Characterization of Copper-Binding Ligands and Copper Speciation in Open Ocean and Coastal Marine Systems using Electrochemical Methods
This dissertation explores the role that organic ligands play in controlling copper speciation in a range of oceanic and coastal environments, using voltammetric methods. Chapter 1 details the analysis of samples collected on the 2013 U.S. GEOTRACES cruise in the Equatorial Pacific from Peru to Tahiti (GP16). This represents one of the largest copper speciation datasets to date, covering 22 full ocean depth profiles. Results demonstrate that copper ligands follow a similar pattern to dissolved copper of increasing concentrations with increasing depth. Free copper ion concentrations ([Cu2+]) also increase with depth. This dataset provides insight into the impact of large-scale ocean circulation and water mass aging with respect to copper speciation, with increasing Cu2+ and decreasing excess ligand concentrations in the oldest waters of the deep Pacific.
In Chapter 2, copper speciation from select samples in the GP16 data set is analyzed via a novel processing protocol that enables competitive ligand exchange titrations at multiple competition strengths to be analyzed as a unified dataset. This method utilizes publicly available processing tools to determine a unified sensitivity for the data set as a whole. In this way the full spectrum of the copper-binding ligand pool can be most accurately characterized at a range of analytical windows, revealing both weak and strong ligands. For GP16 samples, this analysis reveals two classes of copper-binding ligands present throughout the oceanic water column, with the weaker L2 ligands present at higher concentrations. Strong L1 ligands are present at or above dissolved copper concentrations, dominating copper speciation especially in surface waters where L1 concentrations exceed that of dissolved copper.
Copper binding ligands play a vital role in maintaining low concentrations of the Cu2+ ion in coastal environments, which helps buffer such systems from copper toxicity. In Chapter 3, the processing method developed in Chapter 2 is applied to an assessment of the ligand pool and free copper ion levels in San Diego Bay, an impacted urban environment with high concentrations of recreational watercraft coated with copper-based antifouling paint. Sampling campaigns at 7 locations within the Bay took place in Summer 2015, Spring 2016 and Fall 2016. Samples for total dissolved copper and copper speciation were collected, as well as samples for Synechococcus concentrations as a biological indicator of copper toxicity. Results from the three samplings show elevated levels (30 –120 nM) of dissolved copper throughout the Bay and a ligand pool capable of buffering most of the locations within the Bay from reaching toxic levels of the free copper ion. Shelter Island Yacht Basin, however, was consistently found to contain toxic levels of Cu2+, results that show a relationship with the lowest Synechococcus concentrations in two out of the three samplings. Findings of this study are relevant for ongoing remediation and regulation efforts in San Diego Bay.