| In recent years,due to high-density and high-input breeding methods,a large amount of heavy metals and other pollutants have been left in the water,which exceeds the ecological environment carrying capacity of the aquaculture environment itself.As a class of typical pollutants with the characteristics of refractory degradation,migration and transformation,bioaccumulation,and biotoxicity,heavy metals seriously threaten the sustainable development of aquaculture.On the one hand,heavy metals affect the growth and development of aquatic products;on the other hand,heavy metals can be amplified through the food chain and cause harm to human health.Because of the favorable geographical location and environmental conditions,Xiangshan Port has become an important mariculture area in Zhejiang Province and even the whole country.Meishan Bay is located at the gate of Xiangshan Port and is an important area of Xiangshan Port.In recent decades,the infrastructure of Meishan Island has been continuously improved,with rapid development of aquaculture,industry,agriculture,and port transportation.Due to the interfere of anthropogenic activities,a large amount of wastewater carrying pollutants is discharged into Meishan Bay,which causes a great risk to the local ecological environment.In this stduy,Meishan Bay was selected as the research area,and the total amount and fraction of 8 heavy metals(Cu,Zn,Cr,Ni,Pb,Cd,As and Hg)in the surface water,bottom water,surface sediments and core sediments were analyzed,and preliminary exploration of the effect of ecological floating bed aquaculture activities on the restoration of heavy metals and other pollutants.Combined with the physical and chemical parameters of water,the spatial distribution,pollution level and source of heavy metals in water were studied,and the driving factors of heavy metal release and migration in water were analyzed.The pollution levels,spatial and temporal distribution,sources and ecological risks of heavy metals and their fraction in sediments of Meishan Bay were analyzed.Combined with the characteristics of overlying water and the properties of sediments,the factors affecting the release,migration,and transformation of heavy metals in sediments were explored.The sedimentation rate of sediments in different regions was determined by 210 Pb tracer.The historical change characteristics of the total and the different fraction of heavy metals in sediments of Meishan Bay in the past 50 years were analyzed,and the historical change of heavy metal pollution and related influencing factors were analyzed.The changes of nutrients,phytoplankton community structure and heavy metals in water,and heavy metals in sediments were analyzed before and after the placement of the ecological floating bed,so as to preliminarily explore the restoration effect and driving factors of the ecological floating bed on the water of Meishan Bay.(1)In the surface seawater,the concentrations of Cr and Pb at the gate entrance were significantly higher than those outside the bay and inside the bay,and the concentrations of Zn,Hg and Ni outside the bay were significantly higher than those inside the bay and the gate entrance(P<0.05).In the bottom seawater,the contents of Cu,Cr,Pb and As at the gate entrance were significantly higher than those outside and inside the bay(P<0.05).Water quality parameters such as DO,p H,temperature and salinity were the important factors that affect the adsorption effect of heavy metals.The heavy metal pollution in the surface and bottom seawater was mainly Ni and Zn and was more serious outside the bay and the gate.(2)Various physical and chemical parameters of the overlying water,such as temperature,salinity,and nutrient content,were all the important factors that affect the adsorption and release of heavy metals in the sediments of Meishan Bay.The contents of As,Pb,Cr,Zn,Cu and Ni in the sediments of Meishan Bay in summer were lower than those in other seasons,while the concentrations of Hg and Cd in summer were higher than those in spring and autumn.The concentrations of Cu,Zn and Pb in the sediments of Meishan Bay showed a decreasing trend from 2014 to 2019,while Cd exhibited an increasing trend from 2014 to 2017,and a decreasing trend from 2017 to 2019.From 2016 to 2019,the concentrations of As,Ni and Cr showed a decreasing trend,while Hg showed an increasing trend.The contents of As,Cu and Ni in sediments outside the bay were higher than those in the bay and the gate,and the contents of Cd in sediments outside the bay and the gate were higher than those outside the bay and the gate.The results of source analysis showed that As,Cr,Cu,Pb,Ni and Zn were mainly from the natural environment,while part of Pb,Zn and Cd were mainly from the sewage and waste gas produced by ships,land and sea transportation,and part of Hg and Cd were mainly from agricultural production.Level of granularity and anthropogenic were important factors affecting the spatial and temporal distribution of heavy metal enrichment in Meishan Bay.(3)Residual fraction was the main fraction of heavy metals in the sediments of Meishan Bay.Except for the residual fraction,acid-soluble fraction was the main fraction for Cd,reducible fraction was the main fraction for Pb,Cu and Zn,and oxidizable fraction was the main fraction for Pb,Cu and Zn.The evaluation results of the risk assessment criteria and the ratio of secondary and primary phase showed that Cu,Zn,Cr,Ni,Pb,As and Hg in the sediments of Meishan Bay did not present ecological hazards or slight hazards,while Cd posed slightly pollution in most stations of the gate entrance.(4)The sedimentation rates at the gate and the central area were different.The sedimentation rates at the gate entrance were stratified,with the first sedimentation rate of 0.29cm/a(R2=0.87)and the second sedimentation rate of 3.9 cm/a(R2=0.60),while the sedimentation rate in the central area was relatively stable,which was 1.1 cm/a(R2=0.76).Anthropogenic and hydrodynamic force could affect the sedimentation rate to some extent.The results showed that in the early 1970 s,the concentration of heavy metals in sediments remained a stable trend but began to increase in the late 1980 s.From 2014 to 2019,part of metals in the sediments showed a downward trend.Outside the bay,the increase in heavy metal pollution in the environment was related to anthropogenic activities such as industrial production and port transportation;while port transportation,agricultural production and aquaculture were the main factors causing heavy metal pollution inside the Meishan Bay.Residual fraction was the main fraction of the all the heavy metals in columnar sediments.The risk assessment results showed that Cd had a low level of environmental risk,while other heavy metals had no ecological risk.(5)Comparing data from different periods,it was found that after placing the ecological floating bed,the content of TIN in the water of Meishan Bay had decreased significantly,and the nutrient structure of the water in the bay had gradually changed from phosphorus limitation to nitrogen limitation.In terms of seasonal distribution,there were significant differences in phytoplankton abundance,species richness and diversity index among different seasons.The abundance of phytoplankton in spring and summer was higher than that in autumn and winter,and the diversity index and richness index in summer and winter were higher than that in spring and autumn.In the spatial analysis,the abundance and diversity index of phytoplankton in the offshore area were higher than that in the bay,and the hydrodynamic conditions were the important factor affecting the reproduction and population succession of phytoplankton.Environmental factors,such as p H,nutrient,salinity,and temperature were important factors affecting the propagation and distribution of algae.The changes in the nutrient structure of the water in Meishan Bay were the important factors for the succession of phytoplankton communities.(6)In this study,there was no significant difference in the influence of ecological floating bed aquaculture activities on the spatial distribution and season variation of heavy metals in the water,which was related to the low environmental background value.After the placement of the floating bed,the concentrations of some heavy metals and fraction in the sediments increased,and the increase of the total heavy metals was related to the grain size and organic matter of the sediments,while the increase of moveable components of heavy metals was mainly related to the change of nutrient in water,the activity of phytoplankton and the activity of microorganism in the rhizosphere of hippocampus. |
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