Surficial and deep pore water circulation governs spatial and temporal scales of nutrient recycling in intertidal sand flat sediment

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.