Date of Award
Doctor of Philosophy (PhD)
John R. Donat
Certain trace metals are important cofactors in enzymatic systems and are thus, essential for life in the world's oceans. Two of these metals, Zn and Cd, are required by phytoplankton for enzymes that facilitate carbon uptake (Morel and Price, 2003). In seawater the total dissolved concentration of a metal (MTD) is distributed among different chemical species and this chemical speciation dictates a metal's bioavailability. Strong organic metal-binding ligands greatly affect the metal's chemical speciation in the ocean, potentially limiting phytoplankton growth by reducing the concentrations of Zn2+ and Cd2+, the bioavailable forms of these two metals to levels below those required for optimal, or even minimal, growth (Ellwood, 2004).
In this study, Zn and Cd concentrations, distributions and chemical speciation in the Equatorial and Western South Atlantic Ocean are discussed. The focus, initially on Zn and Cd and their complexing ligands, shifts from depth profiles to sources to processes to analysis of specific hot spots. ZnTD and CdTD profiles mimic silicate and phosphate. Three separate, metal-specific, complexing ligands for Zn and Cd are reported, thanks to the use of a novel mathematical tool (TDI). Notable features include potential regional influences on the ZnTD, CdTD and ligand distribution exerted on surface waters near the Amazon River plume and on subsurface waters by a hypoxic region in the Equatorial Atlantic.
As of water masses of more widespread distribution, the influence of factors like the Amazon River particles/Equatorial area high productivity on intermediate water masses and the Congo River shelf matter on deep water masses show high ZnTD and CdTD and very high ligand concentrations (L), in contrast to the ZnTD and silicate rich water masses originated near the Southern Ocean. The ZnTD/silicate ratios show the widespread presence of a ZnTD subsurface local maximum, suggesting Zn regeneration linked to grazing. A study of the ligand suggests some of them are related to primary productivity and some are related to grazing. Some ligands are very refractory and others are transported in intermediate and deep water masses as both their concentrations and binding strengths (logK) decrease as they age (estimated residence time ∼ 130 years), suggesting potential global distributions of a consortium of decaying ligands.
The chemical speciation and bioavailability of Zn and Cd are compared with some inferred Co speciation data. This region of the Atlantic Ocean is not prone to limitation due to the sufficient concentration of free Zn2+ in surface waters, in contrast to regions where intermediate and deep waters are to upwell, which could show limitation due to low Zn2+ and Cd2+.
Carrasco, Gonzalo G..
"Concentrations, Distributions and Chemical Speciation of Zinc and Cadmium in the Equatorial and South Atlantic Ocean"
(2010). Doctor of Philosophy (PhD), Dissertation, Ocean/Earth/Atmos Sciences, Old Dominion University, DOI: 10.25777/k7j4-y537