Regulation Mechanism Of Phytoplankton Growth Zone In The Changjiang River Estuary Under The Influence Of Estuarine Constructions

The estuary and adjacent waters are complex dynamic systems subjected to multiple dynamic factors such as runoff,tides,waves,continental shelf circulation and atmospheric forces,and have complex dynamic processes such as river plume front and maximum turbidity zone.Driven by dynamic processes,nutrients from river and ocean converge,transport and transform in this region,shaping the unique spatio-temporal distribution characteristics of phytoplankton ecosystem.In recent years,environmental disasters in the estuarine and coastal waters increasingly aggravated,such as eutrophication,harmful algal blooms,habitat fragmentation and hypoxia.These phenomena are caused not only by natural factors but also by significant human activities in river basins and estuaries.For example,with the construction of massive major projects such as the Three Gorges Dam(TGD)in the watershed of Changjiang River,the fresh water,sediment and nutrients entering the estuary area have changed,and its impact on the estuarine ecosystem has been widely concerned.Meanwhile,a number of large-scale projects have been built in the Changjiang River Estuary.The Deepwater Navigation Channel(DNC)project was initiated in 1998 and completed in2011,with two long jetties(each measuring about 50 km long)and raising part of the southern jetty to 1.9 m above the mean sea surface level.And a significant amount of tidal flats were reclaimed,the area of Eastern Hengsha Reclamation(EHR)is about158,000 acres,about twice the original area of Hengsha Island.However,the impact of these mega estuarine construction on the dynamic processes of water and sand fronts and the expansion of nutrient transport in the estuary area,as well as the further regulation of estuarine and adjacent sea ecosystems,has not been studied enough.In order to solve the above problems,we analyzed the measured data and remote sensing satellite data,and then carried out the hydrodynamic,sedimentary-ecological coupling numerical simulation study,to investigate the mechanisms of the spatiotemporal variabilities of the phytoplankton growth zone in the Changjiang River Estuary and its adjacent waters under estuarine constructions.To comprehensively evaluate the impact of estuarine constructions on the estuary area,a comparative analysis is conducted to examine the impacts of both the estuarine construction and the Three Gorges Dam(TGD)in terms of dynamics and ecology.In this analysis,two different scenarios with normal and extreme(typhoon)weather conditions are further discussed.The main scientific understandings obtained in this paper are as follows.Long-term scale statistics revealed that the frequency of harmful algal bloom events(HABs)in the east and northeast of the Changjiang River Estuary increased from2000 to 2010,and the locations had a trend to move landward.Furthermore,the probability distribution of phytoplankton algal blooms derived from satellite chlorophyll(Chl)concentration in different periods.During the early stage of estuary construction(from 1998 to 2002),algal blooms in the Changjiang River Estuary were found far offshore and occupied a small area.From 2003 to 2006,during the second phase construction of the DNC,together with the EHR project,the frequency of algal blooms in the Changjiang River Estuary and its adjacent water began to increase and tended to move landward.During the third phase of the construction of the DNC(2006-2010),the locations of the high-incidence area of algal blooms gradually approached the Changjiang River Estuary.The above results showed a correlation relationship between the location changes of algal blooms in the Changjiang Estuary and the period of the estuarine construction However,the spatio-temporal distribution of algal blooms in different years was affected by complex natural factors at the same time,so we need to conduct further tests using numerical model.First,numerical simulation is conducted under normal weather conditions.The results from numerical model showed that the large-scale estuarine construction in the Changjiang River estuary played an important role in the changes of the phytoplankton growth zone.After the construction of estuarine projects,the area of high surface chlorophyll concentration in the north of the river mouth decreased,and the center of high value migrated toward the land by about 10 km.To the south of the estuary,the center of high value chlorophyll concentration seaward by nearly 10 km.These changes were caused by the altered transport of fresh water and nutrients into the ocean in different directions resulting from the construction of estuarine projects,which results in the weakening of the expansion to the northeast of river mouth and the strengthening of the expansion to the south of river mouth.Meanwhile,the construction of estuarine projects weakened the exchange between North Passage and tidal flat,resulting in the reduction of suspended sediment concentration(SSC)in the Turbidity Maximum Zone(TMZ),thus creating more favorable conditions for phytoplankton growth.In addition,the influence of the variations of river discharge and sediment flux,possibly caused by the construction of the TGD,on the phytoplankton growth zone in the Changjiang River Estuary was also studied.The simulation results showed that the TGD mainly affects the SSC in the TMZ and the distribution of nutrients outside the mouth,however,it has a trivial effect on the inner boundary of phytoplankton growth at the outer river mouth.The location of the outer boundary of phytoplankton growth zone oscillates with the variation of the surface front of the river plume.As the construction of the TGD occurred during a similar historical period with estuarine constructions,we need to carefully distinguish their impacts on the dynamics and ecological processes of the estuarine and oastal area.The results of this study showed that the estuarine construction has an important impact on the dynamics and ecological processes of the estuary and its adjacent water.Under typhoon conditions,strong winds enhance the mixing off the Changjiang River Estuary.The distribution of nutrients there changes with the abnormal expansion of the Changjiang River Plume.Meanwhile,the redistribution of SSC changed the thickness of the euphotic layer.Both aforementioned processes,in combination,determine the spatial distribution of phytoplankton growth zone.The simulation results showed that,compared to the normal weather condition,the effect of south expansion of Changjiang River Plume caused by estuarine constructions was more significant during typhoon period,leading to the increased growth of phytoplankton in the coastal water of Zhejiang and Fujian.For example,after typhoon “Chan-hom”(1509)passed through the Changjiang River Estuary,the phytoplankton biomass there increased to twice as much as before the typhoon.Considering the future reclamation of Hengsha Shoal,the combined effects of the typhoon and estuarine constructions will cause a more significant southernabnormal transport.More dilute water will carry more nutrients into the surrounding waters of Zhoushan Islands,resulting in further expansion of phytoplankton growing zone.This may increase the risk of phytoplankton blooms in waters around the Zhoushan Islands.