A combined sensor for simultaneous high resolution 2-D imaging of oxygen and trace metals fluxes

A new sandwich sensor, consisting of an O-2 planar optode overlain by a thin (90 mu m) DGT layer is presented. This sensor can simultaneously resolve 2-D O-2 dynamics and trace metal fluxes in benthic substrates at a high spatial resolution. The DGT layer accumulates metals on a small particle size (0.2 mu m) chelating resin and records the locally induced trace metal flux during the deployment, whereas the planar optode resolves the O-2 dynamic in near real time at the same location in the sediment. Despite its ultrathin composition, the DGT layer has high carrying capacity for trace metals with no saturation problems during application to typical coastal-or contaminated sediments. Combined with laser ablation, accumulated metal fluxes could be resolved at a resolution of similar to 200 mu m, whereas the O-2 images had a resolution of similar to 100 mu m. A 2-D diffusion-reaction model showed that the enhanced smearing and reduced response time of the O-2 signal associated with the additional DGT layer were marginal. To test sensor performance at realistic conditions, it was applied to an artificial burrow system consisting of permeable dialysis tubing flushed with oxygenated seawater. The measurements demonstrated localized mobilization of Ni, Cu, and Pb close to the burrow wall, where O-2 was elevated. The latter was also confirmed for Cu and Pb in natural sediments irrigated by the polychaete Hediste diversicolor. The sandwich sensor has great potential for investigating interrelations between O-2 d ynamics and metal mobilization in complex benthic systems such as burrows, rhizospheres, permeable sands, and microbial mats.