EFFECT OF OCEAN ACIDIFICATION ON THE SPECIATION OF METALS IN SEAWATER

The continued production of CO₂ from the burning of fossil fuel is estimated to increase the pCO₂ in the atmosphere to ~2000 µatm by the year ~2200. The equilibration of the CO₂ in the atmosphere with surface ocean waters will decrease the pH from the present value 8.1 to 7.5 on the total pH scale. Although a number of workers have examined how this decrease in pH can affect the production and dissolution of calcareous organisms in ocean waters, studies on the effect this has on the speciation of metals have not been widely examined. This decrease in the pH of seawater will decrease the concentrations of OH^- and CO₃^2- in the surface ocean, respectively by as much as 82 and 77%. Since these anions form strong inorganic complexes with divalent and trivalent metals, their decrease will increase the concentration of the free or uncomplexed metals in seawater. This will also increase the thermodynamic and kinetic activity of these ions. Since the ionic form of Cu²⁺ is toxic to some organisms, the lower pH may affect the productivity of phytoplankton in the oceans. A decrease in the concentrations of OH^- and CO₃^2- will also slow down the rate of oxidation of Fe(II) with O₂ and H₂O₂ and increase the solubility of Fe(III). These two effects will make iron more available to phytoplankton. Recent studies, however, have shown the decrease in pH makes it more difficult to take up iron. The reasons are not clear why this is the case. Some possibilities for the causes will be discussed.