Investigation On Main Biological And Heavy Metal Pollution Of Bivalve And Evaluation On Ecological Risk In Liaodong Bay

Liaodong Bay is a northeastern bay of the Bohai Sea that is rich in scallops,abalones,prawns,oysters and other shellfish.It is an important marine fishing and artificial breeding hub in the Liaoning offshore area.However,the rivers emptying into the Liaodong Bay are heavily polluted,and the unique geomorphology and limited self-cleaning ability of the bay has locally amplified the pollutants,leading to a significant decline in shellfish populations and extensive ecological damage.The major pollutants in Liaodong Bay are pathogens(bacteria,viruses,parasites),heavy metals and organic chemicals.The biological pollutants can infect and kill marine life,and cause food poisoning upon consumption of the tainted shellfish.Heavy metals and organic pol lutants are also highly toxic,and pose serious health risks to humans.The bivalve molluscs living in the mudflats(e.g.,Mactra veneriformis,Ruditapes philippinarum and Scapharca subcrenata),reefs(Crassostrea gigas and Mytilus edulis)and hard sea beds(Mizuhopecten yessoensis)of the bay filter plankton and seaweed from the sediment or seawater,which enriches heavy metals and microorganisms.Therefore,we studied the biological and major heavy metal pollution levels in the bivalve molluscs in Liaodong Bay from 2013 to 2017 in order to assess the risk to the local ecology and biodiversity,and revise aquatic product safety standards.From 2013 to 2017,seven sampling sites(S1-Puwan,S2-Bayuquan,S3-Erjiegou,S4-Linghekou,S5-Laohekou,S6-Huludao and S7-Shahousuo)were selected in Liaodong Bay.From July to August of each year,the local bivalve shellfish species(C.gigas,M.edulis,P.yessoensis M.veneriformis,R.philippinarum and S.subcrenata)and surface sediments specimens were collected to identify the biological pollutants in the bivalve shellfish and provide a risk assessment.The main objectives of this study were as follows:1.To investigate the biological pollutants in the bivalve shellfish:To identify the biological pollutants in the local bivalve shellfish species:C.gigas,M.edulis and P.yessoensis specimens were collected from seven sampling sites,and the biological contaminants(bacteria,viruses and parasites)were identified by PCR,culture characteristics and bio chemical tests as appropriate.2.To analyze heavy metal pollution in the bivalves and provide a risk assessment:Contents of Cd,Cr and Pb in M.veneriformis,R.philippinarum and S.subcrenata were detected by Graphite furnace atomic absorption spectrophotometry.Contents of As and Hg were detected by Atomic fluorescence spectrophotometry.Pollution degree,distribution characteristics and tissue accumula-tion of heavy metals in bivalve species were analyzed.The potential health risks of these heavy metals were calculated in terms of target hazard coefficients(THQs)and maximum daily consumption(CRmax).3.To survey heavy metal pollution in surface sediments and assess the ecological risk:The heavy metal content in surface sediments was analyzed.By calculating the geo-accumulation index(I_geo),pollution index(C_fⁱ),the pollution degree index(m Cd)and pollution load index(PLI)in surface sediments,the pollution degree of heavy metals in this area was analyzed.By analyzing the potential ecological risk index(E_rⁱ),comprehensive potential ecological risk index(RI)and Sediment quality guidelines(SQGs),the potential ecological risk of heavy metal pollution is evaluated.The main findings are as follows:1.The biological pollutants in bivalve shellfish in Liaodong BayVibrio parahaemolyticus was detected in C.gigas,M.edulis and P.yessoensis at 7 sampling points,and respective infection rates were 10.29%,7.49%and 9.74%.The average infection rate of V.parahaemolyticus in the shellfish species was 9.28%.Furthermore,PCR and sequencing analysis identified hepatitis A virus,norovirus and group A rotavirus in the above bivalves,with respective infection rates of 8.19%,3.62%and 8.81%.The parasite Haplosporidium nelsoni was detected in C.gigas from Puwan and Shahousuo,with an infection rate of 2.1%,and H.costale were detected in Pacific oysters in Puwan(infection rate 1.26%).Mikrocytos mackini was detected in C.gigas and P.yessoensis in Puwan(1.01%),and Marteilia refringens in M.edulis from the Puwan,Linghekou,Laohekou and Huludao sites(1.57%).All parasites were identified by PCR and sequencing.2.Heavy metal pollution in the bivalves in Liaodong BayThe overall distribution of five heavy metals in M.veneriformis,R.philippinarum and S.subcrenata was not significantly altered in the 5 year period from 2013 to 2017 in Liaodong Bay.The concentration of As showed a gradual decline,while that of Cd and Cr decreased significantly.In addition,Hg and Pb levels fluctuated during this period.The As levels exceeded the Class I standard for marine biological quality in Shahousuo.The concentration of Cd was very high in all three shellfish species,and close to class III standard in the R.philippinarum in Huludao and Shahousuo.At other sampling points,Cd levels exceeded class I standard.Cr and Hg levels exceeded class I standard in Puwan and Linghekou respectively,which was significantly higher compared to the other sampling points.The concentration of Pb a t each sampling point exceeded Class I standard,and was highest in the shellfish of Laohekou but lower than class II standard..As,Cr and Hg levels were lower than or close to Class I standard in the shellfish throughout the Liaodong Bay.The concentration of Cd and Pb were between class I and class II standards in all samples shellfish.3.Geographical distribution of heavy metals in bivalve from Liaodong BayThe concentration of As,Cd and Cr in M.veneriformis,that of As,Cd,Cr,Hg and Pb in R.philippinarum,and of As,Cd and Pb in S.subcrenata were significantly higher in the west compared to the east of Liaodong Bay(p<0.05).No significant differences were seen in other heavy metals in each species between the east and west region of Liaodong Bay(p>0.05).The metal pollution index(MPI)of M.veneriformi decreased in the order of S5>S6>S4>S7>S3>S1>S2,that of R.philippinarum decreased in the order S6>S4=S7>S5>S3>S1>S2,and of S.subcrenata as S5>S6>S7>S4>S1>S3>S2.The MPIs in the west(S4-S7)were higher than that in the east(S1-S3),indicating more severe pollution in the former.4.Tissue specificity of heavy metals in the bivalves from Liaodong Bay and health risksThe amount of As,Cr and Hg were significantly higher in the visceral tissues compared to the muscles(p<0.05),whereas Cd levels were higher in the muscle as opposed to visceral tissues(p<0.05).Pb levels were similar in both tissues(p>0.05).The respective THQs of Cd in M.veneriformis,R.philippinarum and S.subcrenata were greater than 1,and higher than that of other heavy metals(<1),indicating that the bivalves offshore from Liaodong Bay were mainly contaminated with Cd and may pose a health risk due to long-term consumption.In addition,the CR_maxs of Pb was relatively high in all the above bivalves,and that of Cd were the lowest compared to other heavy metals,indicating that the consumption of these shellfish species can increase the risk of Cd-related disorders.5.Heavy metals in surface sediments along the coast of Liaodong Bay and ecological risk analysisThe content of As and Cd were only slightly higher in Shahousuo compared to the other sampling points.The levels of Hg in Linghekou,Puwan and Bayuquan,Pb in Erjiegou and Shahoushou were higher,but still lower than the marine sediment quality Class I standard.According to I_geo and C_fⁱ,the pollution degree of Cd was highest,followed by Pb,As,Hg and Cr in that order.Thus,Cd is the main pollutant in the Liaodong Bay area,although the overall pollution level is low as per m Cd and PLI.From 2013 to 2017,E_rⁱ of Cd and Hg decreased slowly with each year,while that of others did not change significantly.The ecological risk of Cd was highest,followed by Hg,As,Pb and Cr.Comprehensive RI analysis show that the ecological risks of various heavy metals have gradually decreased in the last few years,as indicated by the average RI of<130.However,the RI values of Linghekou in 2013 and 2014,and of Shahousuo in 2013-2016 were between 130-260,indicating moderate pollution.,while the other points showed low pollution.Furthermore,the combination of five heavy metals has a small 21%risk of toxicity in terms of both negative biological effects and sediment pollution.Taken together,the health risks of heavy metals in the bivalves and surface sediments are not significant.However,the potential impact of these pollutants in marine ecosystems cannot be ignored and should be continuously monitored.