Nutrient release from an exposed intertidal sand flat

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–2d–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–2d–1for NH₄, 280 and 2500 μmol m–2d–1for PO₄ and 140 and 1700 μmol m–2d–1for 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 m²). Nutrients contained in this water corresponded to 6–25 kg d–1(90 to 350 mg m–2d–1) carbon mineralized during March and 42–223 kg d–1(600 to 3200 mg C m–2d–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.