Nutrient Environment Variations And Diatom Response In Jiaozhou Bay And Yantai Offshore Under The Influence Of Human Activities

The increasing human activities in the context of global climate change have had a great impact on the offshore ecological environment.Jiaozhou Bay and Yantai offshore（Taozi Bay,Sishili Bay and Muping offshore）are two typical sea areas in northern China affected by various human activities,and the intensification of human activities since 1980s has led to the intensification of eutrophication in the two sea areas.Although extensive research has been conducted on the process of nutrient input and the environmental effects of eutrophication,knowledge about the long-term changes in nutrient environments and the response of phytoplankton under environmental protection policies is relatively limited.Against this background,this study analyzes the spatial and temporal changes in nutrient environments and the community response of diatoms in the two marine areas over the past 30 years by examining nutrient concentrations in seawater and coastal river water,as well as the total organic carbon and its stable isotopes(TOC,δ¹³C),total nitrogen and its stable isotopes(TN,δ¹⁵N),phosphorus（P）,biogenic silica（BSi）and sedimentary diatoms in surface and sediment cores from two areas.This is important for clarifying the process of offshore ecological environment change under the implementation of China’s offshore environmental protection policy.The main findings of this thesis are as follows:1.The research results of the water and sediment in Jiaozhou Bay indicate that:（1）In summer 2019,the water quality in Jiaozhou Bay was mainly classified as Class III and IV,and the overstandard rate of organic nitrogen in sediments was 70%,mainly located in the northeastern and eastern coastal areas.The major factors causing this situation were river input,sewage discharge,and the enclosed topography.The mean values ofδ¹³C andδ¹⁵N in surface sediments were-21.42‰and 5.91‰,respectively,indicating that the main source of sediment organic matter in the bay was from marine primary production.Theδ¹³C in the eastern coast was relatively negative,mainly influenced by land-based inputs,while nitrogen in sediments in the northern and eastern parts of the bay was affected by pond aquaculture and sewage discharge,respectively.（2）Salt field development in the northern part of Jiaozhou Bay had little impact on the nutrient environment of the bay before the 1980s.The eutrophication of the bay was accelerated by sewage discharge and pond aquaculture activities from the 1980s to the 2000s,but the eutrophication in the central part of the bay was not significant due to shellfish aquaculture.Since the 2000s,with the implementation of measures such as"Total Control"and aquaculture clearance in Qingdao city,the degree of eutrophication in Jiaozhou Bay has generally decreased.Although the eutrophication in the northern and northeastern parts of the bay has also decreased,it is still worse than that in other areas of the bay.（3）Cyclotella stylorum was the dominant species of sediment diatoms in the central part of Jiaozhou Bay,accounting for an average of 59%of the community,followed by Paralia sulcata.The changes in sediment diatoms responded well to the eutrophication process indicated by nutrient concentration in seawater and sediment biogenic element content in Jiaozhou Bay,showing an increasing trend in abundance and diversity index of the community and a decreasing trend in cell size since the 1980s.2.The research results of the water and sediment in Yantai offshore indicate that:（1）In the summer of 2019,the water quality of the nearshore waters in Yantai was classified as Class I with low nutrients.Nutrient concentrations in the bottom waters around Yangma Island and Muping offshore were 3-4 times higher than in the surface waters due to bivalve aquaculture and water stratification,while in other areas,they were 1-2 times higher.The mean values ofδ¹³C,C/N,andδ¹⁵N were-22.26‰,6.04,and 5.91‰,respectively,indicating that the organic matter in the sediments mainly came from marine autochthonous sources.（2）Since the 1980s,the increasing discharge of sewage and use of fertilizers have exacerbated eutrophication in the nearshore waters of Yantai.Dumping of dredged materials in the harbor has also caused eutrophication in the outer part of Sishili Bay.However,since the 2000s,the pollution discharge has been effectively controlled,and eutrophication in areas such as Zhifu Bay and Sishili Bay has significantly alleviated.The changes in aquaculture models have resulted in significant differences in the environmental impact:eutrophication in the western part of Yangma Island and Muping offshore has continued to worsen since the 1980s,mainly due to river input and pond aquaculture in the estuary and bivalve rack culture,while the effects of measures such as aquaculture reduction and artificial reef construction since 2012 are not yet evident.Eutrophication in the eastern marine ranching area of Yema Island has been partially alleviated since the mid-2000s,mainly due to measures such as the clearance of coastal aquaculture ponds,artificial reef construction,and an overall reduction in river and sewage pollution.（3）In the past 30 years,the absolute dominant species of diatoms in the sediment in the Muping offshore has been C.stylorum,accounting for an average of 65%of the community,followed by P.sulcata,accounting for an average of 10%.In response to the continuous increase in eutrophication in Yantai offshore since the 1980s,the primary productivity in Muping offshore has continued to increase,and the abundance and proportion of small algae such as P.sulcata have also increased.However,the increased biodiversity index of the diatom community in the sediment indicates that the eutrophication in Muping is still in the early stage.Comparing the two areas,the difference in terrain is an important reason for the poorer environmental quality in Jiaozhou Bay than in Yantai offshore.The discharge of sewage and aquaculture activities have been the main causes of eutrophication in both areas since the 1980s,and under the control of nearshore environmental management,eutrophication has gradually eased in both areas after 2000.The environmental management in the later stage of Jiaozhou Bay should increase control of the discharge of wastewater from the northeast river basin and improve the quality control of wastewater discharge to deal with the natural disadvantage of the terrain.In Yantai,it is necessary to focus on controlling point source pollution near Yantai Port and Yangma Island,and combine the monitoring of hypoxia in the water,reasonable control of artificial reef construction volume and bottom seeding density in Marine ranching area.