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The methods used until now have not been able to reliably resolve O 2 concentrations in oceanic oxygen minimum zones below 1-2 µmol L -1

A new tool ('planar optrodes') for measuring fine scale 2-dimensional O2 distributions in benthic communities is presented and discussed. The sensor consists of an 02-quenchable fluorophore cast onto a 25 X 55 mm, 175 pm thick transparent PVC sheet, which can be introduced into sediments and microbial mats. The O2 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 O2 to be resolved with a spatial resolution of 26 pm. 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 O2 distribution in an intertidal sediment. The obtained distribution correlated well with O2 concentration microprofiles measured with an O2 microelectrode. The data showed pronounced differences in the O2 penetration depths due to patchy distri.bution of h~ghly lablle 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.

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