Limited recruitment during relaxation events: Larval advection and behavior in an upwelling system

Abstract

We capitalized on a long-term record of larval recruitment and a distinctive oceanographic signature to reveal how changes in ocean conditions affect larval advection, behavior, and recruitment in a region of strong, persistent upwelling and recruitment limitation. We repeatedly sampled the vertical and horizontal distribution of a larval assemblage and ocean conditions during infrequent relaxations of prevailing upwelling winds near Bodega Bay, California. During prolonged relaxation events, a poleward, coastal, boundary current imported low-salinity surface waters that were devoid of larvae to the study area. The resident larval assemblage was restricted to cold, saline, bottom waters and pushed offshore while diel vertical migrations were suppressed. Hence, changes in oceanographic conditions strongly affected behaviorally mediated larval distributions, revealing the reason that few species recruit during relaxation events in this region. During relaxation events in upwelling regions, poleward coastal boundary currents will force larvae offshore throughout the water column at small headlands and in seaward-flowing bottom currents at straight coastlines, but recruitment may decrease markedly only near estuaries where few larvae will arrive in low-salinity surface waters. Targeted profiling of larval assemblages complements widespread monitoring of recruitment from shore and is necessary to determine how changing ocean conditions affect larval distributions, recruitment dynamics, and the connectivity of populations.

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

“…Nevertheless, benthic larvae in these regions are consistently retained near the coast, typically within a few kilometers of shore. These larval distributions are observed along the US west coast Hameed et al, 2018;Morgan and Fisher, 2010;Morgan et al, 2009aMorgan et al, , 2009bMorgan et al, , 2012Morgan, in press;Nickols et al, 2013;Shanks and Shearman, 2009), as well as upwelling regimes off Chile (Bonicelli et al, 2016;Poulin et al, 2002) and the Iberian Peninsula (Bartilotti et al, 2014;dos Santos et al, 2008;Domingues et al, 2012;MartaAlmeida et al, 2006). Larvae may avoid offshore transport by residing subsurface, where currents are weaker or onshore.…”

“…Larvae may avoid offshore transport by residing subsurface, where currents are weaker or onshore. Depth regulation in stratified flow determines interspecific differences in the distances that larvae migrate from shore (Domingues et al, 2012;Marta-Almeida et al, 2006;Miller and Morgan, 2013;Morgan, 2014;Morgan et al, 2009bMorgan et al, , 2012Morgan, in press;Morgan and Fisher, 2010;Poulin et al, 2002;Shanks and Shearman, 2009). Whether and how postlarvae navigate to suitable settlement sites is unknown.…”

Spatial differences in larval abundance within the coastal boundary layer impact supply to shoreline habitats

“…Inner shelf currents, observed previously on the scale of kilometers, are slower than currents farther offshore, have a higher tendency to move poleward (Kaplan et al 2005), and are associated with increases in invertebrate settlement (Wing et al 1995a,b). Benthic crusta cean larvae can be present during both relaxation and upwelling conditions within areas of topographic retention and along the open coast, and many can be retained in areas within 1 to 3 km of the shore via a combination of physical and behavioral mechanisms (Morgan et al 2009b, Morgan & Fisher 2010, Morgan et al 2012). The present study focused on waters within 1 km of the shore.…”

“…Nearshore retention zones arising from topographic effects on coastal circulation have been observed (Graham & Largier 1997, Wing et al 1998, Roughan et al 2005, and are associated with higher larval abundances and settlement (Mace & Morgan 2006, Morgan et al 2009a. Avoidance of surface waters by larvae can favor retention and decrease offshore transport (Morgan et al 2009b,c, Shanks & Shearman 2009, Morgan & Fisher 2010, Morgan et al 2012 and there is mounting evidence that larval concentrations are high close to shore, even in areas of strong upwelling that are traditionally viewed as being recruitment-limited. Here we investigate this phenomenon closer to shore to see if high abundances extend over the inner shelf and inward to the shoreline.…”

Inverse approach to estimating larval dispersal reveals limited population connectivity along 700 km of wave-swept open coast

“…The ability of larvae to regulate transport has been widely underestimated in this region of strong current flow and it is now evident that larvae regulate depth in stratified currents, thereby controlling rates and directions of cross-shore transport [24][25][26][27][28]. Larvae of most nearshore invertebrates complete development in a coastal boundary layer of slower flow close to shore (less than 6 km) by either remaining beneath faster-moving surface currents or ascending only at night (diel vertical migration) after winds have subsided and currents have slowed [26,27,[29][30][31][32]. These behaviours reduce alongshore transport and population connectivity [24,33].…”