Benthic fluxes of cadmium, copper, nickel, zinc and lead in the coastal environment

Abstract

Fluxes of trace metals across the sediment-water interface were measured in situ at 6 m depth in Gullmarsfjorden, Sweden, using diver-operated stirred benthic flux-chambers. These were equipped so that dissolved oxygen and pH could be maintained near ambient seawater values (regulated chamber) or be allowed to change in response to benthic respiration (unregulated chamber). In the regulated chamber, Cd, Cu, Ni, and Zn were released from the sediment at constant rates both during a winter experiment (water temperature −1 °C) and during a fall experiment (+ 10°C). During the fall experiment, fluxes (in nmol m−2 d−1) of 13 (Cd), 118 (Cu), 209 (Ni), and 1400 (Zn) were measured. In winter, the release rates were lower by factors of 5 and 10 for Cu and Ni but not significantly different for Cd and Zn. Neither release nor uptake by the sediment could be demonstrated for Pb. The pore-water in a diver-collected core was depleted in Cd, Cu, and Zn and slightly enriched in Ni and Pb, relative to the ambient seawater. There was no correspondence between fluxes calculated from porewater profiles and actually measured fluxes; nor could the fluxes be directly related to the degradation rate of organic matter.In the unregulated chamber, initial trace metal release rates were lower than in the regulated chamber. As the oxygen concentration decreased, the metal fluxes decreased as well and were ultimately reversed as sulfide began to appear in the water. The fluxes of trace metals are sensitive to the oxygen regime in the flux chamber because the solubilization of these metals, which takes place in a thin oxic layer near the sediment surface, depends on the oxygen flux across the sediment-water interface.