Impact of flow on oxygen dynamics in photosynthetically active sediments

To assess the influence of boundary flow on interfacial oxygen flux in sediments inhabited by benthic phototrophs, we measured the oxygen distribution and the photosynthetic activity in an intertidal sandy core at flow velocities of 0, 2, 5, 10, and 15 cm s(-1) in a laboratory flume. The impact of flow was large; the effects were reversible and most pronounced in the velocity range of 0 to 5 cm s(-1) With increasing flow velocity, both the maximum oxygen concentration in the sediment and the oxygen penetration decreased significantly The depth of the oxygen concentration maximum was shifted over 1 mm closer towards the sediment surface at the highest flow and the diffusive boundary layer was gradually compressed to a width of 0.2 +/- 0.1 mm. The width of the photosynthetically active sediment layer decreased from 3.2 +/- 0.6 mm in stagnant water to 1.4 +/- 0.3 mm under flow, resulting in an overall reduction of gross photosynthesis. This is explained by a migration of the benthic algae (dominated by pennate diatoms) into deeper sediment layers under flow to avoid resuspension into the water, and thereby impairing photosynthesis. Despite the decrease in photosynthesis, the flux of oxygen into the water column did not change significantly, suggesting that advective processes enhanced the release of oxygen from the sediment under flow conditions. We concluded that boundary layer flow is an important factor controlling photosynthesis and oxygen release in shallow water sediments