Title Larval retention , entrainment , and accumulation in the lee of a small headland : Recruitment hot spots along windy coasts Permalink

We surveyed crustacean larvae in the plankton in the lee of Bodega Head, California, for 2 months during late spring and summer and cross-correlated the larval abundance and stage of development of 16 taxa with 8 indicators of upwelling–relaxation conditions. Larvae in all stages of development for 11 of these taxa were entrained and accumulated in a recirculation feature in Bodega Bay, found in the lee of this small headland as soon as upwelling-favorable winds began, and persisted until winds weakened. The remaining taxa were more prevalent during relaxation events when the recirculation feature dissipated and a poleward coastal current transported larvae into and through the bay. During upwelling conditions, a vertically sheared flow occurs in the lee of the headland that accounts for the accumulation of plankton. However, larval behavior did not have a substantial effect on density and persistence in the lee of the headland, regardless of whether taxa were more prevalent during upwelling or relaxation conditions or whether they developed entirely nearshore or migrated offshore later in development. Thus, larvae and postlarvae of many, diverse taxa are concentrated in the plankton in the lee of small headlands in upwelling areas, which is consistent with previously reported recruitment hot spots in small upwelling bays. These headland-related hot spots increase spatial structure in population density and community structure in recruitment-limited upwelling regions and other wind-driven coasts. Larval transport is a key determinant of the dynamics and structure of marine populations and communities. Temporal and spatial variation in upwelling has been proposed to regulate the intensity of settlement and postsettlement interactions among invertebrates along the western margins of continents (Yoshioka 1982; Broitman et al. 2001; Menge et al. 2004). Strong winds coupled with the Coriolis effect drive an offshore flow of waters in the surface boundary layer, known as Ekman transport (Dever et al. 2006). This offshore transport of larvae may limit the number of recruits to coastal communities and subsequent interactions among them in regions of persistent upwelling. Peak recruitment onshore was proposed to occur infrequently and episodically when winds weaken (relax) or reverse, delivering postlarvae (final larval stage before settlement) onshore and poleward alongshore (Farrell et al. 1991; Wing et al. 1995, Connolly et al. 2001). Subsequent studies have shown that most species remain close to shore throughout development (Poulin et al. 2002; Morgan et al. 2009a; Shanks and Shearman 2009) and recruit onshore during upwelling conditions (Mace and Morgan 2006a; Jacinto and Cruz 2008; Morgan et al. 2009b) or when internal tides are active (Pineda 1994; Shanks 2002, 2006). Further, spatial and temporal variation in surf zone hydrodynamics now has been proposed to limit larval recruitment along upwelling coasts (Bustamante and Branch 1996; Rilov et al. 2008; Shanks et al. 2010). Recruitment hot spots occur in otherwise recruitmentlimited regions along upwelling coasts. Recirculation features form in the lee of capes during prevailing equatorward currents that are driven by upwelling-favorable winds (Graham and Largier 1997; Wing et al. 1998; Largier 2004). Larvae that originate in the lee of capes could be retained there and those originating in other areas could be entrained into recirculation features for as long as equatorward winds persist. When prevailing winds weaken, larvae are transported poleward around capes, supplying larvae to recruitment-limited populations along the open coast (Wing et al. 1995, 2003; Lundquist et al. 2000). Recruitment hot spots also occur in the lee of small headlands (Roughan et al. 2005; Mace and Morgan 2006b). A recirculation feature forms in the lee of the small promontory of Bodega Head, California (Roughan et al. 2005), and more invertebrate larvae are recruited there than on the adjacent open coast (Mace and Morgan 2006b). The low-lying headland does not entirely block prevailing winds, so that surface waters still move equatorward, but recirculation occurs at depth (Roughan et al. 2005). The equatorward flow over the shelf separates from the shoreline at the southern end of Bodega Head, forming a shear zone that entrains water from the bay (Fig. 1a). Onshore winds and flow at the southern boundary of the bay form a poleward pressure gradient in the bay (Roughan et al. 2005). The shear zone and poleward pressure gradient together generate cyclonic (anticlockwise) circulation in subsurface waters in the bay (Roughan et al. 2005). Thus, upwelled water enters the bay through the deeper channel in the south and flows northward along the eastern side of the bay, potentially transporting larvae from the outer coast into this recirculation feature (Roughan et al. 2005). In addition, larvae that hatch in the lee of the headland could be retained there as long as upwellingfavorable winds persist. Retention in the recirculation feature may depend on larvae staying in the lower water column or undertaking diel vertical migrations between surface and bottom currents (Roughan et al. 2005). Larvae * Corresponding author: sgmorgan@ucdavis.edu Limnol. Oceanogr. , 56(1), 2011, 161–178 E 2011, by the American Society of Limnology and Oceanography, Inc. doi:10.4319/lo.2011. 56.1. 0161