Bioaccumulation And Influential Mechanisms Of Heavy Metals In Offshore Seafood

Seafood,the important source of protein and other essential nutrients,has been increasingly consumed by humans.Coastal regions are undergoing various issues including extraordinarily rapid development of marine resource exploitation,sharp increase in populations,and the extensive impact of anthropogenic activities.Various contaminants can be accumulated in the seafood through the food chain.Heavy metals,stable in the aquatic environments,can be assimilated and accumulated in organisms to cause potential hazard so as to attract global attention.This study investigated the distribution,accumulation,health risk,transfer on the food chain and the corresponding influence factors.This study mainly adopted three steps: "comprehensive investigation-factor analysis-mechanism exploration".First of all,a continental-scale investigation in terms of heavy metal pollution in a large amount of seafood was implemented to explore the overall situation and outstanding problems.Moreover,the effect of policies used to prevent and control metal pollution was quantified.Secondly,the representative species for bivalves and fish were selected to explore the impacts of aquiculture environmental and species differences.The dynamic variations of heavy metal contents of scallops during the whole culture cycle were monitored by a transplantation experiment.At last,the transmission and accumulation characteristics of heavy metals among organisms of different trophic levels were investigated by main food web consisting of seafood organisms and artificial seawater exposure experiments.This study could provide supporting information for sustainable development of coastal zone,prevention and control of heavy metal pollution,and health risk control of seafood safety.The main findings of this study included the followings: This study investigated the distribution,accumulation,health risk,transfer on the food chain and the corresponding influence factors.This study mainly adopted three steps: "comprehensive investigation-factor analysis-mechanism exploration".First of all,a continental-scale investigation in terms of heavy metal pollution in a large amount of seafood was implemented to explore the overall situation and outstanding problems.Moreover,the effect of policies used to prevent and control metal pollution was quantified.Secondly,the representative species for bivalves and fish were selected to explore the impacts of aquiculture environmental and species differences.The dynamic variations of heavy metal contents of scallops during the whole culture cycle were monitored by a transplantation experiment.At last,the transmission and accumulation characteristics of heavy metals among organisms of different trophic levels were investigated by main food web consisting of seafood organisms and artificial seawater exposure experiments.This study could provide supporting information for sustainable development of coastal zone,prevention and control of heavy metal pollution,and health risk control of seafood safety.The main findings of this study included the followings:(1)Total concentrations of 8 heavy metals in seafood varied with sampling sites and species with the order of bivalve molluscs > crustaceans > fish.It was found that scallops were the typical metal-polluted organism and metal concentrations in wild fish were higher than those in farmed fish.Estimated daily intake(EDI)was lower than the provisional tolerance daily intake(PTDI)set by WHO and over 97% hazard quotients(HQs)were lower than threshold,indicating that it was safe for humans to consume seafood routinely.Source apportionment analysis demonstrated that the fossil energy consumption(coal combustion and vehicle exhaust),seawater,and metallurgic dust might serve as the possible main sources of heavy metals in seafood.Based on the policy scenario analysis of Water – Food-Energy NEXUS,the fossil-energy-controlling policy and clean coastal water action were beneficial to the seafood safety by reducing target heavy metals in seafood.(2)Fish samples belonging to the same species with similar size but from different aquiculture environments were collected to confirm conclusion in comprehensive assessment.Heavy metal concentrations and stable isotope ratios in wild fishes were higher than those in farmed fishes of same species,showing the wild fish possessed the higher metal pollution level and trophic level.The MPI values were also positively correlated with TL and Bp values,indicating that the higher trophic level and benthos proportion might cause higher metal pollution level.Therefore,different heavy metal accumulation abilities of fish in the aquaculture environment and natural habitat were affected by trophic levels,feeding habits and growth rates.Factory farming under artificial controlled conditions was an effective means to control metal contents in seafood in view of the metal pollution in offshore area.(3)The over-limit ratios of Cd in three farmed scallop samples along the coastal of the Yellow and Bohai Seas reached 96% so that the scallops could be regarded as the hyperaccumulator of metal Cd.C.farreri tended to accumulate more metals than A.irradians and P.yessoensis while the accumulation capacity of tissues followed the order: digestive gland > gill > adductor muscle.Significant spatial distribution of metals in scallops was also summarized: Bohai Sea > South Yellow Sea > North Yellow Sea.The scallops could keep accumulating metals rapidly from the surroundings even in the standard-compliant situation,which should be paid great attention.Therefore,both the strong enrichment of certain metal in typical species and the growth rate difference among species could affect the metal accumulation in seafood.(4)In order to analyze the source and accumulation process of high concentration of Cd in scallops,bay scallops(A.irradias)which has a wide culture range and a fast growth rate was selected as the experimental organism of transplantation experiment.Target metals in scallops and their ambient samples were investigated and the sources of metals were traced.The content of Cd in scallops rose rapidly after being transplanted to the offshore aquaculture farm,indicating the accumulation of heavy metals in scallops was a process of rapid enrichment.During the whole culture cycle,the over-limit of Cd contents from beginning should be paid attention.The concentrations of heavy metals in scallops were mainly affected by the concentrations of heavy metals in seawater and sestons.Principal component analysis showed that seawater(35.7%)and sestons(50.7%)were the main sources of metals in scallop.Therefore,the environmental change was the main factor affecting the accumulation of metals,and the scallop could quickly adjust the balance of metal contents in vivo to adapt to the environmental.(5)In the food web consisting of the major economical seafood in the coastal of Yantai,big gaps appeared between the metal concentrations in invertebrate and fish.The main metal in fish was As while Cd was the major metal to cause the pollution in invertebrate.The MPI of three fish categories showed significant positive correlation with TL and Bp values,indicating the metal accumulations in fish were greatly affected by trophic levels.On the whole food chain,Cr and Hg in seafood showed obvious biomagnifications efficiency while Ni showed a biological dilution.Cu/Zn/As could show an accurate regularity because of the interspecific distinction of fish and invertebrate.Specially,As in invertebrate and Zn in fish could show the biomagnification efficiency.(6)The removal rates of heavy metals in soft tissues of whole scallop by temporary exposure of artificial seawater followed the order: Hg(55%)> Ni(52%)>Cr(46%)> Cu(45%)> As(44%)> Pb(38%)> Zn(27%)> Cd(20%).The proportion of removable-Cd were the lowest in 8 heavy metals to suggest that the Cd mainly existed in a stable fraction in scallop,which also verified the conclusion that scallops were the hyperaccumulator of metal Cd.Artificial seawater temporary exposure and washing had limited purification capacity for heavy metals in soft tissues of scallops,especially in the muscle.Therefore,it was difficult to remove heavy metals from seafood,and the control of source of pollution was the key to improve the quality of seafood.The innovative points of this dissertation were shown as follows:(1)This study comprehensively assessed the heavy metal pollution levels in seafood at large spatial scale,quantifyed the effect of policies on heavy metal pollution control in seafood,and confirmed the important role of "carbon neutral" strategies in improving the quality of offshore seafood.(2)The whole culture cycle of farmed scallops was tracked,and the characteristics of heavy metal accumulation in scallops were explored.Moreover,DGT technique was used to investigate the relationship between the concentration of bioavailable metals in seawater and their accumulations in scallops.(3)This study revealed that there was still a potential health risk of metals in seafood under the standard-compliant environment from the aspects of strong enrichment characteristics of typical species and the magnifying effect of trace heavy metals in the food chain,which could provide reference data for the health risk assessment of seafood consumption and the establishment of safety threshold.