Spatial and temporal patterns of mineralization rates and oxygen distribution in a permeable intertidal sand flat (Sylt, Germany)

Oxygen distribution and benthic mineralization rates were investigated in a permeable intertidal sand flat (permeability: 3.9 X 10(-11) m(2)) in a transect from the low toward the high waterline. At all stations, oxygen penetrated several millimeters to centimeters into the sediments during inundation because of pore-water advection. The wave- and current-driven deep oxygen penetration caused high oxygen consumption rates (OCRs) and high aerobic mineralization rates at all stations. Because oxygen penetration was enhanced during inundation, 71-90% of the daily oxygen consumption took place in that period. OCRs and sulfate reduction rates (SRRs) changed with inundation time of the stations, emphasizing the importance of pore-water advection for benthic mineralization: OCRs were elevated at the lower flat in summer (lower flat 131-187; middle and upper flat 64-108 mmol C m(-2) d(-1)). SRRs increased sharply from the high to the low waterline during all seasons (e.g., in summer: lower flat 18-40; middle flat 8.8-9.4, upper flat 0.5-4 mmol C m(-2) d(-1)). A one-dimensional model of the advective organic matter supply to the sediment could only explain a fraction of the organic carbon required for benthic mineralization. This suggests that either transport of particles via the seawater is more efficient than the one-dimensional concept can explain or that additional carbon was supplied through other sources (e.g., benthic photosynthesis). Mineralization rates were higher in summer than in winter. Only in summer did sulfate reduction contribute > 20% to total mineralization.