| As a major producer of marine ecosystem, the phytoplankton maintain the nutrition and energy flow in the sea area, and the amount of biomass largely determines the sea fisheries performance and aquatic ecological environment. In recent years, China’s economy, especially the rapid development of coastal economy, to bring a heavy burden on the aquatic ecological environment. Such problems began to attention of scientists. And the front zone off the Changjiang Estuary as an important part of China’s Delta area, serious eutrophication, frequent occurrence of red tide, the aquatic ecological environment is threatened. Affected by the complex bottom topography, huge runoff, monsoon and shelf circulation, dynamic process is very complicated and the seasonal variation is obvious of front zone off the Changjiang Estuary. The Changjiang River and the seasonal flow, wind and other key physical processes affect the sea temperature, salinity, nutrients distribution and current Characteristics, thereby further affects the area of phytoplankton biomass and primary productivity. Though, phytoplankton dynamics Study of front zone off the Changjiang Estuary has important scientific and practical significance to the status of eutrophication in the sea area and energy flow, and the external environment and physical processes of the formation mechanism of red tide, sea aquatic ecological rehabilitation.Mainly through historical data analysis, field observations and numerical model simulation, this paper studied the changes and its affecting factors of phytoplankton biomass of front zone off the Changjiang Estuary.According to the historical data of the site survey in 2005 and 2006, phytoplankton growth was mainly controlled by temperature, light and nutrient concentration of front zone off the Changjiang Estuary. The biomass of Dinoflagellates and diatoms sharply increased at the beginning of April. Diatoms had two obvious biomass blooms in April and June, dinoflagellates had bloom only in April. Due to the characteristics of resistant low salt and not resistant high temperature of dinoflagellates, the biomass of dinoflagellates was high from April to June, but the biomass of diatom remained very high from April to September. The growth of dinoflagellates was more limited by the temperature, but the biomass of diatoms was more limited by the light and nutrient. Analysis on the section observation data in 2010, 2011, and 2012, we found that there exist halocline, thermocline and pycnocline in front zone off the Changjiang Estuary in summer. The depth of thermocline and DO vertical gradient at the thermocline was negatively correlated, and between with the vertically minimum DO concentrations was positively correlated. The salinity was lower, but the concentration of dissolved oxygen and chlorophyll-a was higher.Through the field observation dates in March 2016, we determined the biggest area of the change of salinity gradient in the survey period of front zone off the Changjiang Estuary. The location was roughly 30°41′N, 122°41′E and the salinity had a sharp jump between 26-28.8. The salinity front moving with the tide change, the front with the tide to advance and retreat, along the path of the salinity and suspended sediment was changing. The front was a convergence zone, the Changjiang River carries huge nutrient was extended to the sea and gathered. Both sides of the front of the nutrients and phytoplankton biomass had a greatly varies. Through the investigation and study, to explain the change characteristics of nutrient salt and phytoplankton biomass in the transition zone provide support.Based on the analysis of the field observation data and historical data, considering the runoff, tide, shelf circulation and monsoon and other key physical processes, a N2P2 ZD style numerical phytoplankton dynamics model coupled the 3-D high resolution ocean numerical model ECOM-si with additional modules solar radiation, sea surface heat flux and sediment calculation module was established. The ecosystem dynamics of front zone off the Changjiang Estuary was simulated, and to explored the influence of the Three Gorges Project on distribution of nutrients and phytoplankton biomass of front zone off the Changjiang Estuary. Through test, the model was validated stability and has a certain precision, can be applied to ecological dynamics study of phytoplankton of front zone off the Changjiang Estuary.The simulation results show that the difference of salinity were within 2 of front zone off the Changjiang Estuary at the runoff regulation maximum in May and October of Three Gorges Project(TGP). Before and after TGP’s regulation, the distribution of difference of nutrient concentration was a tongue shaped, high concentration was more in about 122°E area and the East has a rapidly decreased. There was a high concentration value in Hangzhou Bay. The difference of nitrogen nutrient concentration was significantly higher than the phosphorus nutrient. The absolute value of nutrients change was decreased from west to east, from the near estuary to the far estuary, and the peak value of nutrients change was delayed from west to east. The low flow year was largest influenced by runoff regulation, and the nutrients increment during low flow year was significantly greater than during high, mean years caused by runoff regulation in all areas, while the nutrients decrement was just the opposite after October in the transition zone and the far estuary. The spatial difference of the total phytoplankton biomass concentration was totally different from the nutrient salt, which was mainly distributed in the shape of spot, and the high value area was mainly in the Changjiang Estuary.The annual variation situation of phytoplankton biomass was relatively consist with the nutrients, with a greatest effect during the low flow year and the peak value was delayed from west to east. In addition, due to the competition between diatoms and dinoflagellates, the increased amplitude of diatom biomass in flow increased season was higher than the decreased amplitude of diatom biomass in flow reduced season, while dinoflagellates was just the opposite. On the whole, the runoff regulation of Three Gorges Project was beneficial to increase the annual mean phytoplankton biomass, but the annual mean increase amplitude was smaller, only about 0.5-1%. |
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