A Modeling Study Of The Phytoplankton Dynamics Off The Changjiang Estuary

Estuary is always the high productivity of the ocean area, they often are an important area of human activity and an important fishing ground lies. The Changjiang (Yangtze River) is China's longest, and its huge runoff with rich nurients make the Changjiang Estuary to be productive waters. There are famous fishing grounds lies in the Changjiang Estuary in history. The rapid economic development and urbanization process of Changjiang Valley accelerated industrial and agricultural waste water and domestic sewage discharge rising waters of the Changjiang Estuary eutrophication, leading to frequent red tide. Damage to the environment coupled with overfishing and other factors, situation of ecological imbalance and individual smaller and lower value of marine economic organisms is serious off the Changjiang Estuary. Affected by the complex bottom topography (such as submarine valley), huge runoff, monsoon and shelf circulation, dynamic process off the Changjiang River is very complicated. Though, phytoplankton dynamics Study off the Changjiang Estuary has important scientific and practical significance to ecological restoration, environmental protection, red tide prevention, sustainable use of fishery resources.Mainly through field observations, data analysis and numerical model simulation, this paper studied the phytoplankton ecosystem characteristics off the Changjiang Estuary, focusing on the analysis of the major limiting factor and the key physical processes on phytoplankton growth.Using data of phytoplankton species identification and cell abundance during the large area survey in July 2006, the cell volume based conversion biomass (chlorophyll-a concentration) was calculated, and were given diatoms and dinoflagellates biomass respectively. Estimation of phytoplankton cell volume based conversion biomass is a feasible method to objectively reflect the phytoplankton biomass, contributing to the species level in the conduct of marine phytoplankton dynamic numerical model study. Phytoplankton biomass off the Changjiang Estuary waters in summer was the highest in the Changjiang Dilute Water (CDW) area, medium off Jiangsu Coast (JSC), and in areas near the river mouth and the Taiwan Warm Current (TWC) were the lowest. Temperature, salinity, water stability and nutrients that affect the distribution of phytoplankton off the Changjiang Estuary were the most important environmental factors.A multidisciplinary observations in the end of August 2009 to early September off the Changjiang Estuary also showed that chlorophyll-a distribution can be divided into four zones:the CDW expansion area, featuring high chlorophyll-a in the surface water on the offshore side of turbidity front, and a hypoxic zone where DO was 1.02-2.00 mg/L underneath the high chlorophyll-a zone; the TWC zone which had strong stratification,with low chlorophyll-a in the surface water and relatively high value in the subsurface water; off the well mixed JSC (Jiangsu Coast) zone, featuring medium chlorophyll-a of 2-6 mg/m3 in the areas of shallower than 20 m; and the stratified strongly Near-shore Diluted Water(NDW) zone, with relatively low chlorophyll-a. The high chlorophyll-a zone west of 123.0°E located in the surface water above the halocline, the relatively high chlorophyll-a zone east to 123.0°E located in the subsurface water above the thermocline. Data analysis show that there is a strong linear relationship between the degree of water stratification (PEAP) and the oxygen level of water. According to the survey data, the dynamics pattern of chlorophyll-a distribution in later summer was obtained.Based on the analysis of observational and historical data, a N2P2ZD style numerical phytoplankton dynamics model coupled the ocean marine model ECOM-si with additional modules of solar radiation, sea surface heat flux and sediment was established. In the model, phytoplankton was divided into two groups, diatoms and dinoflagellatesc. Through sensitivity test, a long sequence sites and large area survey data validation, the model was validated stability and has a certain precision, can be applied to ecological dynamics study of phytoplankton off the Changjiang Estuary.We take the 2006 and 2009 field survey area as a reference and set area 29.5°N-32.5°N,122.25°E-124.0°E as the target area for the phytoplankton dynamics model study. Phytoplankton growth in the target area is mainly controlled by nutrients, annual changes in water temperature and light conditions, and endured the grazing pressure of zooplankton which is on the upper food chain. Diatoms have two obvious algal blooms in spring and late summer, dinoflagellates have bloom only in spring. Water temperature was the key factor of the first bloom occurred time. After July, by the impact of the high water temperature dinoflagellate biomass was at a low level, the basis biomass before the red tide occurrence can not be achieved. In May to September, diatoms maintained a high biomass, which means these months are the time window that red tide of diatoms may occur off the Changjiang Estuary. Growth of diatoms affected more by light and nutrient limitation, and biomass of dinoflagellates affected more by water temperature.DIN concentration in target area affected significantly by the high concentration of DIN during the Changjiang runoff discharged into the estuary while the PO4-P concentration in the Changjiang runoff is relatively low, so PO4-P input by the Changjiang runoff is not the most important source of PO4-P in the target area. The annual average concentration of PO4-P in the target area is more affected by the open boundaries. Nutrient supply from the open boundaries has a great impact on annual average biomass of phytoplankton in the target area, especially the PO4-P input from the Taiwan Strait boundary. Nutrients input of Taiwan Strait boundary is more important than the boundary east of Taiwan, and the effect on diatoms is more than dinoflagellates.