Spatial and temporal variation in barnacle growth in a coastal upwelling system

Abstract

Recent studies suggest that large-scale variation in nearshore primary productivity may strongly influence the dynamics of rocky intertidal communities. Among the hypothesized effects is a link between phytoplankton abundance and the growth of barnacles and other suspension feeders. We tested the strength of this association on the central coast of Oregon, USA, by quantifying water- column chlorophyll a (chl a) concentration and the growth of 2 intertidal barnacles (Balanus glandula and Chthamalus dalli). Barnacles were photographed at 13 d intervals on mid-intertidal growth plates attached in wave-exposed and wave-protected habitats at 2 sites differing in nearshore pro- ductivity: Strawberry Hill (SH) and Boiler Bay (BB), Oregon. B. glandula grew more rapidly in wave- exposed areas than wave-protected habitats, presumably because higher flows delivered more food to passively feeding barnacles. After several months, individuals of both species attained a larger size at SH, the site with consistently higher chl a concentrations. Surprisingly, short-term growth rates only partially matched spatial and temporal variations in phytoplankton. Growth rates were low dur- ing a persistent upwelling event, and increased 2- to 3-fold during the subsequent upwelling relax- ation. At SH, this increase coincided with a major phytoplankton bloom, but growth rates remained high well after chl a concentrations decreased. Moreover, parallel increases in growth were observed at BB, despite the fact that no bloom was recorded at this site. These results imply that factors other than phytoplankton contributed to variation in barnacle growth. Records of barnacle settlement sug- gest that water-column concentrations of larvae increased dramatically at both sites during the period of high barnacle growth. Published analyses of stomach contents indicate that zooplankton may be an important food source in other barnacle species. Yet another factor, water temperature, was also elevated during the period of sustained barnacle growth. Increased barnacle growth during the upwelling relaxation may thus have arisen from the combined benefits of more phytoplankton, more zooplankton, and warmer water temperatures. Incorporating the influence of zooplankton and water temperature into studies of bottom-up influences may improve our ability to explain variation among intertidal communities.