Nearshore Distribution of Barnacle and Mussel Larvae and Oceanographic Mechanisms of Onshore Transport and Delivery

Recruitment of larvae from the plankton is an important determinant of community structure in marine systems. In populations of many marine species, recruitment determines the basic demographic parameters of immigration, emigration, and reproduction. Moreover, the effect of recruitment as an "ecological subsidy" can determine the strength of interactions among species and whether populations are limited by recruitment itself, or by competition or predation. For this reason, understanding the transport of larvae is essential for management and conservation.;In Chapter 2, my coauthors and I investigated the vertical and cross-shelf distribution of barnacle and bivalve larvae with a series of paired day/night sampling cruises off the coast of central Chile. Barnacle larvae were generally found close to shore (within 1.5 km), and the cross-shelf distribution of all taxa varied little despite contrasting upwelling conditions. Since current velocities decrease quickly with proximity to shore, larvae distributed in the nearshore are less likely to be dispersed long distances. Further, the consistent cross-shelf distribution of larvae suggests that they are not necessarily swept on- or offshore by upwelling or relaxation. Depth distributions consistent with classical diel vertical migration (DVM; swimming deeper during the day, shallower at night) were found in barnacle nauplii, but not barnacle cyprids or in bivalve larvae. One potential advantage of DVM is that it may limit offshore transport and thereby increase the odds that larvae will reach suitable habitat when they are competent to settle. Another possible benefit of DVM is that it may increase feeding opportunity in shallow water at night when visual predation risk is low, while providing refuge at depth during the day when visual predation in the upper water column is greater.;In Chapter 3, my coauthors and I undertook a large-scale study in northern Monterey Bay, CA that integrated high-frequency physical and biological sampling to allow resolution of multiple different potential mechanisms of onshore larval transport and settlement. Depending on location within Monterey Bay, three processes were found to be associated with onshore barnacle settlement: regional upwelling, local diurnal upwelling driven by afternoon sea breezes, and the passage of an upwelling shadow front. Based on these findings we propose a novel conceptual model that encompasses oceanographic processes at multiple scales and reconciles apparent inconsistencies between empirical results and existing theories.;In Chaper 4, my coauthors and I developed a simple larval transport model using data on currents, offshore flux of barnacle larvae, and onshore settlement of barnacles from the empirical study in Chapter 3. A parameter set was found that produced model settlement correlated with observed settlement. The fit of modeled and observed settlement was sensitive to model parameters. However, for all parameter sets examined, onshore transport of particles was much greater at depth. This result from the larval transport model is consistent with the conceptual model proposed in Chapter 3. Further, during intervals when onshore transport was observed, model particle trajectories clearly show onshore transport only at depth via two of the mechanisms identified in Chapter 3 (local diurnal upwelling and passage of the upwelling shadow front).