Coupling between subtidal prey and consumers along a mesoscale upwelling gradient in the Galápagos Islands

Our understanding of how communities are regulated diminishes with increasing spatial scale. Because oceanographic processes Supply food, transport propagules, and set abiotic conditions across broad areas, they may explain metacommunity and food web functioning on large scales. Here we test for bottom-up manifestations of an oceanographic process, upwelling, through production Of barnacle (Megabalanus spp.) prey to top-down control by whelks, fish, and urchins across a mesoscale (125 kin) gradient of 28-68% upwelling at 12 subtidal rock wall sites in the Galapagos from 2002 to 2005. Multivariate analysis of in situ flow measurements and the Occurrence of the Equatorial Undercurrent (EUC) distinguished weak (WUP), intermediate (IUP), and strong upwelling (SUP) site groups. Barnacle cover and recruitment were significantly higher at SUP than at WUP sites and decreased with depth from 6 to 15 In. High barnacle recruitment was a persistent feature of SUP sites from 2002 to 2004. This recruitment was significantly related to larval concentrations, vertical flow speeds, and to I lesser extent, onshore offshore flow speeds, while growth rates of barnacles were highest at three SUP and one IUP site. Three-dimensional (3-D) flow explained 39% of the variation in growth. Therefore, bottom-up effects of upwelling occurred via prey production as recruitment and growth of barnacles. Vertical flow was the most important component of the upwelling regime predicting these dynamics. Evidence of population level predator-prey coupling was provided by significant linear relationships between densities of the whelk, Hexaplex princeps, and both the cover and recruitment of barnacle prey. In a mesoscale predation intensity experiment 33-100% of barnacles were eaten by Hexaplex and fish, linking bottom-up and top-down effects. Significantly more barnacles were consumed at SUP than at WUP sites. The broader spatial context for upwelling and predator-prey coupling documented here was provided by the topography of the sea floor offshore, its the maximum depth and bottom slope within 15 km of the sites predicted 40% of upwelling duration. Eighty percent of the SUP sites were close to steep slopes and deep (>1100 m) water where upwelling from internal waves and the EUC is likely.