Climate-driven trends and ecological implications of event-scale upwelling in the California Current System ALISON C. ILES*, TARIK C. GOUHIER*, BRUCE A. MENGE*, JULIA S. STEWART † , ALISON J. HAUPT ‡ andMARGARET C. LYNCH § *Department of Zoology, Oregon State University, Corvallis, OR 97331, USA, †Hopkins Marine Station, Stanford University, Pacific Grove, CA 93950, USA, ‡California Natural Resources Agency, Sacramento, CA 95814, USA, §School of Medicine, University of Virginia, Charlottesville, VA 22908, USA Abstract Eastern boundary current systems are among the most productive and lucrative ecosystems on Earth because they benefit from upwelling currents. Upwelling currents subsidize the base of the coastal food web by bringing deep, cold and nutrient-rich water to the surface. As upwelling is driven by large-scale atmospheric patterns, global climate change has the potential to affect a wide range of significant ecological processes through changes in water chemistry, water temperature, and the transport processes that influence species dispersal and recruitment. We examined long- term trends in the frequency, duration, and strength of continuous upwelling events for the Oregon and California regions of the California Current System in the eastern Pacific Ocean. We then associated event-scale upwelling with up to 21 years of barnacle and mussel recruitment, and water temperature data measured at rocky intertidal field sites along the Oregon coast. Our analyses suggest that upwelling events are changing in ways that are consistent with climate change predictions: upwelling events are becoming less frequent, stronger, and longer in duration. In addition, upwelling events have a quasi-instantaneous and cumulative effect on rocky intertidal water temperatures, with longer events leading to colder temperatures. Longer, more persistent upwelling events were negatively associ- ated with barnacle recruitment but positively associated with mussel recruitment. However, since barnacles facilitate mussel recruitment by providing attachment sites, increased upwelling persistence could have indirect negative impacts on mussel populations. Overall, our results indicate that changes in coastal upwelling that are consistent with climate change predictions are altering the tempo and the mode of environmental forcing in near-shore ecosystems, with potentially severe and discontinuous ramifications for ecosystem structure and functioning. Keywords: California Current System, climate change, coastal ecosystem, environmental forcing, recruitment, rocky intertidal, upwelling Received 10 June 2011; revised version received 6 September 2011 and accepted 16 September 2011 Introduction Eastern boundary current systems, such as the Califor- nia Current System (CCS) in the eastern Pacific Ocean, are among the most productive ecosystems on Earth. Although such regions account for <1% of the ocean surface, they support 20% of global commercial fishery yields (Pauly & Christensen, 1995). The high productiv- ity of these systems is largely dependent upon coastal upwelling, a wind-driven process that promotes the growth of phytoplankton, the base of the coastal food web, by bringing large pulses of deep, nutrient-rich water to the sunlit surface. As the upwelling process is driven by large-scale atmospheric patterns, it is expected to respond to global climate change. In 1990, Andrew Bakun hypothesized that increased concentra- tions of greenhouse gases would drive stronger and more persistent upwelling (Bakun, 1990), a prediction recently confirmed along the coast of California (Garcia- Reyes & Largier, 2010). Coastal ecosystems – and the services they provide – will likely demonstrate a diverse range of significant, complex, and potentially discontinuous responses to changes in the upwelling process (Harley et al., 2006). Our understanding of these responses is critical for successful ecosystem- based management of these important systems. Coastal upwelling occurs when equatorward wind stress along the coast drives surface waters offshore, a phenomenon known as Ekman transport. Surface waters are replaced by subsurface waters that are drawn up from depth along the coast (Huyer, 1983). Periodic reversals of upwelling-favorable winds, termed ‘wind relaxations’, break the upwelling process into a series of upwelling events (Huyer, 1983; Papa- stephanou et al., 2006; Melton et al., 2009). Upwelling events are particularly characteristic off upwelling along the coast of Oregon, with periods of days to Correspondence: Alison C. Iles, tel. + 1 541 737 4565, fax + 1 541 737 0501, e-mail: ilesa@science.oregonstate.edu © 2011 Blackwell Publishing Ltd 783 Global Change Biology (2012) 18, 783–796, doi: 10.1111/j.1365-2486.2011.02567.x