A Preliminary Study Of The Yangtze River Terrestrial Input Change And Response Of The Estuarine Ecological Environment

Terrestrial material inputs of the Yangtze River contribute important roles in the Yangtze RiverEstuary and the East China Sea ecosystems. In recent years, however, human activities, in particularthe dam constructions, produced tremendous impacts on the flux of the Yangtze River into theestuary. The flux variations will cause response of the ecological environment in the Yangtze RiverEstuary (YRE), thereby influencing the entire biosphere. In this paper. The integrative surveys wereconducted in the Yangtze River Estuary and its adjacent waters from1998to2011. Based on thesurvey data as well as the Yangtze River flux (runoff, sediment) data at Datong station since1950,the change trends of terrestrial material input and estuarine ecosystem and the their relationshipswere examined.Since1950, the annual runoff of the Yangtze River keeps equilibrium, and unapparent changetrend was found. However, the sediment discharge significantly reduced, which is mainly caused bydam construction in the upper reaches of the Yangtze River basin. The construction of the ThreeGorges Project (TGP) is the main reason for significant sediment discharge reduction in recent years.In addition, the seasonal distribution of the Yangtze River flux is also strongly influenced by waterstorage of the TGP, so the differences between the flood season and dry season shrinked.The runoff would directly affect the distribution of temperature and salinity. In the investigationarea, the relationship between surface salinity and runoff are negative related，and the reduction ofrunoff is the main reason for the increase in salinity. During different years with similar annualrunoff, the salinity contours move towards east in spring, while towards the entrance in fall. In April,compared with the seawater temperature, the temperature of the runoff is relatively lower, so thetemperature and runoff were negatively correlated. In October, the high temperature of the dilutedwater cause the sea temperature rising, and a positive correlation exist between runoff and thetemperature.Due to the increasing disturbance in seawater and the promoting growth of phytoplanktoncaused by runoff, the positive correlation between runoff and the dissolved oxygen content wasdetected in the investigation area. On the contrary, the sediment discharge brings more pollutants andinhibits the growth of phytoplankton, so its correlation with DO is negative. COD is mainly affectedby runoff. In the in shore, the negative correlation exists between COD and runoff, while, an opposite pattern exists on the offshore side.The dam construction has important effects on silicate content. Silicate content reduced quicklyafter the TGP construction. The spatial distribution of silicate and nitrate showed a decreasing trendfrom nearshore to offshore. The high value area of phosphate is not within the estuary, but has a highdegree of consistent with the high values of suspended solids. The behavior of silicates and nitratesin the estuary are conservative, while the phosphate content is also affected by the particulate matter.The correlations between the nutrients and the Yangtze River inputs have the similar distributioncharacteristics. In spring, nutrient is negatively related to runoff in the area with high phytoplanktonamount, while they have positive correlation in the other area. The relationship between nutrients andsediment showed the opposite distribution pattern. In the waters with strong phytoplankton growth,biological uptake has greater impact on the existing nutrient amount than land-based sources of input,while the situation is opposite in other sea areas. In autumn, phytoplankton growth weakened, andthe nutrients has a positive correlation with runoff in most of the waters, while has negativecorrelation with sediment.The Yangtze River flux has a direct impact on the surface suspended particulate matter contentin the YRE. The construction of the TGP has intercepted most of the sediment from the upperreaches of the Yangtze River. During similar annual runoff year, the content of suspended particulatematter after the Three Gorges dam construction expressed substantial reduction. In the same time, thehigh value area move towards southwest，indicating that the sediment from the Hangzhou Baybecame dominant.The high value of chlorophyll a mainly distributed in the middle of the investigation waters,where conditions of light and nutrition are more suitable for the growth of phytoplankton. The runoffbrings large amounts of nutrients to promote the phytoplankton growth, so its correlation withchlorophyll a is positive. However, the relationship between chlorophyll a and the sediment is morecomplicated. In the high turbidity waters, chlorophyll a and sediment discharge are negativelycorrelated, and in the coastal waters with poor nutrients, they are positively correlated.