Study On Eutrophication Of Tianjin Coastal Water Using Soft Computing Methods

Eutrophication in coastal waters is an important issue to many coastal regions. Accurate assessment and prediction of water eutrophication and determination of the main impact factors are helpful for improving coastal ecological environment and coastal zone management. Due to the rapid increasing economy of Tianjin, a metropolis in the Bohai Economic Circle, the ecological environment of Bohai bay is facing increasing pressure and water eutrophication has become an important issue. In this paper the methods to assess and predict water eutrophication were studied and applied to Tianjin coastal water.The residual tidal current and the migration path of the pollutants discharged from nine outfalls along Tianjin coastline were simulated and analyzed using a well calibrated depth-integrated hydrodynamic and water quality model. The results indicates that the general structure of the residual current in four seasons are basically the same. There is a pair of circulation in Tianjin coastal area, with the circulation to the south of Dagu being clockwise and that to the north is counterclockwise. The migration pathes of the pollutants are obviously affected by the circulations, and show significant trends of longshore transport.The synthetical pollution index method and eutrophication index method were applied to assess the eutrophication of Tianjin coastal water. It was found that DIN, PO_<sup>3--P, BOD as well as Pb are the major pollutants, and the degree of eutrophication varies significantly with the season. The major impact factors were extracted by principal components analysis. Then, the spatial distributions of DIN, PO_<sup>3--P and COD were analyzed using special interpolation method. It was found that the concentration of eutrophication impact factors in the area close to coastline is higher than that in the out sea, and it is higher in the north than that in the south. In addition, the alternately appeared high and low concentrations of DIN and PO_<sup>3--P implies that the land-source pollutant is the main reason for the pollution of Tianin coastal water.Based on the monitoring data and the existing modeling experience, a structural equation model?SEM? was developed to discover the causality between the eutrophic factors and chlorophyll a. According to the causal relationship between the variables provided by SEM, a eutrophication model was built for the Red Tide-monitoring Area in Tianjin coastal area, and the model performance was tested using the monitoring data. The study demonstrates that in the Tianjin coastal area nutrients have greater influence on the eutrophication than the physical factors. In recent years, inorganic phosphorus has become the restriction factor influencing the growth of phytoplankton in the Tianjin coastal water. However, the effect of silicate on phytoplankton growth has been becoming more and more significant.