California Sea Grant College Program

Identifying where and how larvae accumulate in space and time is key to understanding the dynamics of benthic marine populations and communities and for locating marine reserves, particularly in recruitment-limited upwelling regions. The interaction of alongshore coastal currents with large headlands has been shown to increase the accumulation of planktonic organisms through the formation of headland eddies. Larval settlement of benthic invertebrates and circulation around Bodega Head, a small headland in an upwelling region, was investigated to determine if similar processes are at work around both large and small headlands. During the upwelling seasons of 2000 through 2004, invertebrate settlement rates and physical variables, including wind, current, and water properties, were monitored to identify spatial and temporal settlement patterns, circulation features, and larval transport mechanisms. A larval accumulation zone was identified in the lee of the headland where crabs, mussels, and barnacles settled in greater abundance than along the exposed coast. Oceanographic sampling revealed a sub-surface recirculation feature in the lee of Bodega Head. This recirculation feature strengthened as upwelling increased providing a mechanism for larvae to accumulate in an area thought to have extensive offshore transport. During relaxation this recirculation feature weakened allowing the potential export of larvae to adjacent habitats. High frequency sampling revealed interspecific differences in larval transport mechanisms. Strongest correlations with physical variables that are 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. Further, C. magister and C. antennarius/productus settled at different depths and exhibited different spatial distributions, suggesting that these two congeners employ different strategies for returning to adult habitat. Non-cancrid crab settlement was correlated with tidal periodicity, indicating the importance of multiple taxon-specific delivery mechanisms. We suggest that both small and large headlands predictably accumulate larvae and are particularly effective during periods of strong upwelling. Thus larval accumulation zones should be included in networks of marine reserves in upwelling regions where advection of larvae may limit recruitment to adult populations.