The Effects Of Seawater Carbonate Variation On Key Physiological Activities Of Two Economically Important Molluscs

The seawater carbonate system plays an important role in the global carbon cycle.The surface-ocean pH has decreased 0.1 units since pre-industrial times due to CO₂emissions,and seawater carbonate parameters such as[HCO₃^-],[CO₃^2-]and calcium carbonate saturation have also been changed.Seawater carbonate system in coastal habitats is much more complex and variable due to terrestrial runoff and human activities.This change may affect mariculture especially when superimposed with other environmental factors such as temperature and heavy metals.It is of great ecological significance to study the physiological response of marine economically molluscs to seawater carbonate variables.In this paper,we used physiological index measurement,scanning electron microscopy（SEM）,Raman spectroscopy,etc.to study:1)the physiological response of Pacific abalone Haliotis discus hannai,an economically important species in China,to ocean acidification and warming;2)the physiological response of clam Ruditapes philippinarum,a common intertidal bivalve in China,to ocean acidification and heavy metals（Cu and Cd）;3)the early embryonic development and calcification of Pacific abalone in seawater with different carbonate system.To understand the effect of large-scale mariculture on the carbonate system variations,we conducted five cruises from May to September in the semi-closed Sanggou Bay,which is famous for its large-scale mariculture.This study point out the important role of seawater carbonate system in shellfish physiology and the ability of large-scale mariculture to regulate seawater carbonate system,and clarified the potential of integrated multi-trophic aquaculture（IMTA）model in alleviating ocean acidification.The main results are as follows:1.The effects of seawater acidification and warming on the respiratory of larval Pacific abalone.Oxygen consumption rate（OR）of larvae in the trochophore and veliger stage were measured in dark conditions,in control seawater and acidified seawater（pH_T 7.8 and 7.5）and rapidly raised the seawater temperature from 20℃ rearing temperature to 30℃.The temperature series was 20,22,24,26,28 and 30℃.Warming and acidification both affected larvae respiration significantly.OR of trochophore larvae in control ranged from5.20 to 7.97ng·ind^-1·h^-1and in veliger larvae it ranged from 7.29 to 10.59ng·ind^-1·h^-1during continuous warming.OR of trochophore and veliger larvae declined firstly and rose to the highest level later and declined finally following the warming incubation,the highest value occurs at 28℃both.There is no significant difference in oxygen consumption rates between larvae in control treatment and pH_T 7.8 treatment at 20-24℃,a standard commercial farm temperature range.pH_T7.5 treatment negatively affected respiration of trochophore larvae,and affected respiration of veliger larvae positively in the opposite.2.Physiological response of clams to the acute stress of seawater acidification and heavy metals（Cu and Cd）incubation.We determined the interactive effects of low pH_T（7.7 and 7.3）and the addition of heavy metals（Cu or Cd）on the physiological functions[including oxygen consumption rate（OR）,filtration rate（FR）and heart rate of clams.The experiment was conducted under laboratory conditions with pH_T 8.1 as the control treatment.Addition of Cu at concentrations of 0.06 and 0.60 mg·L^-1 suppressed OR,FR,and heart rate（P<0.05）.The clams’heart rate decreased from 23.8 bpm（beats per minute）to 10.5 bpm after being exposed to 0.06 mg·L^-1 Cu for 20 min.No significant effect of Cd(0.03,0.30 mg·L^-1)was found on OR or FR（P>0.05）,however clams incubated in the 0.30 mg/L Cd group had a significantly higher heart rate（39.0 bpm）than the control（25.2 bpm）（P<0.05）.Low pH_T had no effect on OR and FR（P>0.05）,but clams’heart rate in the pH_T 7.3treatment decreased at the beginning of incubation and recovered rapidly.There was no significant interaction between the pH and heavy metal addition（P>0.05）.3.Early embryonic development of Pacific abalone in response to seawater carbonate variation.We created different levels of pH_T,[HCO₃^-],[CO₃^2-]andΩ_arag through manipulating seawater total alkanity（TA）and dissolved inorganic carbon（DIC）to determine the exact carbonate system component by which growth and calcification of larval Pacific abalone are affected.Our results suggest,[HCO₃^-]and[CO₃^2-]had no effects on development of larvae soft tissues in the trochophore stsge,while low pH_T（<7.7）and higher pCO₂（>1872.80μatm）affected malformation rate and hatching rate respectively.The malformed larvae can be irregular,incomplete or no shell.Shell growth strongly related with[CO₃^2-]in period between trochophore and veliger stage,and the larval shell length and thickness both increased with the increase of[CO₃^2-].pH and[HCO₃^-]had no effects on calcification in this period,but high pCO₂（>1872.80μatm）may do harm to shell structure.4.Calcification of adult Pacific abalone in seawater with low-carbonate system.We changed seawater TA(385.5μmol·kg^-1)to create low-carbonate system seawater which the values of pCO₂,[HCO₃^-],[CO₃^2-]andΩ_arag are very low but keep pH_Tstay at～8.0,and studied whether adult Pacific abalone can calcify or not.A certain size glass slide was inserted into cavity between the mantle and shell,and the Ca CO₃ deposition on the surface of the glass slide was checked by scanning electron microscopy,elemental analysis and Raman spectroscopy.On the 2nd day of breeding,the control and treatment abalones both secreted organic substrate.On the 4th day,the control abalone has created Ca CO₃ and treatment abalone has not.At the end of the experiment（day 8）,aragonite was detected by Raman spectroscopy in control group,and in treatment group it was not.Otherwise the shell of treatment abalone was corroded by seawater.The experimental results indicate that the shell formation of Pacific abalone is extremely dependent on the seawater carbonate system.5.Effect of large-scale kelp and bivalve farming on seawater carbonate system variations in the semi-enclosed Sanggou BayBoth kelp and bivalve farming induced significant spatiotemporal variations in the carbonate system within the bay.When cultured kelp reached its highest biomass in May,the maximumΔDIC,ΔpCO₂ andΔpH_T between the seawater from the kelp farming area and the non-farming outer bay area was-156μmol kg^-1,-102μatm and 0.15 pH units,respectively.However,no significant effect of kelp farming on seawater total alkalinity（TA）was observed.Kelp farming also caused the carbonate system variations of seawater from the bivalve farming area.Bivalve farming significantly reduced seawater TA,indicating that fast deposition of calcium carbonate occurred in the bivalve farming area.Although bivalve respiration released CO₂ into seawater and elevated seawater pCO₂level and reduced seawater pH_T,surprisingly,seawater dissolved inorganic carbon（DIC）reduced significantly in the bivalve farming area.These results indicated that bivalves fixed a larger amount of inorganic carbon by calcification than that released into seawater by respiration.Overall,large-scale kelp and bivalve farming are important biological drivers of variations in the carbonate system within the semi-enclosed Sanggou Bay.Altered carbonate systems by kelp farming may favour calcification of farmed bivalves and provide an essential refuge for these species during the future ocean acidification.