Strong copper complexation in an organic-rich estuary: the importance of allochthonous dissolved organic matter

Abstract

River input of allochthonous organic matter dominates the strong complexation capacity of dissolved copper in the organic-rich Cape Fear River (CFR) estuary, North Carolina. This slightly stratified estuary is characterized by conservatively mixed dissolved organic carbon (DOC=200–1200 μM C), high river flow, and low biological productivity. Copper speciation data measured using competitive ligand equilibration–cathodic stripping voltammetry (CLE–CSV with 8-hydroxyquinoline (8-HQ)) for seven Cape Fear estuarine transects revealed that strong (mean detection window α Cu8HQ=10 5.2) Cu-complexing ligands range in concentration from 7 to >200 nM (at fixed K CuL′=10 13.5), are conservatively mixed below the turbidity maximum zone ( S∼5), and exist in substantial excess of dissolved Cu levels (3–25 nM). Strong ligand and DOC concentrations exhibited strong linear correlations among transect samples at DOC concentrations <1000 μM C ( r 2=0.93, p≪0.01) and among all size fractions of ultrafiltered estuarine samples ( r 2=0.94, p≪0.01). A 300 μM C solution of Cape Fear River humic substances isolated using C 18 extraction exhibited a strong Cu ligand concentration of 143 nM (at fixed K CuL′= 10 13.5). Comparison with speciation data from estuarine transects indicates C 18 isolated humics could account for 100% of the typical strong Cu-complexation capacity in the Cape Fear estuary.