Long-term Variations Of The Key Environmental Factors And Their Ecological Effects In The Changjiang Estuary

Previous studies show that long-term variatons of runoff, sediment and nutrientfluxes of Changjiang cause a series of effects to the estuary ecosystem in nearly half acentury. Influences are mainly manifested in the ecological environment,eutrophication of the Changjiang Estuary and Adjacent Coastal Waters is aggravatinggradually, sea area in excess of the Fourth Water Quality Standard increasesignificantly, and the phenomenon of hypoxia is increasing severe in bottom water,which bring about the frequent occurrences and expanded area of Red Tide, dominantspecies of Red Tides evolve from single diatom to diatom-dinoflagellates alternately.However, with regard to the long-term variations of ecological environment caused bythe fluxes variation of Changjiang is still lack of quantitative analysis, especiallyresearch about the response of Changjiang Estuary ecosystem to the significantreduction of sediment flux from Chanjiang to the adjacent coastal area in recent yearsis still blank. Also study about biological community response to the variation ofChangjiang estuary environment is lack of a systematic description. Above all, thereis no clear understanding and in-depth discussion of influence or main control factorscontributing to the rapid growth of phytoplankton biomass in the recent30years.Therefore, in order to obtain the dynamic change law and identify the main controlfactors affecting the ecological environment in Changjiang estuary, it is necessary tostudy the long-term variations of ecological environment and their ecological effectsin Changjiang Estuary.Withal, related data about runoff, sediment and nutrient fluxes in the past60yearsare collected, combined with the historical documents and field investigation data ofsurface salinity, Total Suspended Solids (TSS), nutrients (Dissolved inorganicnitrogen, phosphate and silicate) in Changjiang Estuary and Adjacent Coastal Waters, variation of ecological environment caused by the variation of material flux fromChangjiang to the sea are described quantitatively, and the ecological effects resultedfrom variation of ecological environment in Changjiang Estuary and Adjacent CoastalWaters are elucidated through the long-term evolutions of phytoplankton biomass andcommunity structure, benthic species number and biomass. Finally, the main controlfactors in the variation and evolution process of the Changjiang Estuary ecosystem areobtained preliminarily. Main conclusions are as follows:1. Long-term variation of the material fluxes from Changjiang to the seaRunoff of the Changjiang shows little change in the recent half a century. Unlikerunoff, the sediment flux presents decrease trend overall, this is ascribed to the declineof sediment content in Changjiang water, especially after entering the21st century,speed of reduction is accelerated significantly. Three kinds of nutrient content inChangjiang water show different variation trends. Before the1980s, DIN and DIPconcentrations in Changjiang water fluctuate around20μmol/L and0.5μmol/L upand down, respectively. After1980s, DIN shows rapid increase until the beginning of21st century, then the growth rate slows down with the concentration around120~140μmol/L. However, since the middle1980s, DIP concentration increases slightly, afterentering the21st century, DIP still show the significant trend of increase. Unlike DINand DIP, concentration of SiO3-Si in Changjiang water decreases obviously,especially after the1980s, speed of decline accelerates significantly. After enteringthe21st century, SiO3-Si concentration maintains around100μmol/L, its variation isno longer obvious.2. Long-term variations of key environmental factors in the ChangjiangEstuary caused by the variation of material fluxes from Changjiang to the seaIn the recent30years, the material fluxes variations of Changjiang result incorresponding changes of environmental factors in Changjiang Estuary. Under thepremise of steady runoff, horizontal distribution of sea surface salinity changes littlein Changjiang Estuary. Boundary of Changjiang Diluted Water basically appearsaround the area of123.5°E. According to data of29cruises, TSS content showssignificant decline with a rate of about-51%in the past30years, and the corresponding annual average rate is-1.7%. TSS concentration isoline of20mg/L(main part of Changjiang Diluted Water) shifts to the direction of estuary obviously.Meanwhile, nutrient data of61cruises show that increase rate of DIN, PO4-Pconcentration and decrease rate of SiO3-Si are103%,102%and-33%, with thecorresponding annual average rate of3.32%,3.19%and-1.03%, respectively.Changes of the nutrients concentration result in the variation of Si/N比值/P ratio. Inparticular, N/P比值ratio changes little around45; Si/P比值ratio become more andmore close to16, which is accordance with Redfield ratio; but Si/N比值ratio isqualitatively changed after2005, it lead to the concentration of silicate less than theDIN, which is conducive to the competition and growth of non-diatom phytoplankton(e.g. dinoflagenate) in Changjiang Estuary.3. Ecological effects caused by the variation of ecological environment in ChangjiangEstuaryAccording to chlorophyll-a data of50cruises from1981to2011, phytoplanktonbiomass characterized by chlorophyll-a concentration presents exponential growth.70cruises data of phytoplankton cell density by net sampling and water sampling inChangjiang Estuary and Adjacent Coastal Waters shows the same change trend withchlorophyll-a. After2003, growth trend of phytoplankton with small size and smallcell (mainly dinoflagellates) is more significant. Meanwhile, abundance ofdinoflagellates shows a rising trend, while the abundance of diatom with the silicateas the main nutrition reduces gradually, especially after entering the21st century,dinoflagellates as the dominant species appears frequently in the survey results. Inaddition,32cruises data of benthos in Changjiang Estuary and Adjacent CoastalWaters show reduations of total number of species and biomass, while decline ofspecie numbers is more apparent. In recent years, proportion of polychaetes withsmall size and short life cycle increases, and takes the place of the echinoderms andmollusks with bigger size and long life cycle gradually. 4. Conditions of TSS and nutrient become more and more beneficial to thegrowth of phytoplankton in Changjiang Estuary, eventually lead to the frequentoccurrence of Red Tide with wide distribution range, and gradually appears inlow salinity waters near the estuaryIn the recent30years, Using data of22summer cruises, relationship betweenenvironmental factors in surface water of the Changjiang estuary are investigated.Results show that salinity with high values of chlorophyll-a not only shows thetendency of shift to the direction of estuary, but also the corresponding scope ofsalinity is expanded significantly. In low (S=10) and middle salinity (S=20, S=25),corresponding concentrations of DIN and PO4-P increase, while TSS concentrationdecreases. In relatively high salinity (S≥30), growth of corresponding concentrationsof DIN and PO4-P and decline of concentration of TSS are more significant. AlthoughSiO3-Si concentration decreases, it is not the limitation to the growth of phytoplanktonin the recent30years. Finally, environment of Changjiang Estuary is more suitablefor the growth of phytoplankton, which is the reason of frequent occurances of RedTide with wide range and shifts to the estuary with low salinity.5. The main influencing factors of phytoplankton biomass growth inChangjiang Estuary exists seasonal and regional differencesFor interannual variation, growth of phytoplankton biomass is mainly influenced byincreasing nutrient fluxes and decreasing sediment flux from Changjiang to the sea. Indifferent season, decreasing sediment flux contribute more to phytoplankton biomassgrowth in spring, while there are no significant correlation between material fluxesand increasing phytoplankton biomass. For different salinity area, in low salinitywaters near estuary, reduction of sediment flux affect more of the phytoplanktonbiomass growth than others; in the plume front area with middle salinity, growth ofphytoplankton biomass could be ascribed to the increasing nutrient fluxes anddecreasing sediment flux; in the waters off the estuary with high salinity,phytoplankton biomass growth is mainly caused by the increasing nutrients fluxes,and function of decreasing sediment flux is relatively weak.