In situ study of bromide tracer and oxygen flux in coastal sediments

Laboratory and in situ experiments were performed to assess the use of brom ide as a tracer for in situ studies of benthic solute exchange. Bromide was used in the benthic chamber lander 'Elinor' for flux measurements in coast al sediments of the German Eight, Kiel Bight and Skagerrak (28-700 m water depth). Tracer and total oxygen uptake were monitored simultaneously in the same chamber incubation. Concurrently, in situ oxygen micro-profiles were recorded at the same locations by the profiling lander 'Profilur'. Deployme nt in an anoxic silt (Kiel Eight) confirmed that in the absence of bioturba tion and advection, tracer transport into the sediment was driven solely by molecular diffusion. This flux could be well described by a simple box mod el accounting for molecular diffusion only. In oxic sediments (German Eight and Skagerrak) enhanced exchange of bromide tracer due to bioirrigation pa rallelled enhanced oxygen uptake equivalent to a 4-fold molecular diffusive flux. Our experiments showed that incubations can be short. Depending on i rrigation activity of the fauna, however, incubation length should exceed 3 h in order to provide a useful data base for flux calculations. The method demonstrating caveats is discussed and indicate possible improvements. The results show how the bromide tracer addition can be used as a tool for det ermining solute fluxes exceeding diffusive flux in benthic chamber incubati ons. (C) 1999 Academic Press.