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

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 αCu8HQ=105.2) Cu-complexing ligands range in concentration from 7 to >200 nM (at fixed KCuL′=1013.5), 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 (r2=0.93, p 0.01) and among all size fractions of ultrafiltered estuarine samples (r2=0.94, p 0.01). A 300 μM C solution of Cape Fear River humic substances isolated using C18 extraction exhibited a strong Cu ligand concentration of 143 nM (at fixed KCuL′= 1013.5). Comparison with speciation data from estuarine transects indicates C18 isolated humics could account for 100% of the typical strong Cu-complexation capacity in the Cape Fear estuary.