U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993, Water-Resources Investigations Report 94-4015

Distributions and Benthic Flux of Dissolved Sulfides in the Oxic Water Column of San Francisco Bay, California

Abstract

Chemical speciation affects the bioavailability of metals and organic ligands to planktonic and benthic organisms, as well as the partitioning of these solutes between phases. Our previous work in San Francisco Bay indicated that sulfide complexation with metals can be of particular importance because of the thermodynamic stability of these complexes. Although the water column of the Bay is consistently well-oxygenated and typically unstratified with respect to dissolved oxygen, the kinetics of sulfide oxidation could exert at least transient controls on metal speciation. Our initial data on dissolved sulfides in the main channel of both the northern and southern components of the Bay consistently indicate submicromolar concentrations (from <1 to 162 nanomolar) as one would expect in an oxidizing environment. However, chemical-speciation calculations over the range of observed sulfide concentrations indicate that these trace concentrations in the Bay water column can markedly affect chemical speciation of ecologically significant trace metals, such as cadmium, copper and zinc. In contrast to longitudinal concentration gradients for dissolved organic carbon, dissolved sulfide concentrations were typically elevated at depth suggesting a primary benthic source. Subsequent flux core and in-situ flux chamber experiments support this hypothesis.