Coastal upwelling, larval recruitment, and the dynamics of upper intertidal barnacle communities

Understanding the mechanisms underlying recruitment variability in marine organisms and the effects of this variability on community dynamics is critical to our understanding of coastal ecosystems. This research addresses: (1) the role of biophysical interactions in generating recruitment variability, and (2) the effects of recruitment variation on marine community dynamics.;Zooplankton distributions across coastal upwelling fronts off central California were examined during the spring and summer of 1993 and 1994. The important influence of upwelling on the larval dynamics of nearshore invertebrates was clearly demonstrated. During upwelling, meroplanktonic larvae were swept offshore by Ekman drift, but their transport was impeded by upwelling fronts. In contrast, many holoplanktonic taxa had their shoreward distributions truncated at these features. This resulted in complementary cross-shelf distributions of holoplankton and meroplankton. Accumulations of some taxa were observed at upwelling fronts. The late-stage larvae of many meroplanktors occurred at greater depths than earlier larval stages. A hypothesis that relies on the deep onshore flow associated with coastal upwelling is presented to explain these observations, and to explain recruitment patterns of intertidal barnacles in central California and the Pacific Northwest.;The effects of different recruitment densities on barnacle community dynamics were examined at four upper intertidal sites off central California. Biweekly recruitment and community development were measured concurrently for two years. In recruitment-limited sites, community composition and dynamics were determined by recruitment patterns, but in recruitment-saturated sites post-recruitment processes more strongly influenced community structure. Where recruitment was low, free space was abundant, recruitment and cover were strongly correlated, competition was weak, and mortality was low. But where recruitment was high, free space was rare, recruitment and cover were poorly correlated, and intense competition produced increased mortality of Balanus glandula and Chthamalus spp. New observations added by this research include: (1) demonstration of the correlation between recruitment and cover at low recruitment sites; (2) a breakdown of the Balanus-Chthamalus competitive hierarchy; (3) evidence that high recruitment, followed by growth and intense density-dependent mortality, is sufficient to produce population cycles; and (4) an examination of the correspondence between recruitment history and present-day size distributions.