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 5 1.3–1.8 3 10211 m2) 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 m22 d21 and exceeded the solute exchange rates caused by hydrostatic wave pumping (38 L m22 d21) and initial molecular diffusion (corresponding to 10–12 L m22 d21). 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. Physical and biological transport link the biogeochemical processes in the water column and sediment. Whereas molecular diffusion and, locally, also bioturbation are the major transport mechanisms in the cohesive, fine-grained deep-sea deposits (Berner 1980; Aller 1982, 2001), solute transport caused by pore-water flows increases in importance in permeable sandy shelf sediments. Here, boundary layer flows, interacting with sea-bed topography, induce pressure differences at the sediment-water interface that lead to pore-water motion in permeable sediments. The ensuing advective transport can exceed transport by molecular diffusion by several orders of magnitude (Huettel and Webster 2001). In areas where water depth (D) is smaller than half the wavelength (l) of the surface gravity waves, oscillating flows are generated at the sediment-water interface by the wave orbital-water motion (e.g., Denny 1988). Webb and Theodor (1968, 1972) showed, by injecting dyed water into coarse sandy nearshore sediment and observing its reappearance at the sediment surface, that such oscillating boundary flows could drive sediment-water–interfacial fluxes. The trajectories of pore-water particles under a rippled bed over one wave period were calculated by Shum (1992). His results suggested that the zone of advection extends several ripple heights below the sediment surface over a wide range of wave conditions and sediment characteristics. Indications that surface gravity waves may be relevant for the 1 Corresponding author ([email protected]).