Resistance of rocky intertidal communities to oceanic climate fluctuations

early years. The response to SSWD was not the primary focus of the present study but was included because it was a major event that was concurrent with the focal climate fluctuations. A more extensive investigation of this SSWD event will be reported elsewhere. Averaging across sites and capes, mobile gastropods including limpets (Lottia species), her- bivorous snails (primarily Littorina spp. and Tegula funebralis) and predatory snails (primarily Nucella ostrina and N. canaliculata) were quite variable over time (S4, S5 and S6 Figs in S3 Appendix). Some trends were notable (i.e. bivariate trends in Fig 8 showing an R2 of >0.10, in Table 3b), including increased limpets with windier NPGO in the high and mid zones, increased limpets with windier NPGO and higher upwelling variability in the high zone, and decreased predatory snails with warmer air temperatures in the mid zone. Limpets and whelks are relatively small and short-lived (e.g., 2–5 yr; [120–122]) so are likely susceptible to environ- mental stress. Thus, these species may be indicators of climate change, suggesting that the modest patterns noted above warrant further investigation. We were surprised by the lack of responses of algal taxa to climate fluctuations, since they are primary producers that should conceivably be sensitive to variation in nutrients, tempera- ture and environmental stress [123]. In particular, we expected kelps to be responsive to oce- anic climate since others have noted negative responses to El Niño and temperature [64,124].

Further, subtidal bull kelp (Nereocystis luetkeana) populations in Oregon and northern Cali- fornia collapsed just adjacent to several of our sites due to a combination of warm water and sea urchin overgrazing [75,125]. However, despite the clear effects of climate on nearby subti- dal kelps, the intertidal kelps (primarily Hedophyllum sessile) in our study were markedly con- sistent. This may be linked to their ability to to perennate and regrow from their holdfasts each year [108] unlike the nearby subtidal canopy-forming kelps [126]. The only notable trends in algal/plant taxa were an increase in the mid-to high zone species Endocladia muricata in the mid zone only during years of high air temperature (Fig 8, Table 3b), which could indicate a seaward shift in hot years. Surfgrass (Phyllospadix scouleri and P. serrulatus) abundance also increased with upwelling intermittency in the low zone (Fig 9, Table 3c), which may be linked to higher nutrients in the water during these years [16]. Limited inferences for long-term stability and the potential for imminent change It is possible that the 14-year duration of our study may have been too short to detect climate- induced change, especially in relation to the longer-period climate cycles like PDO and NPGO (20–30 year and 7–15 year cycles, respectively). For example, the declines in mussel cover and increases in algal cover referred to earlier occurred over 20 years [54], and a ~30 year study recently found that west coast intertidal communities were shifting northward at about 3.5–5

km/yr [98]. While it is certainly true that further decades of study are necessary before we can make concrete determinations, several other studies performed at similar decade-long time- scales have detected sensitivity of ecological processes to oceanic climate fluctuations in this system during this period [20,36,54,62]. This suggests our approach is robust, and if strong and immediate effects of these cycles were occurring, we should have been able to detect them. On the other hand, it is likely that there are responses to climate change occurring in this system that we were not able to detect with the indicators of annual community structure, functional group abundance, and diversity we measured here. For example, our plots were not fixed in space, so we were unable to detect small changes in cover of any given species over time. We also did not focus on rarer species that may be responding to climate change. Also, unlike latitudinally-focused studies that have detected range shifts over dozens or hundreds of intertidal sites [67,98,99] our 14-site study was well within the range edges of most of our PLOS ONE Resistance of rocky intertidal communities PLOS ONE | https://doi.org/10.1371/journal.pone.0297697 May 29, 2024 26 / 34