Resistance of rocky intertidal communities to oceanic climate fluctuations

can successfully detect the ecological consequences of climate oscillations operating on multi- decadal scales (e.g. PDO, NPGO). Increasing evidence suggests that rocky intertidal community structure is already respond- ing to climate change. Smith et al. [66] documented lower diversity in California rocky inter- tidal communities in 2002 compared to the 1960s and 1970s and suggested either a warm phase of PDO or warming global temperatures were responsible. Poleward range shifts of rocky shore species has occurred in northern Europe [67], and the relative dominance of northern versus southern species has shifted in California [68]. Globally, many coastal ecosys- tems have shown sometimes dramatic susceptibility to warming climate [e.g. 69–72]. Here, we examine if annual shifts in intertidal community structure in Oregon and northern California correlate to oceanic environmental variation to identify which, if any, environmental factors may drive community change. By identifying these drivers, we may be better equipped to pre- dict and perhaps mitigate the effects of changing climate on these iconic communities. Approach Apart from the simple hypotheses that rocky shore communities did not (null) or did (alterna- tive) respond to recent changes in ocean climate, our approach was primarily focused on dis- covering patterns. To determine if intertidal community structure changed during a period of intensifying climate change [35], we used a 14-year dataset (2006–2020) taken at 11 sites in Oregon and northern California spanning 294 km (183 miles) of coastline. We investigated

whether these communities were sensitive to yearly variation in PDO, NPGO, El Niño, upwell- ing intensity, upwelling variability, air temperature and water temperature during this time. We also included a time series of sea star wasting disease occurrence in our analysis because it caused coast-wide declines in the abundance of the keystone predator, the sea star Pisaster ochraceus [9], so may have caused strong ecological effects in this system [73,74]. Though wast- ing disease was more severe in warmer locations [75], there is either no or modest evidence that the outbreak was triggered by environmental factors [9,10,76–78]. Because the periodicity of some climate oscillations (e.g., PDO, NPGO) is typically longer (10–30 yr) than most eco- logical data sets, our ability to investigate the full extent of their importance is limited. How- ever, when multidecadal datasets are unavailable, examining the relationships between ecological patterns and yearly variation in these climate indices over several years can provide insight into their potential importance [20,54,62]. We quantified percent cover of sessile taxa and density of mobile taxa yearly in the high, mid and low intertidal. We asked 1) how have intertidal community structure, abundance of focal functional groups, and diversity varied over time? We then investigated sensitivity of these communities to climate variation by asking 2) have the abundance of focal functional groups, rate of community change, or diversity cor- related with yearly fluctuations in PDO, NPGO, El Niño, upwelling intensity, upwelling vari-

ability, air and water temperatures, and sea star wasting disease? Materials and methods Community surveys Starting in 2006, summer (June-August) community surveys were conducted annually at 11 sites on 4 capes (i.e., headlands or regions) from central Oregon to northern California (Fig 1, S1 Table in S1 Appendix). From north to south, Capes included Cape Foulweather (sites Fogarty Creek and Boiler Bay), Cape Perpetua (sites Yachats Beach, Strawberry Hill and Toka- tee Klootchman), Cape Blanco (sites Cape Blanco North and South, Port Orford Heads and Rocky Point), and Cape Mendocino (sites Cape Mendocino North and South). Scientific col- lection permits for this work were granted by the Oregon Department of Fish and Wildlife PLOS ONE Resistance of rocky intertidal communities PLOS ONE | https://doi.org/10.1371/journal.pone.0297697 May 29, 2024 4 / 34