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Dramatic changes in rocky intertidal communities associated with oceanographic regime shifts.
Menge, Bruce1, 1 Department of Zoology, Corvallis, OR, USA
ABSTRACT- Marine community theory predicts that community structure and dynamics should vary both with rates of propagule input (larval transport model or LTM) and food (phytoplankton and detritus) subsidies (bottom-up model or BUM). Both models predict that with increases in subsidies, rates of recruitment, predator abundance, and predator effect should all increase. They make opposite predictions for prey abundance, which increases with subsidies in the BUM but decreases with more recruitment in the LTM. Because larval transport rates and food subsidies in coastal habitats vary over tens to thousands of km, tests of such models have lagged. Recent studies in New Zealand are generally consistent with both models, except that contrary to the LTM prey abundance increases rather than decreases with recruitment. Studies on the US west coast, in concert with a recent oceanographic regime shift, provide an opportunity to examine these predictions across appropriate scales of space and time. The Pacific Decadal Oscillation (PDO) is a large scale (Pacific Ocean Basin) long-term (25 year) cycle of change in surface temperature. In about 1998, the Northeast Pacific shifted from a warm phase to a cold phase. Long-term studies of intertidal community dynamics underway since 1989 (Oregon) and 1995 (California) have revealed major changes in several factors. In Oregon, but not California, upwelling intensity increased, average water temperature decreased, and concentration of phytoplankton, recruitment of mussels, and abundance of mussels on the shore all increased. Prior research suggests that the increase in phytoplankton results from higher nutrient inputs due to more intense upwelling, and that this higher level of food supply enhances mussel larval growth and survival. Observed changes to date are consistent with all predictions of the bottom-up model and, again excepting prey abundance, the larval transport model. Ongoing investigation through the PISCO project will permit a detailed mechanistic understanding of the ecological consequences of these changes.
Key words: marine community theory, rocky intertidal, Pacific Decadal Oscillation, coastal ecosystem dynamics