Planar optrodes: A new tool for fine scale measurements of two-dimensional O-2 distribution in benthic communities

A new tool ('planar optrodes') for measuring fine scale 2-dimensional O-2 distributions in benthic communities is presented and discussed. The sensor consists of an O-2-quenchable fluorophore cast onto a 25 x 55 mm, 175 mu m thick transparent PVC sheet, which can be introduced into sediments and microbial mats. The O-2 sensitive fluorescence emitted by the planar optrode was measured with a high resolution CCD camera. A 3-point calibration was performed for each pixel in the acquired images to calculate the 2-dimensional oxygen distribution with high accuracy. Calibrated images covered an area of 13 x 17 mm and the set-up allowed the 2-dimensional distribution of O-2 to be resolved with a spatial resolution of 26 mu m. The sensitivity of the sensor had excellent long-term stability. The planar optrode was used to record the oxygen equilibration between small gas bubbles and surrounding water with a temporal resolution of 20 s. The sensor was also used to measure the vertical and horizontal O-2 distribution in an intertidal sediment. The obtained distribution correlated well with O-2 concentration microprofiles measured with an O-2 microelectrode. The data showed pronounced differences in the O-2 penetration depths due to patchy distribution of highly labile organic carbon and showed differences in thickness of the diffusive boundary layer due to small topographic structures on the sediment surface. Potential applications of the planar optrode are presented and discussed.