Limnol. Oceanogr., 45(3), 2000, 534–549

In flume and field experiments we demonstrate that interfacial water flows, generated when bottom currents interact with sea bed topography, provide a fast and efficient pathway for the transport of suspended phytoplankton into subsurface layers of permeable sandy sediments. The advective transport, associated with small mounds and ripples as commonly found on shelf sediments, increased penetration depth of unicellular algae (Dunaliella spec.) into sandy sediment (permeability k 5 4 3 10211 m2) up to a factor of 7 and flux up to a factor of 9 relative to a smooth control sediment. The pore water flow field produced a distinct distribution pattern of particulate organic matter in the sediment with subsurface concentration maxima and zones depleted of algae. Flux chamber simulations of advective transport of algae into sands of different grain sizes revealed increasing fluxes, algal penetration depths, and degradation rates with increasing permeability of the sediment. Two experiments conducted in intertidal sand flats confirmed the importance of the advective interfacial transport of phytoplankton for natural settings, showing permeability-dependent penetration of planktonic algae into embedded sand cores of different grain sizes. The significance of our results is discussed with respect to particulate organic matter flux and mineralization in shelf sands, and we suggest the concept of a decomposition layer. In contrast to muddy sea beds with low permeabilities, where transport of solutes is mainly driven by diffusion, water can flow through marine sands, providing a fast carrier for the exchange of substances between the water column and the upper sediment layers. Surface gravity waves cause pressure oscillations that increase fluid exchange at the sediment–water interface and dispersion of solutes within the bed (Webb and Theodor 1968; Riedl et al. 1972; Harrison et al. 1983). Bottom currents deflected by sediment topography create small horizontal pressure gradients that force water into the bed upstream and downstream of protruding surface structures and draw pore water to the sediment surface where the pressure is lowest (Fig. 1) (Savant et al. 1987; Thibodeaux and Boyle 1987; Huettel and Gust 1992a). Huettel et al. (1996) showed that the interfacial water flows can carry particulate tracers several centimeters into sands. Shells of sea scallops on sandy sediment increase the deposition of diatoms (Pilditch et al. 1998). Permeable sands are most common in coastal environments (Riggs et al. 1996), and relict sands cover approximately 70% of the continental shelves (Emery 1968). In these nearshore waters, high nutrient concentrations boost phytoplankton growth to generate about 30% of the total oceanic primary production in a zone covering less than 10% of the world’s ocean area (Walsh 1988; Wollast 1991). Up to 50% of the organic matter produced in the water column on the shelf is decomposed at the seafloor (Rowe et al. 1988; Wollast 1991; Bacon et al. 1994). Wind, waves, and tidal currents cause deep mixing of the water column, carrying phytoplankton cells to the bottom (Jones et al. 1998). Velocity and turbulence of bottom currents in the 1 Corresponding author ([email protected]).