Geochemical Characteristics Of Nutrients And Environmental Assessment In Xiangshan Bay

Due to excessive development of marine breeding and exploitation of marine resources in the past years, natural balance of many crude marine aquaculture bases has been severely destroyed. As a result, the eutrophication degree and the frequency of red tide occurring of the water body increased. It is a challenging task to ensure the balance between environmental pollution and human demand for marine products, and ensure the sustainable development of marine breeding industry. As the most important domestic marine aquaculture base, Xiangshan Bay has also suffered great losses from red tide. This paper presents geochemical data of nutrients of Xiangshan Bay, followed by the discussion of the health level of the ecological environment, the evaluation of the impact of manmade effects on the regional environment, and the estimation of the cultivation capacities of the region, in the hope of providing scientific basis for the sustainable development of marine aquaculture and environmental management in this area.In this paper, the concentrations of different forms of nutritive elements in water and surficial sediments in Xiangshan Bay from May, 2005 to April, 2006, are analyzed. The temporal and spatial variation characteristics of nutritive elements of different forms and trophic structure have been discussed. The influential factors and controlling rules of the variations have been studied. The health level of the ecological environment of Xiangshan Bay was evaluated by back-propagation artificial neural network. Results are as follows.Concentrations of nitrogen and phosphorus in water body are high all year, with an increasing trend on the year by year basis. On the month scale, nutrients of different forms show high values in spring, followed by autumn and summer, with the lowest contents in winter. In the aspect of horizontal distribution, the overall variation of nutrients of different forms showed a decreasing trend from the inner bay to the outer bay. For nutrients'concentration in surficial sediments, variation showed a reverse rule compared with that of water body in time axis, i.e., contents are higher in autumn than that in spring. In the aspect of spatial distribution, variation patterns are similar to that of water body, with higher contents in two tributary bays.For Xiangshan Bay, main sources of nutrients are continental runoff input and the discharge of mariculture waste water, followed by marine life and atmospheric sedimentation. Influential factors controlling the temporal and spatial variation patterns of nutrients in water body are varied. For nitrates, dominant factors include plankton, water temperature, pH and dissolved oxygen (DO). Plankton and pH controlled the distribution of phosphates. For surficial sediments, the influencing factors of nutrients are different between spring and autumn. Marine aquaculture and sediments property controlled the spatial distribution of nutrients in spring, while marine aquaculture and physico-chemical factors of the overlying water are the main factors in autumn.The cyclic processes of nutrients in Xiangshan Bay are as follows: from spring to autumn, the surficial sediments is important sink of nutrients, alleviating water eutrophication effectively; from autumn to the following spring, the surficial sediments, on the other hand, acts as source of nutrition, keeping the nutrients of the water body at a high level.Based on National Sea Water Quality Standard, and the maximum and minimum value of available investigation data, relative standard of sea water has been proposed. Based on this relative standard, the health standard of the ecological environment of the Xiangshan Bay is constituted. Appraisal of the regional ecological environment with BP neural networks showed that, the ecological health level of Xiangshan Bay is really awful. Government departments concerned should take actions immediately to cut down the pollution of the ocean ecosystem.