Biological and physical coupling in the lee of a small headland: contrasting transport mechanisms for crab larvae in an upwelling region

Abstract

Delivery mechanisms for crab postlarvae along upwelling coasts have been inferred from weekly sampling, but more frequent sampling is needed to better relate highly variable oceanographic conditions to postlarval supply. Settlement of 8 crab taxa was measured in Bodega Bay, California every 2 d from 12 May through 3 July, 2002. Abundance of postlarval and juvenile settlers was cross-correlated with physical variables. Four upwelling and 4 relaxation events were recorded as fluctuations in wind stress and sea temperature during this period. Transitions to and from upwelling conditions in Bodega Bay were brief and temperature changes occurred rapidly. The strongest correlations between crab abundance and physical variables indicative of upwelling and relaxation conditions were observed for Cancer magister, which settled primarily during relaxation-favorable conditions, and for Cancer antennarius/productus, which settled primarily during upwelling-favorable conditions, suggesting interspecific differences in delivery of postlarvae to adult habitat. Weak correlations with upwelling-favorable conditions were observed for Pugettia producta/ richii and Pagurus spp. Settlement of Hemigrapsus nudus, Petrolisthes cinctipes, and P. eriomerus exhibited significant correlations with changes in tidal height. These results highlight the existence of multiple taxon-specific delivery mechanisms of closely related taxa in one small geographic region.

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

“…The effect of the boundary current on a diverse larval assemblage was observed three times during typical relaxation conditions, revealing the process behind the 13-yr record of low recruitment by most species during relaxation events-both inside Bodega Bay and off an exposed headland (Wing et al 2003;Mace and Morgan 2006a;Morgan et al 2009a). Our results should apply to the many other small headlands and bays (Morgan et al 2011), as well as straight coastlines in upwelling regions.…”

“…Larval avoidance of the surface lens of warm, lowsalinity water was especially apparent when diel vertical migrations were suppressed. Only larvae of C. magister undertook diel vertical migrations when the boundary current was present, and it is the only crustacean that recruits more during relaxation conditions in some years (Wing et al 2003;Mace and Morgan 2006a). Thus, we have Fig.…”

“…Although larval recruitment has been monitored for 13 yr in a region of strong, persistent upwelling on the west coast of the USA, processes responsible for inter-annual variation in recruitment to coastal populations and communities are still not resolved (Wing et al 2003;Mace and Morgan 2006a;Morgan et al 2009a). Prevailing northwesterly winds force surface waters offshore due to the Coriolis effect (Ekman transport), lowering sea level at the coast and forcing upwelling of bottom waters and southward alongshore transport (Largier et al 1993).…”

“…To test these hypotheses, we repeatedly profiled the vertical and horizontal distributions of nearly the entire larval assemblage of nearshore benthic crustaceans relative to ocean conditions for 2 months during the peak upwelling season, over alternating upwelling and relaxation conditions and diel cycles. We sampled a small embayment where larvae are entrained during upwelling (Roughan et al 2005;Mace and Morgan 2006b;Morgan et al 2011), as well as in the open waters of the adjacent continental shelf, to determine the effect of the boundary current on larval distributions throughout nearshore coastal waters along topographically complex coastlines in upwelling regions.…”

Recruitment of coastal fishes and oceanographic variability in central California

“…This is one of the most important mechanistic hypotheses relating the physical transport of pelagic larvae to the nearshore environment (Roughgarden et al, 1988;Cury and Roy, 1989;Farrell et al, 1991). In most ecological studies of invertebrate recruitment, upwelling dynamics have been examined at relatively fine spatial and temporal scales (Gaines and Bertness, 1992;Wing et al, 1995a;Mace and Morgan, 2006a), from which mechanisms of larval transport could be inferred. However, fisheries studies involving upwelling dynamics commonly associate the strength and timing of upwelling throughout a season with an index of year class strength (Larson et al, 1994;Laidig et al, 2007).…”