Pore water transport and microbial activity in intertidal Wadden Sea sediments

Pore water transport and its implications for mineralization and nutrient release from the sediment were studied in intertidal permeable sands of the German Wadden Sea. During ebb tide, the hydraulic gradient developing between the pore water level and the faster dropping sea water level caused drainage and seepage of pore water from the sediment near the low water line. As a result, pore water nutrient and DIC concentrations were independent of the season and up to 15-times higher than at the upper flat. Nutrient fluxes associated with the seepage exceeded 5 to 8-fold those fluxes caused by the combined effects of diffusion, advection and bioirrigation during inundation and may enhance primary production in the Wadden Sea. The drainage affects deep sediment layers and is characterized by long flow paths and pore water residence times. This 'body circulation' is only active during low tide and can act as buffered nutrient source to the ecosystem.During inundation of the tidal flat, oxygen penetrated deeper into the sediment than during exposure, driven by hydrodynamic forcing. Oxygen consumption rates were high and sulfate reduction contributed 3-25 % to total mineralization. In another intertidal sand flat, oxygen consumption and sulfate reduction rates decreased from the low water line towards the upper flat and were closely linked to inundation time. The advectively flushed surface layer of the sediment is characterized by short flow paths and low pore water residence time. An immediate feedback of benthic mineralization to the ecosystem can be provided by this 'skin circulation' during inundation. Intertidal sands were also shown to support relatively high benthic primary production while submersed. Gross photosynthesis was on average 4 and 11 times higher in the net autotrophic fine and coarse sand than in the net heterotrophic mud, despite higher chlorophyll content in the mud. Light limitation was less severe at the sandy sites, and two to three times more light was available to the microalgae in the sands than in the mud. The advective flushing of the permeable sands removed chlorophyll decomposition products and may have enhanced benthic photosynthesis by counteracting a possible CO2 limitation of the microalgae.