Copper ion activity and concentrations of voltammetrically defined dissolved, surface bound, and tightly bound colloidal copper in samples from San Diego Bay were measured independently using potentiometric and stripping voltammetric methods, respectively. 'Free' copper, calculated from activity, was approximately three orders of magnitude less than the concentrations of the dissolved, surface bound and tightly bound copper fractions (indicating a high degree of complexation) and over-all, was well correlated to them, suggesting a common, heterogeneous, copper matrix, of probable organic nature, whose distribution is controlled by physical mixing. This finding supports the aggregate or 'onion' model of copper speciation in seawater. Within the bay, we also encountered environments in which copper activity was not directly correlated to concentration and we attribute this to local production of organic ligands by microalgae.