Surfzone hydrodynamics as a key determinant of a latitudinal gradient in marine communities
The ability of microscopic larvae to replenish populations in dynamic marine environments has been a long-running topic of debate for regulating marine communities. The discovery of a latitudinal difference in larval recruitment along the west coast of the USA in the 1980s stimulated a resurgence of interest in the effect of oceanographic conditions in establishing community dynamics. Larvae of nearshore species are considered to be especially susceptible to offshore transport in the strong, persistent upwelling conditions off California, limiting onshore recruitment and reducing the intensity of community interactions relative to the weaker upwelling of the Pacific Northwest. Recent critical tests of this paradigm demonstrated that larvae of most species actually complete development in high densities close to shore raising the possibility that spatial variation in surf zone hydrodynamics may primarily determine whether larvae can cross the surf zone to replenish communities. We tested this hypothesis by determining larval recruitment at shores with different surfzone hydrodynamics along the West Coast and comparing entire plankton communities relative to physical conditions inside and outside of the surf zone daily for one month each at two shores with different surf zone hydrodynamics. We investigated how larvae cross the two surfzones using three-dimensional biophysical models.
Recruits and adults of rocky shore invertebrates were orders of magnitude more abundant on dissipative shores in Oregon than reflective shores in California, indicating that different surfzone hydrodynamics may underlie the latitudinal gradient in larval recruitment. Opposite cross-shore distributions of plankton communities occurred at the two intensively studied sites: all plankters were more abundant inside the surf zone at the dissipative shore with high water exchange, and they were more abundant outside the surf zone at the reflective shore with reduced water exchange. Hence, spatial variation in hydrodynamics determines the ability of plankters to enter and remain in the surf zone. Modeled larvae recruited onshore in two ways: benthic streaming near the bottom when winds are calm and wind-driven transport of surface-dwelling larvae to the turbulent surf zone followed by sinking to the bottom boundary layer. Thus, spatial variation in surf zone hydrodynamics may play a key role in regulating larval recruitment to nearshore communities along the West Coast where much of the theory of marine community ecology has been developed.