Advective pore-water exchange driven by surface gravity waves and its ecological implications

The effects of surface gravity waves on pore-water release from permeable sediment (k = 1.3-1.8 X 10(-11) m(2)) in shallow water were studied in a wave tank. Our tracer experiments demonstrated that shallow-water waves can increase fluid exchange between sandy sediment and overlying water 50-fold, relative to the exchange by molecular diffusion. The main driving force for this increased exchange are the pressure gradients generated by the interaction of oscillating boundary-flows and sediment wave ripples. These gradients produce a pore-water flow field, with a regular pattern of intrusion and release zones, that migrates with ripple propagation. The ensuing topography-related filtering rates in the wave tank ranged from 60 to 590 L m(-2) d(-1) and exceeded the solute exchange rates caused by hydrostatic wave pumping (38 L m(-2) d(-1)) and initial molecular diffusion (corresponding to 10-12 L m(-2) d(-1)). Wave-induced filtration is ecologically relevant because permeable sandy sediments are very abundant on the continental margins and can be converted into effective filter systems, which suggests that these sediments are sites for rapid mineralization and recycling. We propose that the wave influenced continental shelf may be subdivided,into two zones: a shallow zone (water depth < wavelength/2), where wave orbital motion at the sea floor creates ripples and causes topography related advective filtering; and a deeper zone (wavelength/2 < water depth < wavelength), where wave pumping enhances interfacial exchange by hydrostatic pressure oscillations.