Resistance of rocky intertidal communities to oceanic climate fluctuations

differences were not readily apparent in nMDS visualizations (Fig 6a–6c) and the temporal patterns varied with site in all zones and with cape in the low zone (S3a-S3c Table in S1 Appen- dix). In the high zone, only 2007 differed from each of the other years (p < 0.005), but this was because only Boiler Bay was sampled that year. In the mid zone, only 2012 differed from 2017 and 2018 (p = 0.038 and 0.044, respectively). On the other hand, in the low zone several years differed from one another, particularly 2006, 2008, and 2019 compared to other years. Community associations with oceanic climate Community and functional group patterns. In all zones, all oceanic climate fluctuations were related to community structure (p < 0.03 in most cases), but with the exception of years since sea star wasting disease in the mid zone (15.9% of variance explained), the amount of variance explained by each was low (ranging from 0.6 to 8.6%; Table 2). In the high zone, no single factor accounted for more than 3.8% of the variation in community structure, and together the 8 factors drove only 16.1% of the variation in community structure (Table 2a). In the mid zone, environment-community associations were stronger (Table 2b, p < 0.012). Col- lectively, the climate indices, upwelling and temperature accounted for 25.5% of the variance, with NPGO and PDO contributing the most strongly (8.6% and 6.1% of variation, respec- tively). As noted above, the years since sea star wasting accounted for a further 15.9% of the

variance explained, bringing the total variance explained by environment to 41.3% in the mid zone. In the low zone, all factors except upwelling variation and maximum daily air tempera- ture varied with community structure, but individual effects were weak with sea star wasting disease, NPGO and mean upwelling being the strongest drivers (Table 2c, 6.2%, 4.5%, and 4.2%, respectively). Collectively, environment explained only 20.1% of the low zone variance. Analysis of correlations between the most temporally variable functional groups (S4, S5 and S6 Figs in S3 Appendix) and the 8 oceanic environmental variables showed that several groups appeared sensitive to the environment in the high and mid zones, but fewer were sensitive in the low zone. In the high zone, both herbivorous snails and limpets were sensitive to several environmental variables (Fig 7; Table 3a), and the strongest trends were increases with windier phases of the NPGO (R2 = 0.157 and 0.122, respectively), decreases with years since SSWD (R2 = 0.239 and 0.151, respectively), and limpet abundance increased with higher upwelling vari- ability (R2 = 0.164). Finally, fucoid algal abundance in the high zone was weakly negatively cor- related with upwelling variability and mussel abundance was weakly positively correlated with PDO (Fig 7; Table 3a; R2 = 0.077 and < 0.001, respectively). Mid zone limpet abundance varied similarly to that in the high zone, increasing with windier NPGO and increased upwelling variation (R2 = 0.107 and 0.109, respectively),

decreasing with years since SSWD (R2 = 0.131), and additionally decreasing in warmer phases of PDO (R2 = 0.222, Fig 8, Table 3b). Mid-zone predatory snail abundance was lower when maximum air temperature was warmer (R2 = 0.117), mussel abundance increased with years since SSWD (R2 = 0.136), and the alga Endocladia muricata was more abundant with increased maximum air temperature (R2 = 0.255, Fig 8, Table 3b). In the low zone, individual functional groups were generally less sensitive to varying environments (Fig 9, Table 3c), with the only notable trends being decreased limpets with warmer sea surface temperatures (R2 = 0.123) and decreased surfgrass (Phyllospadix spp.) with increased upwelling variability (R2 = 0.111). Temporal trajectories of community structure. Trajectories of annual community shifts showed that communities did vary over time, but no consistent trend emerged within any zone or among years (Fig 6d–6f). In the high zone, the rate of community change was not cor- related with any of the 8 environmental variables (Table 4a). In the mid zone, community PLOS ONE Resistance of rocky intertidal communities PLOS ONE | https://doi.org/10.1371/journal.pone.0297697 May 29, 2024 15 / 34