Dynamics of coastal meta‐ecosystems: the intermittent upwelling hypothesis and a test in rocky intertidal regions

The intermittent upwelling hypothesis (IUH) predicts that the strength of ecological subsidies, organismal growth responses, and species interactions will vary unimodally along a gradient of upwelling from persistent downwelling to persistent upwelling, with maximal levels at an intermediate or “intermittent” state of upwelling. To test this model, we employed the comparative‐experimental method to investigate these processes at 16–44 wave‐exposed rocky intertidal sites in Oregon, California, and New Zealand, varying in average upwelling and/or downwelling during spring–summer. As predicted by the IUH, ecological subsidies (phytoplankton abundance, prey recruitment rates), prey responses (barnacle colonization, mussel growth), and species interactions (competition rate, predation rate and effects) were unimodally related to upwelling. On average, unimodal relationships with upwelling magnitude explained ∼50% of the variance in the various processes, and unimodal and monotonic positive relationships against an index of intermittency explained ∼37% of the variance. Regressions among the ecological subsidies and species interactions were used to infer potential ecological linkages that underpinned these patterns. Abundance of phytoplankton was associated with increases in rates of barnacle colonization, intensity of competition and predation, and predation effects, and rates of barnacle recruitment were associated with increases in mussel growth, barnacle colonization, and species interactions. Positive effects on interactions were also seen for rates of colonization, competition, predation, and predation effects. Several responses were saturating or exponential, suggestive of threshold effects. These results suggest that the IUH has geographic generality and are also consistent with earlier arguments that bottom‐up effects and propagule subsidies are strongly linked to the dynamics of higher trophic levels, or top‐down effects, as well as to nontrophic interactions. The ∼50% of the variance not explained by upwelling is likely due to more regional‐to‐local influences on the processes examined, and future efforts should focus on incorporating such effects into the IUH.