Morphodynamic Processes And Biological Effects Over The Intertidal Mudflats

The Intertidal flat is located in the sensitive area of continental and ocean interaction.It is widely distributed in estuaries and coastal areas,and is one of the regions with high primary productivity on earth.It is also a hot area of global climate change research.As an important reserve of land resources in coastal areas,the intertidal mudflat plays an important role in breeding,wetland development,shoreline protection,biodiversity maintenance and other aspects,and has high economic and ecological values.Storm events are one of the most common natural disasters in coastal zones.However,in recent years,storm disasters caused by global climate change occur frequently,which bring inestimable losses to the social economy of coastal areas.The intertidal mudflat is the ecosystem most directly affected by storm events,such as tidal flat topography,geomorphic shape,hydrodynamic condition,sediment composition characteristics,macrobenthic community,etc.,which will be affected to varying degrees by storm events.Therefore,it is necessary to quantitatively characterize the sedimentary geomorphic processes of intertidal mudflat,which can help to evaluate the impact of hydrodynamic conditions on organisms,deepen the understanding of the intertidal mudflat ecosystem,and provide reference for the maintenance of biodiversity in the region.In this study,we were used advanced instruments to observe the intertidal mudflat of East Chongming,Dafeng Doulong Port and Qingshuigou in The Yellow River Estuary from July 17,2020 to August 21,2020,October 5,2021 to October 12,2021,and November 5,2021 to November 9,2021,respectively.The observation contents include:1)Using ADV to obtain the field high frequency velocity data;2)Using RBR-Wave to obtain the data of water depth and wave;3)OBS and RBR-TU were used to record the suspended sediment concentration and temperature of the water.4)Obtain the relative elevation change of bed by ADV ranging function;5)Collect sediment samples and analyze sediment components and water content in laboratory;6)Macrobenthic and swimming fauna samples were collected to investigate the intertidal fauna community.Through the field observation and sampling,we got data of sediment composition,water power variation,suspended sediment concentration,erosion-deposition of bed and large benthic animal.To explore the relationships among hydrodynamic processes,sediment initiation,bed level changes and macrobenthic community in intertidal mudflat under different weather conditions.The main conclusions are as follows:(1)Under the calm weather condition,the bed level of intertidal mudflat changes significantly during tidal cycle and spring-neap tides.During the tidal cycle,the bed of East Chongming intertidal mudflat erodes during the period of sharp rise and fall,and the bed is often deposition in the steady period of high tide.At the neap and spring tides,there are significant differences in the change of bed.During the spring tide period,the suspended sediment concentration is up to 5 kg/m~3,and the sediment supply is abundant.The bed of the East Chongming intertidal mudflat is gradually silted.However,during neap tide period,the suspended sediment concentration decreased significantly,and the mean suspended sediment concentration decreased to about 0.6 kg/m~3,resulting in insufficient sediment supply and the bed of the East Chongming intertidal mudflat gradual erosion.(2)During extreme weather events such as gale,cold wave and storm,the bed of intertidal mudflat erodes obviously.Due to the influence of gale,the erosion amount of bed in East Chongming intertidal mudflat and Dafeng Doulong port intertidal mudflat reaches 50 mm.Due to the influence of cold wave storm,the erosion amount of bed in the intertidal mudflat of Qingshuigou reaches 15 mm.After the storm weather such as gale and cold wave,the bed of intertidal mudflat will quickly silt back.In two tidal cycles after gale,the intertidal mudflat of Dafeng Doulong Port in Jiangsu province gradually recovers to the state before erosion,the backfill volume is about 50mm.In the Qingshuigou intertidal mudflat,the back amount of silt reached half of the erosion amount in just four hours after the cold wave.(3)Macrobenthos samples were collected in the East Chongming intertidal mudflat on July 18,2020(before gale)and August 19,2020(after gale).Six species were collected before gales,and the species diversity index H,species richness index d and evenness index J were 1.69,1.01 and 0.65,respectively;Four species were collected after gales,and the species diversity index H,species richness index d and evenness index J were 1.09,0.79 and 0.55 respectively.Macrobenthic species were more abundant before gales.After gales,deposition occurred in the bed,and the deposition thickness was 40 mm.The macrobenthos inhabit on the surface of the bed from 50 mm to 100 mm,the change of bed was the main reason for the difference between the two samples.(4)The gale enhanced the hydrodynamic conditions of the intertidal mudflat in Dafeng Doulong Port,Jiangsu Province,resulting in an increase of 0.54 m in the water depth,the maximum wave height reaches 0.8 m(0.28 m higher than normal weather period),and the average velocity from less than 0.1 m/s to 0.32 m/s.Before gale,the relative importance index(IRI)of spot tedtail goby was less than 20.In the gale period,the relative importance index(IRI)of spot tedtail goby increased sharply,reaching the maximum value of 60.11.The gale changed the species distribution of swimming animals in the region,and the spot tedtail goby became the dominant species,suggesting that the intertidal swimming fauna community was significantly responsive to gale-weather.