Plankton accumulation and transport in propagating nonlinear internal fronts

Abstract

Accumulation and transport of plankton in fronts propagating across-shore is a process of considerable ecological importance for many inhabitants of the littoral zone, since it links the offshore larval pool with the juvenile and adult inshore habitat. Several field studies have shown that larval plankton accumulates in fronts, but have failed to give a conclusive proof that effective Lagrangian transport takes place. A few process-oriented numerical studies have lent support to the idea, but the scope of their results is limited by the two-dimensional nature of the flows considered and by the simple model used to account for the behavior of plankton. In this paper, we relax both constraints. We solve the three-dimensional Navier-Stokes equation to compute the time dependent velocity field, and we use an empirically based model for the behavior of plankton. Our results show that accumulation and transport is possible, even for larvae characterized by sustained swimming speeds that are small compared with the speed of propagation of the front. We introduce a simple model to characterize the accumulation along the front, which includes both entrainment and detrainment. The model accurately represents accumulation calculated from the numerical runs, and provide a simple tool to estimate transport under a variety of circumstances. We also investigate the spatial distribution of plankton along and across the front and show that it is very patchy and dependent on the swimming speed of plankton, with important implications for interpreting results from field experiments.

Effect of nearshore surface slicks on meroplankton distribution: role of larval behaviour

“…Romano 1995, Leichter et al 1998. Irrespective of their physical origin, frontal structures that exhibit a foam line or surface slick are characterized by a convergence in surface flow and associated subduction in which upward-swimming or buoyant organisms and objects may accumulate, provided that their upward velocity is faster than the downward movement of water (Franks 1992, Largier 1993, Pineda 1999, Helfrich & Pineda 2003, Shanks & Brink 2005, Scotti & Pineda 2007.…”

Swimming behavior and velocities of barnacle cyprids in a downwelling flume

“…Cross-shelf larval transport could be mediated by physical mechanisms that accumulate planktonic larvae in surface convergent zones that subsequently propagate onshore (Shanks 1983, Helfrich & Pineda 2003, Scotti & Pineda 2007. Convergent zones are associated with physical features such as internal waves (Shanks 1983), internal tidal bore warm fronts (Pineda 1994), upwelling relaxation fronts (Farrell et al 1991), and estuarine fronts (Eggleston et al 1998).…”

“…The effect of vertical buoyancy or swimming capabilities on the accumulation and transport of plankton by propagating fronts has been investigated in laboratory and numerical modeling studies (Helfrich & Pineda 2003, Scotti & Pineda 2007. Particles (and planktonic larvae) retained in surface layers with sufficient buoyancy (or vertical swimming abilities) experienced accumulation and shoreward transport in convergent zones associated with frontal features (Helfrich & Pineda 2003).…”

Behavioral responses of invertebrate larvae to water column cues

“…During the planktonic phase, vertical position is relevant as its regulation can permit larvae to accumulate in, or escape from, vertically sheared flows or fronts. Such flows may facilitate larval transport to nearshore settlement environments [17,18,19,20], or under different conditions, act as a barrier [21,22]. When larvae are preparing to settle, vertical position can be especially important in permitting them to identify and investigate suitable settlement sites (e.g., [23,24,4]).…”

“…Vertical position can be especially important for larvae accumulating in vertically sheared flows or fronts, as such flows may facilitate larval transport to nearshore settlement environments (e.g. [17,18,19,20]), or under different conditions, act as a barrier (e.g. [21,22] [46] reported downward swimming in eight-armed sand dollar plutei in strong shear flows.…”