The Coastal Boundary Layer: Pattern, Mechanism, and Ecological Effects of Decreased Alongshore Transport
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 has been available regarding transport over the innermost portions of the continental shelf and its impacts on larval distributions and population connectivity. I quantified velocity in nearshore waters at 5 sites along the California coast and investigated characteristics relevant for dispersing larvae. Mean depth-averaged 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) that increase approximately linearly with distance from shore, provides a possible explanation for the observed logarithmic velocity pattern. I incorporated these gradients in alongshore velocity and mixing into a 2-dimensional Lagrangian particle-tracking model to explore effects of the CBL on dispersal and self-retention for a variety of sites and life histories. Incorporating a CBL decreased mean dispersal distances up to 56% and was more profound for shorter pelagic larval durations (PLD) and gentler bathymetric slopes associated with broader CBLs. Most notable is that the presence of a CBL increased self- retention by as much as three orders of magnitude, which indicates that ignoring the reduced velocities in the CBL may overestimate population connectivity. These model results were echoed by measurements of planktonic communities within the field. I measured cross-shore distributions of crustacean larvae within the CBL in northern California (between 250 and 1100 m from shore) and found high larval abundances within the CBL, peaking inshore of the 30 m isobath (1 km from shore). However, abundances decreased substantially in the inner portion of the CBL, and there were distinct larval communities between the nearshore and the rest of the CBL. These patterns persisted across sample dates, suggesting that the spatial structure of nearshore larval communities is robust to changes in physical conditions. High larval abundance within the CBL coupled with the potential of the CBL to reduce alongshore larval transport suggests that larvae may exhibit behaviors that interact with recirculating flow features to elevate local retention. Because of these consequences, CBLs should be considered in future models of coastal transport, as they appear to have large effects on population dynamics of marine species.