The time-dependent vertical and horizontal distribu-tion of 02-concentration in a photosynthic sediment revealed spatial heterogeneity at a mm scale with "hotspots" of intense photosynthesis and respiration. On a horizontal scale, sediment-water 02-exchange may vary by a factor of 3 and 10 for 02-uptake and 02-release, respectively over distances of 2 mm. Results also show that steady state 02-fluxes are obtained only sever-al hours after large changes in light intensity. Diel meas-urements of 02 and inorganic C exchange rates revealed that anaerobic metabolites (e.g., metallic sul-phides) were immobilised during darkness resulting in an 02-debt. When the sediments were subsequently illu-minated 02-consumption therefore exceeded that needed for carbon remineralisation. Consequently the sediment-water 02-exchange represents an underesti-mate of net photosynthesis during the light period as well as of mineralisation processes in the dark; these processes are more accurately estimated by fluxes of dis-solved inorganic carbon. Such an approach also allow for a more accurate estimate of the coupling between respiration and photosynthesis in the light.
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