Study On Physical Emergency Treatment Measures For Resting Eggs In Ship Ballast Sediments

With the rapid development of shipping networks,the transfer and discharge of ballast water and sediment from vessels has become a major means for aquatic nonindigenous species(NIS)invasion.In response to this situation,there are various international measures for ballast water treatment have been created to successfully reduce the potential for NIS invasion.However,there are fewer studies on the efficacy of these measures on ballast sediment and the accumulated data are insufficient.Ballast sediments are formed by the sedimentation of suspended substances such as organisms,organic fractions,and soils contained in the ballast water,especially for invertebrate resting eggs in sediments that constitute a particularly challenging life stage to eliminate.The salinity and temperature treatments are the most frequently applied ballast water physical treatment on ships.Therefore,based on the existing measures,this study aims to explore the reasonable salinity and temperature treatment measures that can eliminate the hatchability of resting eggs in ship ballast sediments from different habitats.It will prevent invertebrates’ resting eggs from invading the original ecosystem and disrupting the ecological balance.The main components are as follows.In this study,based on the existing measures for ballast water treatment,the active zooplankton and sediments from freshwater were treated to multiple concentration and multiple times of brine exposure in laboratory.The maximum diversity hatching experiments were conducted to examine the hatching of resting eggs isolated from the sediments.Thus,the survivability of the active zooplankton and the hatching strategy of the resting eggs were assessed.The results showed that active zooplankton collected in freshwater experienced rapid mortality after experiencing changes of salinity in estuarine or marine environments.However,the hatching rates of invertebrates’ resting eggs from freshwater after exposure to brine below 35 psu were not significantly different from that of the controls.Therefore,the related measures of ballast water exchange or saltwater flushing,which are currently widely used for ballast water treatments,might not be effective for resting eggs from freshwater.When increasing the salinity to 100 psu,the hatching rate of resting eggs showed a significant decrease,but failed to achieve complete inactivation.Meanwhile,the longer the exposure time,the lower the hatching rate of resting eggs.In addition,variations in hatching rates were also observed among the resting eggs of the different taxa in this study.These differences could be transferred through ballast sediments and affect their colonization potential.Thus,to minimize the risk of invasion and to achieve more than 90%inactivation of resting eggs of freshwater invertebrates,it might be necessary to add salt to the ballast tanks to expose the sediment to 200 psu of brine for more than 20 days or to saturated brine for more than 10 days.Ships sailing from marine ports can arrive in freshwater or marine ports,but BWE measures are virtually ineffective for individuals carried by such ships.In this study,we conducted brine exposure experiments on marine zooplankton and sediments in the laboratory.The survival of individuals among marine zooplankton in the short term and the hatching rate of invertebrates resting eggs were examined.The results indicated that marine zooplankton were not tolerant to short-term brine exposure.Compared to resting eggs from freshwater sediments,marine resting eggs were apparently more resistant to brine disposal.Hatching appeared to be significantly reduced when exposed to water with different salinity from the original habitat.The longer the exposure time,the better the inhibition effect.The first hatching time also appeared to be delayed to varying degrees.The species,Labidocera rotunda,dominated this trend.Therefore,the onboard ballast treatment measures such as BWE or saltwater flushing were able to substantially reduce the risk of invasion by marine NIS in the form of resting eggs,though it did not completely eliminate it.To completely eliminate the risk of invasion by resting eggs should add salt to expose the sediment at 200 psu brine for more than 10 days or at 265 psu for more than 5 days.The choice of killing organisms in ballast tanks by raising the temperature on board is one of the most effective,environmentally friendly and cost-efficient methods.In this study,thermal exposure experiments were conducted in the laboratory based on the temperature range that could be achieved in ballast water by using waste heat from the ship’s engine or by adding a heat exchanger.The inactivation effect of heating treatment on zooplankton active community and resting eggs from different habitats were investigated.The results revealed that the zooplankton active community was significantly intolerant to the heating treatment.The hatching characteristics of zooplankton and their resting eggs from different habitats were almost consistent after different temperature and time exposures.The hatching rate of resting eggs decreased significantly with increasing temperature and length of exposure,and there was a corresponding delay in first hatching time.The hatching rate of freshwater invertebrates’ resting eggs decreased after exposure at 40℃,but was similar in the treatment groups after more than 2 days of exposure.Incubation rate decreased with extended exposure time as the temperature was raised to 50℃.When exposed at 60℃,no hatching was achieved in 10 minutes.Resting eggs found in marine sediments showed a similar hatching trend.Therefore,the treatment using waste heat on board might be an efficient method to treat ballast sediments,but it needs to heatat 50℃ for 2 hours or 60℃ for 10 minutes to achieve more than 95% inactivation.In conclusion,this study found that brine exposure as an emergency treatment measure might not be sufficient to have a devastating effect on resting eggs hidden in ballast tank bottom sediments,but that thermal exposure could achieve the ultimate goal of complete inactivation of resting organisms.This study provided a reliable evidence for the ballast water and sediment emergency treatment measures to eliminate potential invasion risks in ship ballast sediments during sailing.However,in practice,the high cost and safety of achieving salinities above 200 psu or heating sustained to 60℃ throughout ballast water need to be examined.Therefore,in the future,ballast water and sediment treatment should be further studied and evaluated in the fields on board,and can be considered in combination with other physical or chemical treatment measures to achieve the best results.