The Environmental Capacity Study Of Nitrogen And Phosphorus Nutrients In Laizhou Bay Based On A Three Dimensional Ecosystem Dynamic Numerical Model

In recent years, a large amount of nutrients is discharged into the Laizhou Bay byrivers with the development of the country’s economy. As it strongly enhances thegrowth of the phytoplanktons, the ecosystem of the Laizhou Bay could be destroyedby the eutrophication status and its developing trend. According to our country’scondition, cutting off all the discharges of the nutrients into the sea is unpractical. Inorder to reduce the damage to the marine environment, the nutrients discharges shouldbe optimized with the reasonable analysis and assessment of the nutrientsEnvironmental Capacity (EC) in the sea. The innovations in the thesis would be theestablishment of the EC calculational method of the nonconservative substance andthe successful application of the newly developed method on the bay.The nutrients EC of the Laizhou Bay should be calculated based on an ecosystemdynamical model. And the PIC (Particle-In-Cell) method is used in the three-dimensional ecosystem model, which is based on the N-P-Z-D (Nutrients-Phytoplankton-Zooplankton-Detritus) module. Compared with the observed data, theannual cycle of the nutrients and phytoplankton (PPT) in the bay is successfullyreproduced by the new model. In addition, the spatial and seasonal distributions of thenutrients and PPT in the bay are revealed using the model results. The studies of thenutrients limitation in2000s and the effects of the Yellow River and the XiaoqingheRiver on the ecosystem in the bay are also discussed.Recently, the most popular EC calculation method is the discharge optimizationmethod. The ascertained response factor field of the research area should be providedin the optimization method. As the specific seasonal variability of the nutrients existsin the bay, the corresponding nutrients response factor field changes over time. As a result, the nutrients EC couldn’t be directly calculated by the discharge optimizationmethod.To obtain the ascertained nutrients response factor field, the monthly averagednutrients concentration data and the monthly maximum nutrients concentration dataare adopted respectively, which both meet the applicable condition of the optimizationmethod. Both of the data are used to obtain the nutrients response factor field, andthen calculate the nutrients EC. The results show that the linear programming methodcould be used with the monthly averaged concentration field, while the nonlinearprogramming method would be used with the monthly maximum concentration field.In our model, there is about half a month’s time that the daily concentrationexceeds the monthly averaged concentration. Under this condition, even though thenutrients discharge is arranged according to the nutrients EC, which is obtained fromthe monthly mean concentration field, the ecosystem of the bay can still be destroyed.Therefore the monthly maximum concentration field is used to calculate the respondfactor field. In other words, the nonlinear programming method should be used tocalculate the EC of the nonconservative substance, including the nutrients.There are interactional effects between the DIN and DIP. If the DIN (or DIP) inthe bay is in different concentration conditions, the corresponding DIP (or DIN) ECmay be different, which is a complex process. The simplified means are adopted, asthe point of this thesis is to discuss how to replace the linear programming with thenonlinear programming. So in the condition of keeping the DIP (or DIN) dischargeunchanged, the corresponding DIN (or DIP) EC in the bay is calculated. Based on themarine functional division in the Bohai Sea, the water quality near all the pointsources should be Grade II. The EC and SEC (Surplus EC) of the nutrients in the bayare calculated using the nonlinear programming. In the condition of keeping the DIPdischarge unchanged, the corresponding DIN EC in the bay is21843t/a, while theSEC is-8264t/a, which means the DIN discharge should be restricted. In the conditionof keeping the DIN discharge unchanged, the corresponding DIP EC in the bay is1794t/a, and the SEC is1485t/a, which means more DIP could be discharged into the bay.In the end, the effects of the different hydrodynamic conditions on the ecosystemand the nutrients EC of the Laizhou Bay are also discussed. Strong hydrodynamiccondition of the Yellow River can reduce the eutrophication of the bay, when thefluxes of the discharged nutrients from the Yellow River keep unchanged. Meanwhile,weak hydrodynamic condition of the Yellow River can also reduce the eutrophicationof the bay, while the nutrients concentrations of the discharged water amounts fromthe Yellow River keep unchanged. It means that it is possible to improve theenvironmental quality of the Laizhou Bay by controlling the hydrodynamic conditionand the nutrients discharge of the Yellow River. In addition, the DIN concentration ofthe bay would be undervalued with the barotropic model, in which only the tides areconsidered. While the DIN concentration of the bay would be overestimated with thebarotropic model. Also, the DIN concentration of the bay would decrease if no wind isincluded in the model.