Larval transport from the pelagic environment to inner shelf benthic habitats is a crucially important step for marine species with bipartite life cycles, yet our understanding of the underlying mechanisms remains limited. Several have been proposed, and are supported by field studies, but as yet no consensus exists on how to integrate these into a general conceptual framework. Here, based on the results of an extensive and intensive field campaign, we propose a new conceptual model of larval transport and settlement that incorporates oceanographic processes at multiple scales. The study was conducted during the summer upwelling season of 2007, and focused on linking the physical processes involved in larval transport and delivery in northern Monterey Bay to the ecological outcome of settlement. In conjunction with the collection of continuous nearshore physical oceanographic data, offshore flux of barnacle larvae and intertidal barnacle settlement were monitored at 1- and 2-day intervals at 4 cross-shore transects arrayed along a 16-km stretch of coastline extending north of Santa Cruz, CA during the entire summer upwelling season (May through September).
Conditions during the summer period were typical of strong upwelling. Depending on location within the bay, settlement of barnacle larvae (Balanus spp. and Chthamalus spp.) was linked to a combination of regional upwelling, local upwelling driven by a diurnal sea breeze, and the passage of a cross-shore upwelling shadow front. Temperatures across the front changed by as much as 5° C within a few meters. Settlement at the northern sites (BDB and SHB) was higher during periods when the upwelling shadow front crossed the site. At the two central sites (SHB and TPT) settlement was associated with regional upwelling, while at the two sites generally east of the front (TPT and LHP) settlement was greater during diurnal upwelling. Abundances of adult intertidal barnacles were consistent with observed settlement patterns suggesting that nearshore oceanographic regimes can affect community structure at the scale of kilometers. Our results, as well as those of previous research, are consistent with our newly proposed conceptual model.