmol/L and28.9μmol/L, respectively, and2.54μmol/L,11.5μmol/L and16.8μmol/L respectively in summer. The average concentration of reactive phosphate (SRP) in the spring and summer both are0.06μmol/L, which below the threshold of phytoplankton growth. Furthermore, the nutrient ratios deviates the Redfield value, so that phosphorus becomes the absolute and relative nutrient limiting factor in the studied waters. Concentration of dissolved silicate (DSi) is2.12μmol/L in spring and up to5.18μmol/L in summer since the runoff input supplement. Nutrient especially nitrogen concentration in coastal waters of Guangxi is keeping an upward trend in the period from1990to2010.In spring, the main form of DIN is nitrates (NO3-N), which occupied a proportion of55.0%while in summer is ammonium (NH4-N), accounting for49.8%; The main forms of DN and total dissolved phosphorus (DP) are dissolved organics, accounting for63.8%-77.8%and77.0%-88.5%, respectivel; And the main form of TN is dissolved species, while the total phosphorus (TP) in spring is mainly particulates and dissolved form in summer. Hence the dissolved organic nutrients play an important role in the ecosystem of the studied area. As the result of the analysis of nitrogen and phosphorus cycling, the biochemical purification capacity of the water is stronger in spring than summer.Meanwhile, the study found the place that under maximal nutrients pressure is Lianzhou Bay (Nanliu River estuary), then is the entrance of Qiongzhou Strait and the water near Fangcheng Harbor when combined single indicators and comprehensive index method to assessment the nutrients pressure on each region; When assessed by potential eutrophication level model, for the gulf which total dissolved content is very high but the inorganic content is below the threshold for phytoplankton growth, the evaluation would be more reasonable to use the total dissolved nutrients rather than dissolved inorgnic, and the results of this study shows the studied waters is in a moderated nutrition state, but phosphorus limitation was existed in spring; At the same time, this paper applied pressure-state-response model evaluated the nutrients pressure strength of the entire sea half-quantitatively, with a result is moderate.Employing the principal component analysis method to analyze the surveyed data, the results show that the key factors affecting the ecological environment of the northern Beibu Gulf are dissolved oxygen (DO) and DN, and nitrogen nutrient revealed the maximum pressure; Critical areas are located in the northwest of Hainan Island (the entrance of the Qiongzhou Strait), southeast waters of Weizhou Island and the waters near Fangcheng Harbor take the second place; Spring is the key season, which need control the input of nitrogen, and followed by summer that need control the concentration of phosphorus.