The Study Of The Seasonal Occurrence Mechanism Of HABs In The Changjiang Estuary And Its Adjacent Sea

In recent 20 years, with the development of economy and the increasing population around the watershed of the Changjiang, more anthropogenic wastewater rich in nutrients have been discharged into the East China Sea (ECS) through the river mouth. Consequently, eutrophication has become an overwhelming phenomenon in the Changjiang estuary and its adjacent sea, and harmful algal blooms (HABs) are occurring more frequently. To understand the mechanism of HABs in Changjiang estuary and its adjacent sea, a three dimensional ecosystem model of the phytoplankton competition is developed in this thesis. The sensitive model parameters related to the phytoplankton growth process are optimized based on the field culture experiments results. Then, the model is validated by comparing calculation results with the field data. After that, the effects of irradiation, temperature and nutrients on the process of HABs are investigated. A train of study results are obtained which can definitely provide valuable scientific evidence and theoretical foundation for revealing the occurrence mechanism of HABs in Changjiang estuary and its adjacent sea.1 A three dimensional ecosystem model of phytoplankton competition is developed in this paper, which takes account into the main phytoplankton community in the ECS and the main factors affecting the growth of phytoplankton. The calculation results from the proposed model agree well with the field experiment observations, qualitatively and quantitatively.(1) Based on the MASNUM wave-tide-circulation coupled model, a three dimensional ecosystem model of phytoplankton competition is developed in this paper. Phytoplankton is divided into two groups: diatoms and dinoflagellates. The effects of nutrients(dissolved inorganic nitrogen (DIN),PO₄-P,SiO₃-Si), irradiation and temperature on the phytoplankton growth are evaluated. The model parameters are confirmed through three steps. Firstly, sensitive parameters of the model are selected by sensitivity analysis. Secondly, the sensitive parameters selected and related to the growth process are calibrated based on the field culture experiments. Lastly, the sensitive parameters are optimized with the observations of the mesocosm experiments carried out in May 2002.(2) The calculation results show that, compared with field observations and historical data, the model simulates the distribution and seasonal patterns of phytoplankton biomass and nutrients (DIN,PO₄-P,SiO₃-Si) in 2002 very well, qualitatively and quantitatively. Besides, the model can also simulate the seasonal patterns of phytoplankton biomass and nutrients in 2005 very well. These results indicate that the logical structure and parameter values of the proposed model are reasonable.2 With the ecosystem model developed, the controlling factors of seasonal occurrence of HABs are investigated by calculating the variation coefficients of irradiation, temperature and nutrient limitations. The effects of irradiation, temperature, and nutrient input (from land and open sea) on the HABs occurrence in Changjiang estuary and its adjacent sea are also studied with numerical experiments. Through above researches, the control mechanism of irradiation, temperature and nutrients on the occurrence and succession of HABs are basically established.(1) The controlling factors of seasonal occurrence of HABs in Changjiang estuary and its adjacent sea are studied by calculating the variation coefficients of irradiation, temperature and nutrient limitations. The results suggest that temperature is the controlling factor for the total HABs and HABs caused by dinoflagellates. While HABs caused by diatoms are controlled by irradiation, temperature and nutrients together, of which the irradiation, temperature and nutrients have effluences ranging from maximum to minimum. During the period of May and June when HABs occurs, temperature is the controlling factor for dinoflagellates, and phosphorus and irradiation are the controlling factors for diatoms. In July and August, temperature and phosphorus are the controlling factors for diatoms.(2) Several numerical experiments are conducted to investigate the effects of temperature on the occurrence of HABs. The results indicate that the scale of dinoflagellate HABs is the largest when the temperature is 20℃; When the temperature is lower than 20℃, with the temperature increasing, the scale of dinoflagellate HABs increases; When the temperature is higher than 20℃, the scale of dinoflagellate HABs reduces gradually as the temperature increases; The dinoflagellate HABs can't occur when the temperature is higher than 28℃or lower than 12℃. As for the diatoms HABs, temperature does not control its occurrence when the temperature is between 12℃and 24℃; however, when the temperature is higher than 24℃, the scale of the diatom HABs reduces with the increase of temperature, and decreases to be zero when the temperature is higher than 30℃.(3) Several numerical experiments simulating the period are conducted to investigate the effects of irradiance on the occurrence of HABs. The results indicate that the scale of dinoflagellate HABs is the largest when the irradiance is150 W·m-2, while HABs caused by dinoflagellates disappear when the irradiance is lower than 50 W·m-2. As for the diatoms, no HABs occur when the irradiance is lower than 100 W·m-2. However, when the irradiance increases to 150 W·m-2, the diatom HABs occur and the scale of them enlarges with the irradiance increased.(4) Based on the model developed, the effect of the nutrient input from land is investigated through several numerical experiments. The results indicate that the scale of HABs enlarges as the nutrient input flux from land increases. It is possible to prevent the HABs by controlling the land nutrient input. As the input flux of DIN and PO₄-P decreases to 30% of 2002's, diatom HABs do not take place, and the dinoflagellate HABs, with a very small scale, exist for only one day. When the input flux of DIN,PO₄-P decreases to 20% of 2002's, dinoflagellate HABs don't take place. As the DIN/PO₄-P of the nutrient input from land decreases and the SiO₃-Si/DIN increases, the percentage of diatom HABs increases. According to the results above, the increase of input flux of DIN and PO₄-P from land may be responsible for the frequent occurrence of HABs in the ECS, and the increase of DIN/PO₄-P ratio and the decrease of SiO₃-Si/DIN in the Changjiang River may be responsible for the increase of dinoflagellate HABs in the ECS in recent years. (5) Numerical experiments are conducted to investigate the influence of nutrient input from Taiwan Strait, Heichao and Yellow Sea on the occurrence of HABs. The results show that the nutrient input from Taiwan Strait and Yellow Sea have a great impact on HABs in July and August and a less one in May and June. The nutrient input from Yellow Sea has an obvious impact on the HABs both in July and August and in May and June.