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

Recent studies suggest that large-scale Variation in nearshore primary prod uctivity may strongly influence the dynamics of rocky intertidal communitie s. Among the hypothesized effects is a Link between phytoplankton abundance and the growth of barnacles and other suspension feeders. We tested the st rength of this association on the central coast of Oregon, USA, by quantify ing water-column chlorophyll a (chl a) concentration and the growth of 2 in tertidal 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 productivity: Strawberry Hill (SH) and Boiler Bay (BB), Oregon. B, glandula grew more rapidly in wave-exposed areas than wave-protected habitats, pres umably because higher flows delivered more food to passively feeding barnac les. After several months, individuals of both species attained a larger si ze at SH, the site with consistently higher chi a concentrations. Surprisin gly, short-term growth rates only partially matched spatial and temporal va riations in phytoplankton. Growth rates were low during a persistent upwell ing 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 chi a concentrations decreased. More over, parallel increases in growth were observed at BE, despite the fact th at no bloom was recorded at this site. These results imply that factors oth er than phytoplankton contributed to Variation in barnacle growth. Records of barnacle settlement suggest that water-column concentrations of larvae i ncreased dramatically at both sites during the period of high barnacle grow th. Published analyses of stomach contents indicate that zooplankton may be an important food source in other barnacle species. Yet another factor, wa ter 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 zooplank ton, and warmer water temperatures. Incorporating the influence of zooplank ton and water temperature into studies of bottom-up influences may improve our ability to explain variation among intertidal communities.