The coastal boundary layer: predictable current structure decreases alongshore transport and alters scales of dispersal

• Published in: Marine ecology. Progress series (Halstenbek) 2012-09, Vol.464, p.17-35
• Main Authors: Nickols, K. J., Gaylord, B., Largier, J. L.
• Format: Article
• Language: English
• Subjects: Boundary layers; Coasts; Diffusion coefficient; Eddy viscosity; Flow velocity; Larvae; Marine; Momentum; Shorelines; Terraces; Velocity
• ISSN: 0171-8630; 1616-1599
• DOI: 10.3354/meps09875

Dispersion of planktonic propagules connects shoreline populations of many marine species, and considerable effort has been directed at understanding this process. However, gaps in knowledge persist. In particular, relatively little information is available regarding transport over the innermost portions of the continental shelf. We quantified velocity in nearshore waters at 5 sites along the California coast and investigated flow patterns relevant for dispersing larvae and algal spores. Mean depth-averaged alongshore velocities increased with distance from shore at all sites. This repeated and consistent ‘coastal boundary layer’ (CBL) pattern exhibits a logarithmic profile that resembles that associated with the ‘law of the wall’ of smaller-scale turbulent boundary layers, despite differences in spatial dimension and governing physics. A tentative scaling of dominant terms in an alongshore momentum balance suggests nontrivial levels of lateral stress, but small cross-shore gradients in this quantity. Such a trend of near-constant lateral stress, when combined with simple representations of horizontal mixing (i.e. eddy viscosity parameterizations) that increase approximately linearly with distance from shore, provides a possible explanation for the observed logarithmic velocity pattern. Incorporating these cross-shore gradients in alongshore velocity and cross-shore mixing in a 2-dimensional particle-tracking simulation leads to a decrease in mean alongshore transport of larvae and an increase in the variance in dispersal distance. Most notably, the CBL is predicted to dramatically increase self-replenishment.