Organisms' Tolerance To Salinity&Pollution And Regime Shifts Of Coastal Brackish Wetland In Tianjin

Coastal wetlands are distributed in the coastal zone that is ecotone of land and sea,and play multiple ecosystem services,including intercepting and storing pollutants into the sea,alleviating marine disasters,maintaining biodiversity,etc.Tianjin is located at the top of Bohai bay and it is the sink of Haihe river,Yongding river,Chaobai river,Daqing river and other rivers into the sea.The coastal wetlands are fully developed.In the past few decades,affected by the population,urbanization process and the rapid development of marine economy,the coastal zone of Tianjin has witnessed drastic changes in land use,as well as the reduction of wetland water quantity,increase of salinity,deterioration of water quality,sharp shrinkage of area and intensification of artificialization.Overlap stress of salt and pollution is an important feature of coastal wetlands,which is different from the upstream inland freshwater wetland.At present,the main problems of ecological management and wetland restoration in coastal areas are unclear coastal zone boundaries and unclear salt-containing wetland steady-state conversion mechanism.Based on the influence of ocean on land,this paper establishes the land boundary of Tianjin coastal zone.Taking brackish water(0.5-30‰)wetland as the research object,the tolerance gradient of main pollutants and salinity of plants in coastal zone and submerged plants,zoobenthos,zooplankton and phytoplankton was studied.The thresholds of alternative states and indicator species of biological community under salt and pollution stress were carried out to provide support for ecological restoration of coastal wetlands.The main research contents and results are as follows:(1)Indicators and methods for the delimitation of the landward boundary of the coastal zone were determined,and the landward boundary of the Tianjin coastal zone was delimited from a larger scale.Based on the idea of the sea-land ecotone,the main processes of ocean influences on land,such as coastal erosion,sea level rise,storm surge water increase,typhoon,soil salinization,groundwater salinization/semi-salinization,salinity rise,etc,were selected.This paper analyzed the spatial scopes of coastal erosion,storm surge water increasing,soil salinization,and groundwater salinization/semi-salinization in Tianjin,and finally determined the landward boundary of Tianjin coastal zone by the distribution area of underground salt water/semi-salinization.(2)Based on the gradient of soil salinity,water salinity and salt-tolerant indicator species from intertidal zone to inland of three seashore rivers in Tianjin,the coastal and inland boundaries of the rivers were determined.It was found that the salt content of Tianjin coastal wetlands was high in the southeast and low in the northwest,and the spatial heterogeneity was large.From the perspective of soil salinity,water salinity,and spatial distribution of riparian plants and aquatic organisms,Duliu river,Haihe river main stream and Yongding river were the boundaries between coastal wetlands and inland wetlands at 40-57 km,32-45 km and 18-32 km respectively.(3)The response relationship between a variety of biological groups and salinity and pollutants has been studied.It was found that salinity,Na+,Cl-,TN were the main soil chemical factors that determined the distribution of coastal plant communities.Nutrients and salinity were the main environmental factors that affected the distribution of submerged plants.Myriophyllum sp.may indicate cleaner water body,Potamogeton crispus,Ceratophyllum demersum and Potamogeton pectinatus may indicate low salt and medium high sewage body,while Ruppia maritima may indicate high salt and high sewage body.Salinity,Cl-,Na+,TN,Chl a and TP were the main water environmental factors that affected the distribution of zoobenthos,zooplankton and phytoplankton.Salt-adapted organisms were distributed in the estuaries of the sea,while pollution-adapted organisms were found in the upper reaches of rivers,tributaries and in the still water areas of ponds.(4)The salt and pollution adaptability of littoral plants and submerged plants was analyzed,and the indicator species and response thresholds of littoral plants,zoobenthos and plankton were determined.Different species had different tolerances to salt content.With the increase of soil salt content,the coverage of riparian plants showed different variation trends,such as rise,fall,parabola and inflection point.Ruppia maritima and Potamogeton crispus may be considered as salt-tolerant and pollution-tolerant species,while Myriophyllum sp.may be considered as clean species.The threshold of zoobenthos community was 0.66-7.36‰ salinity and 1.87-3.58 mg/L TN,and the threshold of zooplankton community was 1.99-7.36‰ salinity and 0.48-0.59 mg/L TP.The phytoplankton community threshold was 0.11-0.24 mg/L TP.Perinereis aibuhitensis,larva,Limnodrilus claparedianus,Chironomus pallidivittatus,and Rissoina bureri can be regarded as brackish water tolerance species,Cyclopoidae strennus,Nitocra lacustris,Difflugia globulosa,and Cyclopoidae vicinus can be treated as brackish water sensitive species.Most phytoplankton were pollution-resistant species,which began to grow rapidly when TP exceeds 0.11 mg/L.(5)The relationship of the water quality,clean or turbid states and the response factors was analyzed,and the alternative states threshold of the brackish water ecosystem was studied.It was found that the water quality of the wetland was poor in the northwest and good in the southeast,and the main factors were organic pollutants.The alternative states of coastal brackish water wetland were mainly affected by Nutrients,submerged plants and biological community structure.The initial transformation threshold was 0.66‰ salinity,0.11 mg/L TP,and 1.71 mg/L TN.According to the density ratio of zooplankton to phytoplankton,three steady-state zones were divided:cladoceratin-dominated zone,al gal-dominated zone of other zooplankton,and algal-type zone.The salinity of cladoceratous algal control zones was less than 1.20‰,that of other zooplankton algal control zones was less than 7.00‰,that of TP was less than 0.48 mg/L,and that of algal type was less than 7.36‰.The transformation threshold of cladoceratous algal control zones to other zooplankton algal control zones was 0.50-2.30‰.The boundary of Tianjin coastal zone determined in this study provides a spatial basis for the integrated planning of land and sea,which serves to focus and solve the problems of coastal wetlands.The determined biological salt-tolerance,pollution-tolerance adaptability and alternative states can be applied to the evaluation of wetland ecological status on a local scale,and provide a basis for the determination and quantitative grading of key indicators for the ecological integrity evaluation of coastal wetlands.