Benthic primary production and O2-CO2 dynamics in a shallow-water sediment:: Spatial and temporal heterogeneity

The time-dependent vertical and horizontal distribution of O-2-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 O-2-exchange may vary by a factor of 3 and 10 for O-2-uptake and O-2- release, respectively over distances of 2 mm. Results also show that steady state Op-fluxes are obtained only several hours after large changes in light intensity. Diel measurements of O-2 and inorganic C exchange rates revealed that anaerobic metabolites (e.g., metallic sulphides) were immobilised during darkness resulting in an O-2-debt. When the sediments were subsequently illuminated O-2-consumption therefore exceeded that needed for carbon remineralisation. Consequently the sediment-water O-2-exchange represents an underestimate 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 dissolved inorganic carbon. Such all approach also allow for a more accurate estimate of the coupling between respiration and photosynthesis in the light.