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Oxygen fluxes in shallow marine sediments exceeded calculated oxygen fluxes based on molecular diffusion of oxygen. Oxygen fluxes measured with bell jars in a stagnant lagoon, combined with oxygen gradients measured with microelectrodes (bell jar/microelectrode method), resulted in apparent sediment hffusion coefficients 2 to 3 times higher than the molecular coeff~cient. We have developed a new method to calculate the apparent sediment diffusion coefficient from oxygen gradient measurements and independently measured downward-directed oxygen production fluxes, both measured with microelectrodes (microelectrode gradientlproduction method). Diffusion coefficients calculated In this way differed at most 13 % from the apparent sediment diffusion coefficients calculated using the bell jar/microelectrode method. The microelectrode gradient/production method was applied to emerged sediments of the Oosterschelde estuary (SW Netherlands), and apparent diffusion coefficients for these sediments exceeded molecular diffusion by 2 to 5 times. Dark oxygen gradients measured with microelectrodes were used to calculate the oxygen consumption rate of the sediment. The Q l o value for this rate ranged from 1.3 to 2.7. Two types of oxygen consumption rates were distinguished. A light-independent oxygen consumption rate depending on diffusion from the overlying water or air was found, varying between 0.63 to 3.65 mm01 O2 m-2 h-' at different stations. This consumption increased by 12 to 14 % of the gross oxygen production during the emersion period in light, in the presence of benthic microalgal production. A net outflux of maximum 45 mm01 O2 m-2 was found for the entire emersion per~od, whereas over a 24 h period production and consumption of oxygen were in equilibrium. Thus, mineral~zation and respiration processes within the sediment consume most of the oxygen produced by benthic microalgae on a daily basis.

marine ecology. progress series

Apparent sediment diffusion coefficients for oxygen and oxygen consumption rates measured with microelectrodes and bell jars: applications to oxygen budgets in estuarine intertidal sediments (Oosterschelde, SW Netherlands)