This study addresses organic matter decomposition in permeable sediment of a sloping intertidal sand flat (German Wadden Sea) affected by current-induced pore water exchange and pore fluid drainage. Seasonal and spatial scales of aerobic and anaerobic mineralization were investigated at 2 sites, one near the water line and one on the upper flat. Hydrodynamic forcing during inundation caused deeper oxygen penetration through flushing of the uppermost sediment layer. This flushing resulted in higher areal oxygen consumption rates and lower depth integrated sulfate reduction rates in the submerged flat compared to the rates measured during exposure. Mineralization rates in the top 15 cm of the sediment were similar between both study sites and ranged from 38 (winter) to 280 mmol C m -2 d -1 (summer), with sulfate reduction contributing 3 to 25% to total mineralization, depending on the season. At the upper flat, these seasonal differences were reflected in the pore water concentrations of nutrients, dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC). Near the low water line, however, pore water nutrient and DIC concentrations were independent of the season and up to 15 times higher compared to the values recorded in the upper flat. The differences in concentrations of metabolic products between the 2 sites resulted from a low tide drainage extending deep below the uppermost flushed layer and causing seepage of pore water near the low water line. Mineralization and nutrient release in these permeable intertidal sediments is affected by 2 circulation processes that work on distinctly different temporal and spatial scales: (1) rapid 'skin circulation' through the uppermost sediment layer during inundation that is characterized by short flow paths, low pore water residence time and immediate feedback to the ecosystem, and (2) slow 'body circulation' through deeper sediment layers during low tide that is characterized by long flow paths and pore water residence times, and acts as a buffered nutrient source to the ecosystem.

In this article, we describe the dynamics of pH, O 2 and H 2 S in the top 5-10 cm of an intertidal flat consisting of permeable sand. These dynamics were measured at the low water line and higher up the flat and during several seasons. Together with pore water nutrient data, the dynamics confirm that two types of transport act as driving forces for the cycling of elements (Billerbeck et al. 2006b): Fast surface dynamics of pore water chemistry occur only during inundation. Thus, they must be driven by hydraulics (tidal and wave action) and are highly dependent on weather conditions. This was demonstrated clearly by quick variation in oxygen penetration depth: Seeps are active at low tide only, indicating that the pore water flow in them is driven by a pressure head developing at low tide. The seeps are fed by slow transport of pore water over long distances in the deeper sediment. In the seeps, high concentrations of degradation products such as nutrients and sulphide were found, showing them to be the outlets of deep-seated degradation processes. The degradation products appear toxic for bioturbating/ bioirrigating organisms, as a consequence of which, these were absent in the wider seep areas. These two mechanisms driving advection determine oxygen dynamics in these flats, whereas bioirrigation plays a minor role. The deep circulation causes a characteristic distribution of strongly reduced pore water near the low water line and rather more oxidised sediments in the centre of the flats. The two combined transport phenomena determine the fluxes of solutes and gases from the sediment to the surface water and in this way create specific niches for various types of microorganisms.

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Oxygen distribution and benthic mineralization rates were investigated in a permeable intertidal sand flat (permeability: 3.9 3 10 211 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 22 d 21 ). 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 22 d 21 ). 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.

We studied pore water seepage and associated nutrient release in the Janssand intertidal sand flat (North Sea) during exposure at low tide. The hydraulic gradient developing at ebb tide between the pore water level in the elevated sand flat and the water level in the tidal gully generated interstitial water flows toward the seepage zone with velocities ranging from 0.5 (March) to 0.9 cm h-1 (July). Pore water was discharged from an approximately 20 m wide zone near the seaward margin of the flat at rates of 2.4 (March) and 4.2 l m-2 d-1 (July). Nutrient and dissolved inorganic carbon (DIC) concentrations of the seepage water exceeded those measured in the pore water of the upper section of the flat by 10-and 5-fold, respectively. Nutrient effluxes through seepage reached 1100 and 7600 µmol m-2 d-1 for NH 4 , 280 and 2500 µmol m-2 d-1 for PO 4 and 140 and 1700 µmol m-2 d-1 for Si(OH) 4 in March and July, respectively. Benthic flux chambers revealed that nutrients and DIC were released from the still submerged sediment as soon as the ebb tide exposed the upper section of the elevated flat. A conservative estimate based on our measurements suggests that 168 000 l (March) to 294 000 l (July) pore water are discharged each day from the sandy northeastern margin of the Janssand (3.5 km length, 70 000 m 2). Nutrients contained in this water corresponded to 6-25 kg d-1 (90 to 350 mg m-2 d-1) carbon mineralized during March and 42-223 kg d-1 (600 to 3200 mg C m-2 d-1) during July. Our study indicates that the Janssand intertidal flat does not accumulate organic matter but releases mineralization products that can account for all the organic matter that is potentially filtered through the permeable beds during a tidal cycle. Nutrient fluxes associated with seepage exceeded 5-to 8-fold those fluxes caused by the combined effects of diffusion, advection and bioirrigation during inundation, emphasizing the importance of sand flat drainage to the nutrient cycles in the Wadden Sea.