The distribution and speciation of copper across different biogeochemical regimes

2013-10-03Copper (Cu) is an essential micronutrient that functions as a cofactor in many important enzymatically-mediated pathways including denitrification, methane oxidation and ammonia oxidation. Yet it can also be a potent toxicant, inhibiting phytoplankton reproduction and growth rates, at picomolar-level concentrations. In natural waters, over 99.9% of dissolved Cu is complexed by strong organic ligands of biological origin. As a result, concentrations of the bioavailable fraction, Cu²⁺, are often over a thousand-fold lower (~10⁻¹⁴ – 10⁻¹³ mol L⁻¹) than dissolved Cu concentrations. The two main controls on the distribution of dissolved Cu, and Cu²⁺ by extension, are organic complexation and scavenging by particles and biological processes. This thesis examines the distribution and speciation of Cu in the North Atlantic and eastern tropical South Pacific (ETSP) Oceans and in Hood Canal, an estuary in Puget Sound, WA, to better understand how regimes with very distinct biogeochemistries influence Cu cycling. It also explores the relationship between Cu bioavailability and nitrogen cycle processes in the ETSP and Hood Canal, two regimes with high nitrification activity. ❧ Cu speciation was characterized using competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV), and total dissolved Cu concentrations were measured using isotope dilution inductively coupled plasma mass spectrometry (ID-ICP-MS). Overall, results indicate that Cu²⁺ concentrations are maintained at uniformly low levels throughout the water column by strong organic ligands and scavenging, regardless of location, but particularly in areas with high nitrification activity. These processes buffer Cu²⁺ concentrations to a narrow, "steady-state" range that rarely varies by more than two orders of magnitude. Dissolved Cu concentrations were significantly more variable but generally exhibited the profile characteristics identified by other researchers. ❧ In the ETSP, Cu²⁺ levels typically reached their lowest values near the chlorophyll and primary nitrite maxima layers, indicating heavy drawdown by ammonia-oxidizing archaea (AOA) and nitrate-reducing diatoms. Dissolved Cu concentrations in offshore waters, away from the Peruvian coastline, were some of the lowest measured values to date. In the Hood Canal, dissolved Cu levels, though much higher than in open ocean waters, were almost unchanged from over 30 years ago, suggesting that anthropogenic inputs have not greatly affected this system. Cu²⁺ concentrations in the upper water column often approached an experimentally-determined threshold below which ammonia oxidation by AOA may become Cu-limited. Although photoinhibition seems like the more plausible control on nitrification activity, it seems likely that Cu bioavailability also plays a role. Finally, in the North Atlantic Cu²⁺ concentrations remained low through the water column and across the transect, rarely exceeding 10⁻¹³ mol L⁻¹. The lowest concentrations were measured in the nutrient-rich upwelling regime off the Mauritanian coastline while the highest concentrations were measured at depth within the subtropical central gyre region. Dissolved Cu profiles exhibited many unique features and showed clear geographic trends while still mostly conforming to the general properties identified by other researchers in the Atlantic and Pacific Oceans