High spatial resolution measurement of oxygen consumption rates in permeable sediments

Abstract

A method is presented for the measurement of depth profiles of volumetric oxygen consumption rates in permeable sediments with high spatial resolution. When combined with in situ oxygen microprofiles measured by microsensors, areal rates of aerobic respiration in sediments can be calculated. The method is useful for characterizing sediments exposed to highly dynamic advective water exchange, such as intertidal sandy sediments. The method is based on percolating the sediment in a sampling core with aerated water and monitoring oxygen in the sediment using either an oxygen microelectrode or a planar oxygen optode. The oxygen consumption rates are determined using three approaches: (1) as the initial rate of oxygen decrease measured at discrete points after the percolation is stopped, (2) from oxygen microprofiles measured sequentially after the percolation is stopped, and (3) as a derivative of steady-state oxygen microprofiles measured during a constant percolation of the sediment. The spatial resolution of a typical 3 to 4 cm profile within a measurement time of 1 to 2 h is better with planar optodes (˜0.3 mm) then with microelectrodes (2 to 5 mm), whereas the precision of oxygen consumption rate measurements at individual points is similar (0.1 to 0.5 µmol L–1 min–1) for both sensing methods. The method is consistent with the established methods (interfacial gradients combined with Fick’s law of diffusion, benthic-chambers), when tested on the same sediment sample under identical, diffusion-controlled conditions.