Accumulation of particles in propagating fronts

Field observations imply that accumulation and advection of invertebrate larvae and other plankton by propagating fronts provides an efficient mechanism for the cross-shelf transport and subsequent coastal settlement of larvae and plankton. Gravity currents propagating into a heavier, uniform-density fluid are known to have near-surface velocities u that exceed the propagation speed c of the gravity current nose. It is shown by simple dam-break laboratory experiments and through corresponding numerical solutions of two-dimensional stratified flow and Lagrangian particle advection that this characteristic of gravity currents can lead to accumulation in the gravity current nose of particles that originate well behind the gravity current head. However, efficient particle accumulation and transport with the speed c only occurs for particles with sufficient vertical buoyancy or vertical swimming capability. Weakly buoyant or neutral particles are swept down and back away from the gravity current head by the circulation in this region. A dam break into an ambient fluid with stratification (two-layered) could result in the transport of some particles that are initially trapped in the forepart of the leading disturbance, an undular bore. However, this situation does not result in significant accumulation of particles at the leading edge of the disturbance. The numerical solutions show that particles that originate in the dense fluid ahead of the gravity current are much less likely to be accumulated at the gravity current nose. A simple scaling criterion for the minimum vertical buoyancy velocity for efficient accumulation and transport of buoyant particles based on the gravity current and particle characteristics is developed and compares favorably with the numerical results. The implications of these results for transport of larvae in coastal environments are discussed. The dynamics of marine benthic invertebrate populations are greatly affected by larval transport. For near-shore populations, larval transport prior to settlement includes at least three processes: cross-shore advection, because larvae are generally very small and cannot control their cross-shore position by swimming alone; larval behavior, because by swimming vertically, larvae might be able to exploit some depth-dependent cross-shore flows; and accumulation by convergent circulation in propagating features, because cross-shore advection of larvae is often dependent on the retention of larvae at the propagating features. Several hypotheses of onshore larval transport, namely surface slicks over linear internal waves (Shanks 1983), internal bore warm fronts (Pineda 1994), and upwelling relaxation fronts (Farrell et al. 1991), invoke concentration of larvae at propagating features. These hypotheses state that larvae become concentrated at convergent features aligned parallel to the coastline and that larval transport occurs as these linear features propagate shoreward. The interplay between transport and accumulation is poorly understood; issues include (1) the dependency of transport on accumulation, and of accumulation on larval behavior, and (2) whether the frontal larvae originate onshore or off