Asymmetric mixing transport: A horizontal transport mechanism for sinking plankton and sediment in tidal flows

A mechanism is described for the net horizontal transport of sinking plankton or sediment by oscillatory tidal flows. The mean motion of the plankton or particles is driven by the modulation of vertical mixing between flood and ebb. Cold water forced over warm water on the flood tide creates enhanced vertical mixing and resuspension of sinking particles higher into the water column. On ebb, the converse occurs, and sinking particles are lower in the water column. Since friction retards the tidal flow near the bottom, this leads to a net horizontal transport toward the less dense water. Because less dense water tends to be shallower, this will tend to move sinking plankton and sediment toward the crest of banks, toward the coast, and up embayments, even in the absence of any mean currents. A one-dimensional Eulerian model of this horizontal transport is developed and is compared to particle motion in a fully nonlinear two-dimensional model with an advanced turbulence closure scheme. The Eulerian model can, in most circumstances, predict the net horizontal motion of the particles as a function of their sinking speed and observable oceanographic properties. The mean horizontal speed of sinking particles is greatest when the sinking speed of the particles is about one-seventieth of the tidal velocities, or equivalently, about one-third of the mean turbulent velocity scale u*. This horizontal transport of plankton and particles requires no behavior more complicated than simple sinking to achieve net horizontal speeds of several kilometers per day for realistic planktonic sinking speeds. It is possible that such a mechanism is a factor in larval recruitment and retention in coastal regions, for example, scallop larvae on Georges Bank.