Resistance of rocky intertidal communities to oceanic climate fluctuations

expectations for upwelling are mixed [12,33,34]. Warming of both air and sea temperatures is ongoing and is expected to continue with climate change [12,35]. The oscillations in the metrics above provide a regime of past climactic variability that may help us understand marine community sensitivity or resistance to future climate change. For example, if communities were notably sensitive to environmental fluctuations over past decades, we may assume that communities will respond readily to climate change in the near future [36,37]. Conversely, if communities remain unchanged despite repeated climate fluctu- ations or perturbations, we may assume that these communities are more resistant [38]. While this line of inquiry may provide some insight for the near future, we admittedly do not know whether the relationships between increasing climate stress and community structure are lin- ear. For example, communities may exhibit tipping points [39–41] where even gradual envi- ronmental change can lead to sudden community responses. Also, climate-related, prominent drivers of community change such as extreme events like heat waves, disease outbreaks, and natural disasters are clearly increasing in frequency [42–45]. Though predicting the effects of such episodic events is challenging, we may be able to infer whether communities will be responsive to future climate changes by testing their sensitivity to past variation in their envi- ronment. These include responses to extreme events in the CCLME, like the exceptionally

strong El Niño event in 2015–2016 [46] and the marine heat wave dubbed ‘the blob’ that hit the Northeast Pacific in 2014–2015 [47]. Sensitivity of processes and communities to oceanic climate variation Along the West Coast of North America, spatial differences in average oceanic conditions are known to be important drivers of intertidal community structure [25,48–53]. For example, community structure clearly varies along this coastline, and this has been attributed to several environmental drivers including temperature (air and sea), nutrient concentrations, upwell- ing, upwelling variation, precipitation, and tidal range [48,50,53]. Clearly, the average oceanic climate can have a strong influence on intertidal communities in different places, but here we focus on whether interannual fluctuations in oceanic conditions relate to community structure at a given location over time. Overall, we are interested whether intertidal communities are sensitive to interannual oceanic climate fluctuations, or instead remain consistent despite such changes. Recent evidence suggests that multiple ecological processes at population and organismal levels are sensitive to oceanic variation along the Oregon California coasts. For example, monthly mussel recruitment tended to vary with NPGO and upwelling but not PDO or ENSO [36,54]. Barnacle recruitment also appears to be sensitive to upwelling [34,55,56], but not in all cases [57–59]. However, order-of-magnitude increases in mussel recruitment associated with

changing climate regime in the early 2000s [20] did not translate into changes in community structure [20,36]. Further, while mussel recruitment was sensitive to environmental condi- tions, mussel cover was not, suggesting a decoupling between recruitment and community structure dynamics [60]. Oceanic climate can also affect growth rates and physiology of intertidal species. Higher phytoplankton abundance and warmer water during warm-phase PDO and El Niño years cor- related with increased growth rates of mussels [61,62]. The kelp Egregia menziesii grew faster in a strongly upwelled region north of Point Conception, California than it did in the weakly upwelled region south of Point Conception [63]. Changes in seasonal and yearly upwelling, nutrients, water temperature, ENSO and PDO correlated to changes in growth rates and Car- bon:Nitrogen ratios in multiple dominant macrophyte species along the Oregon Coast [64,65]. The studies above also highlight how studies lasting just several years to one or two decades PLOS ONE Resistance of rocky intertidal communities PLOS ONE | https://doi.org/10.1371/journal.pone.0297697 May 29, 2024 3 / 34