Effects Of Salinity On Metal Bioaccumulation And Toxicity In Moina Mongolica Simulated Using The Toxicokinetic-Toxicodynamic Model

Estuarine waters are rich in biological resources and are important areas of fishery production.At the same time,estuaries are the channels through which various pollutants(including metals)are transported by rivers into marine ecosystems.Water pollution and ecological risk are usually higher in estuarine waters than in other waters.The ecological risk of metals depends not only on their concentrations but also on the water chemistry.Due to the interaction of runoff and tidal current,water chemistry,such as salinity,is spatially and temprorally dynamic and complex,which makes it difficult to establish water quality standards and assess the ecological risk for metals in estuarine waters.In this study,we used the waterflea Moina mongolica as a model organism to simulate the effects of salinity on metal bioaccumulation and toxicity under the framework of the toxicokinetic-toxicdynamic(TK-TD)model.Firstly,we studied the effects of salinity on Cu bioaccumulation and acute toxicity.At salinity 10?30,uptake rate of Cu decreased with the increasing salinity,indicating that the effects of water chemistry(e.g.,complexation,competition)played a dominant role in influencing Cu uptake.At the lowest salinity(i.e.,5),the uptake rate of Cu was lower than expected,indicating that salinity also affected Cu uptake through physiological effects on the organisms.The 48 h median lethal concentration(LC50)of Cu was 96,153,61 ?g L-1 at salinity 5,15,25,respectively.Cu was more toxic to M.mongolica at the higher salinities(i.e.,25?30),attributing to the higher intrinsic sensitivity of the organism to Cu under the conditions of higher salinity.The TK-TD model can well separate and quantify the dual effects of salinity on Cu and predict the acute toxicity of Cu under various salinity conditions.TK-TD model was a good framework to tease apart the various effects and to simulate the overall effects of salinity.Secondly,we studied the effects of salinity on the bioaccumulation and toxicity of six metals(i.e.,Ni,Cu,Zn,Ag,Cd,Pb).We found that the effects of salinity on the uptake rates of metals were not always consistent with the prediction of the biotic ligand model(BLM),which predicts that increasing salinity would consistently reduce metal bioaccumulation.Uptake rates of Ni,Cu,Zn,Cd and Pb reached the maximum at intermediate salinity(salinity 10).When the salinity was higher than 10,the uptake rates decreased monotonically with increasing salinity,indicating that water chemistry effects of salinity dominated in this salinity range.In contrast,when the salinity was lower than 10,the uptake rates increased with increasing salinity,indicating that physiological effects of salinity dominated in this salinity range.Salinity effects on Ag uptake rates showed different trends from other metals,with the highest Ag uptake reached at salinity 20.Salinity had significant effects on the toxicity of the six studied metals but to di:fferent extents and with different trends.The toxicity of Ni,Cu,Zn and Pb were the lowest at intermediate salinities(15?20),while the toxicity of Ag and Cd were lowest at lowest and highest salinities,respectively.Effects of salinity on the bioaccumulation of metals were not consistent with the effects on their toxicity,indicating that salinity also affected the intrinsic sensitivity of the organisms.The comparative studies showed that the dual effects of salinity on toxicity existed for various metals.Thirdly,we studied the chronic toxicity of Cu and the related mechanism at different salinities.Three salinity levels(5,15,and 30)and three Cu concentrations(0,5,and 20?g L-1)were included.The survivorship of M.mongolica was higher at the intermediate salinity(15).At salinity 5,20 ?g L-1 of Cu reduced the survivorship significantly;in contrast,20 ?g L-1 of Cu showed no significant effects on the survivorship at salinity 15 and 30.Salinity had little effects on the intrinsic rate of natural increase(r)of M.mongolica.The r was significantly reduced by 20 ?g L-1 of Cu only at salinity 5.The clearance rate was not affected by salinity in the studied range or by Cu exposure upto 20 ?g L-1.Both salinity stress and Cu exposure affected the activity of Na+/K+-ATPase.The enzyme activity was lowest at salinity 5 and highest at 15.At salinity 5 and 30,20?g L-1 of Cu significantly reduced the enzyme activity.The activity of Na+/K+-ATPase can indicate the tolerance of M.mongolica to the Cu stress.In conclusion,salinity has dual effects on the toxicity of metals,which affects not only metal bioaccumulation,but also the intrinsic sensitivity of organisms.Salinity also has dual effects on the bioaccumulation of heavy metals,not only through water chemistry effects including anionic complexation and cationic competition,but also by physiological effects on organisms.The above effects can be separated and quantitatively simulated by the TK-TD model.The model can thus be used for better establishment of water quality standards and ecological risk management of estuarine waters.