Resistance of rocky intertidal communities to oceanic climate fluctuations

For temperature trends, both mean maximum air temperature (Fig 3a) and mean water temperature (Fig 3b) generally increased during the study period. The strongest increases occurred for maximum daily air temperature at Rocky Point, Port Orford Heads, Cape Blanco North, and Yachats Beach (P < 0.01, with R2 = 0.22. 0.12, 0.07, and 0.09, respectively). As reported earlier [12], a period of elevated water temperature occurred during the confluence of the 2014–16 marine heat wave and the 2015–16 El Niño (Fig 3b). As expected, warmer water temperature was correlated to stronger PDO (R2 = 0.523, P = < 0.001), but no other correla- tions of temperatures with climate oscillations were significant. Mean daily water temperatures were similar between capes and between sites (Fig 3b). Maximum daily air temperatures were generally warmer at Cape Blanco that the other capes, and air temperatures were fairly similar among sites within capes (Fig 3a). The exception was Cape Blanco, where maximum daily air temperatures were typically lower at Port Orford Heads and higher at Rocky Point and Cape Blanco South compared to the Cape Blanco North site. Spatial versus temporal community variation Community structure was more strongly associated with spatial (zone, cape and site) than temporal variables (year) (Table 1). As expected, zones were clearly separated visually (Fig 4), as were capes and often sites (Fig 5). However, visual separation among years was much less clear (Fig 6a–6c). Within each zone, cape and site were associated with more community vari-

ation than was year (S3a-S3c Table in S1 Appendix; High: 35.6% vs. 19.9%, Mid: 33.2% vs. 19.4%, and Low: 48.4% vs. 14.7%, for spatial vs. temporal variation, see calculation in methods). Community structure among intertidal zones As indicated above, community structure was strongly associated with intertidal zone, which contributed 55.3% of model fit when analyzing the contributions of zone, cape, site and year (Fig 4, Table 1; PERMANOVA: Zone: p = 0.001). In SIMPER analyses, the strongest drivers of within-zone community similarity (i.e., contributing >5%) for the high zone were bare space, barnacles, herbivorous gastropods, and fucoid algae (Fig 4; S3d Table in S1 Appendix; S1 Fig in S3 Appendix). In the mid zone, mussels, limpets, whelks, bare space, and large barnacles drove community similarity (Fig 4; S3e Table in S1 Appendix; S2 Fig in S3 Appendix). The low zone was typified by bare space, articulated coralline algae, crustose coralline algae, limpets, the kelp Hedophyllum sessile, and fine branching red algae (Fig 4; S3f Table in S1 Appendix; S3 Fig in S3 Appendix). Geographic variation in community structure Taxon abundance often differed by cape and site (Fig 5). Although community structure did not differ by cape in the high zone (Fig 5a; S3a Table in S1 Appendix) and functional group rel- ative abundance was similar among capes (S3d Table in S1 Appendix), community structure differed among sites within capes (Fig 5a; p < 0.007 for all site comparisons). These trends sug- gest that high zone variation was controlled at a localized scale.

In the mid zone, among-cape differences (Fig 5b; S3b Table in S1 Appendix) were driven by separation of Cape Blanco from Capes Foulweather and Perpetua (p = 0.023, p = 0.014, respectively), which did not differ from each other (p = 0.311). As in the high zone, sites dif- fered within capes (Fig 5b; S3b Table in S1 Appendix; p < 0.018 for site comparisons). While dominant mid-zone species were often similar among capes, the strongest contributors to community separation among capes were mussels and limpets at Cape Blanco (Fig 5b; S3e Table in S1 Appendix; 58.9% and 31.2% contribution, respectively), predatory snails and bare PLOS ONE Resistance of rocky intertidal communities PLOS ONE | https://doi.org/10.1371/journal.pone.0297697 May 29, 2024 10 / 34