Larval Transport and Coastal Upwelling: An Application of HF Radar in Ecological Research

Abstract

b o x . . . F O R MANY MARINE FISH and invertebrate species, near-surface currents strongly affect the likelihood of surviving as a planktonic larva and arriving at a suitable location, say, a rocky coastline, a kelp forest, or an estuary, to begin life as a juvenile or adult (i.e., to recruit to a population). High-frequency (HF) radar is a recent addition to ecologists' remote sensing toolbox that offers the ability to observe oceanographic processes directly affecting larval ecology at scales appropriate for understanding recruitment dynamics in marine populations. In this article, we report on our application of HF radar in research focusing on larval ecology at coastal upwell ing fronts off central California. Coastal upwell ing regions, such as that of f the western United States, pose an interesting set of ecological questions that can be addressed with data f rom HF radar. During active coastal upwelling, the same currents supplying nutrients that support high productivity put planktonic larvae at risk of being swept offshore and away from coastal habitats. Indeed, some fish and invertebrate populations exhibit reduced recruitment success during periods of increased offshore advect ion (Bailey, 1981; Roughgarden et al., 1988). Coastal upwell ing fronts, which form between cold upwelled water near the coast and warmer, fresher surface waters offshore, may reduce offshore transport of planktonic larvae, thereby facilitating nearshore retention and return of larvae to coastal recruitment sites. To test this hypothesis and to assess the utility of HF radar as an ecological tool, we sampled zooplankton along transects spanning coastal upwelling fronts, monitored recruitment to intertidal barnacle populat ions during the upwelling seasons of 1993 and 1994, and compared our data to coincident HF radar observations. We used radial data obtained from a single SeaSonde HF radar deployed at Granite Canyon, CA