The effect of ocean acidification on the organic complexation of iron and copper

Abstract

Trace metal biogeochemistry is projected to be affected by ocean acidification. The understanding of the effects is of particular interest, as trace metals such as Fe and Cu are known for their significant biological roles. Dissolved Fe and Cu in seawater occur predominantly in the form of metal organic complexes. This thesis has investigated the influence that ocean acidification may have on the chemical forms (speciation) of dissolved Fe and Cu in seawater, focusing on their organic complexation, in order to provide a first insight into the possible future changes in their cycling and bioavailability. Competitive Ligand Exchange Adsorptive Cathodic Stripping Voltammetry (CLE-ACSV) was used as an analytical technique throughout this PhD work to determine Fe and Cu organic binding ligand characteristics. Organic Fe complexation was determined at current seawater pH in the high latitude North Atlantic (HLNA), which is an area of climate interest due to the importance of iron limitation on phytoplankton productivity and hence the carbon cycle. Main findings indicate that iron biogeochemistry in surface and subsurface waters of the HLNA is controlled by a combination of phytoplankton iron uptake and microbial iron binding ligand production, whilst in deep waters ligand saturation was evident, suggesting that additional Fe would be removed by scavenging or precipitation. The effect of ocean acidification on organic complexation of Fe and Cu was determined in the northwest European shelf seas, during the first UK Ocean Acidification consortium programme research cruise. Results suggested that a decrease in surface ocean pH will potentially result in a reduction of the free and inorganic metal fraction (Fe'), and also in an increase in the organically complexed iron fraction. Direct impacts on Fe bioavailability, however, are difficult to quantify, as the overall iron solubility, and hence its bioavailability, is controlled by the interrelationship between inorganic solubility, organic complexation, redox chemistry, and phytoplankton-trace metal feedback mechanisms. No significant effects were observed of a decrease in pH on the organically complexed Cu (II) fraction, or on the overall free and inorganically bound fraction (Cu'). Consequently, it is not clear so far whether Cu ligand production will be affected by ocean acidification, or the possible effects on its toxicity. In addition, surface water trace metal distribution in the northwest European shelf seas was assessed. Dissolved metal concentrations of Cd, Cu, Fe, Ni and Zn appeared to be significantly influenced by riverine inputs in the study area; whereas surface seawater pH was not evident as a controlling factor. The diversity of the chemical and biological processes controlling Fe and Cu biogeochemistry, and the way in which they will be altered by ocean acidification, is likely to be complex.

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