Alongshore variation in barnacle populations is determined by surf zone hydrodynamics

Abstract

Abstract. Larvae in the coastal ocean are transported toward shore by a variety of mechanisms. Crossing the surf zone is the last step in a shoreward migration and surf zones may act as semipermeable barriers altering delivery of larvae to the shore. We related variation in the structure of intertidal barnacle populations to surf zone width (surf zone hydrodynamics proxy), wave height, alongshore wind stress (upwelling proxy), solar radiation, and latitude at 40 rocky intertidal sites from San Diego, California to the Olympic Peninsula, Washington. We measured daily settlement and weekly recruitment of barnacles at selected sites and related these measures to surf zone width. Chthamalus density varied inversely with that of Balanus, and the density of Balanus and new recruits was negatively related to solar radiation. Across the region, long-term mean wave height and an indicator of upwelling intensity and frequency did not explain variation in Balanus or new recruit densities. Balanus and new recruit densities, daily settlement, and weekly recruitment were up to three orders of magnitude higher at sites with wide (>50 m), more dissipative surf zones with bathymetric rip currents than at sites with narrow (<50 m) more reflective surf zones. Surf zone width explained 30-50% of the variability in Balanus and new recruit densities. We sampled a subset of sites <5 km apart where coastal hydrodynamics such as upwelling should be very similar. At paired sites with similar surf zone widths, Balanus densities were not different. If surf zone widths at paired sites were dissimilar, Balanus densities, daily settlement, and weekly recruitment were significantly higher at sites with the wider, more dissipative surf zone. The primary drivers of surf zone hydrodynamics are the wave climate and the slope of the shore and these persist over time; therefore site-specific stability in surf zone hydrodynamics should result in stable barnacle population characteristics. Variations in surf zone hydrodynamics appear to play a fundamental role in regulating barnacle populations along the open coast, which, in turn, may have consequences for the entire intertidal community.

Shoreward swimming boosts modeled nearshore larval supply and pelagic connectivity in a coastal upwelling region

“…Whether recruiting larvae enter the surf zone depends on alongshore variation in surfzone hydrodynamics caused by the interaction of breaking waves with coastal morphology. Orders of magnitude more larvae occur in surf zones on gently sloping, rather than steep, shores due to the presence of bathymetric rip currents Shanks et al, 2017). The substantial inconsistencies between our results and the ecologically based dispersal estimates suggest that a higherresolution modeling effort, capable of resolving the additional physical processes in the coastal boundary layer and surf zone, is needed to accurately predict the dispersal of benthic larvae, the corresponding connectivity of benthic subpopulations, and the design of MPAs.…”

Surf zones regulate larval supply and zooplankton subsidies to nearshore communities

“…The absence of rip currents, the reflective nature of the narrow energetic surf zone and weaker undertow resulted in an exchange rate of surf zone water that was consistent with dissipative beaches when alongshore currents were present (Brown ). In a coastwide survey of 40 sites, recruitment was greater at dissipative than intermediate shores with rip currents and least at reflective shores and one dissipative shore that lacked rip currents (Shanks et al in press b ).…”

“…Larvae and other zooplankters may enter the surf zone in near‐surface onshore wind‐driven currents, Stokes drift and internal waves, whereas those near the bottom may be transported shoreward by benthic streaming (Fujimura et al ; Shanks et al ; Navarrete et al ). Larval settlement and planktonic food may be greater on dissipative than reflective shores, and they may be greatest on dissipative and intermediate shores where rip currents increase exchange of water outside and inside the surf zone (Shanks et al in press b ). In turn, the subsidies of larvae and food determine the intensity of top‐down processes in communities (Menge et al ).…”

“…Our surveys of larval recruitment along the west coast of the U.S.A. indicated that spatial variation in surf zone hydrodynamics is a key determinant of the supply of larvae and food for benthic filter‐feeders (Shanks et al , in press b ). The most recent survey included rock benches as well as rocks embedded in beaches, which commonly occur along the West Coast.…”

Nearshore Larval Retention and Cross-Shelf Migration of Benthic Crustaceans at an Upwelling Center

“…Individual based biophysical models revealed that Stokes drift and benthic boundary layer streaming coupled with downward swimming in response to turbulence from waves facilitated onshore larval delivery beaches (Fujimura et al, 2013(Fujimura et al, , 2014(Fujimura et al, , 2017(Fujimura et al, , 2018. The striking differences in surfzone oceanography at dissipative and reflective shores predictably affect larval recruitment and productivity of inshore communities Shanks et al, 2017, in press) and appear to explain recruitment limitation at many sites in California Shanks et al, 2017;. Together with the above results on larval distributions in upwelling areas, this suggests a shift in paradigms from the past idea that upwelling necessarily limits recruitment to new ideas that recruitment limitation and population connectivity may be better explained by alongshore differences in surfzone circulation Morgan, in press).…”