Resistance of rocky intertidal communities to oceanic climate fluctuations

Determining the responses of complex ecological communities comprised of a network of interacting primary producers, herbivores, predators, omnivores, and detritivores is typically untenable. However, some complex communities are easier to study than others. For example, rocky intertidal community members are relatively small, typically have slow-moving or sessile habits, mature and reproduce relatively quickly, and have a compact habitat that enables research at a community scale [6,7]. One approach for assessing community responses to global climate change is to amass long-term datasets of whole communities and explore relationships with past environmental variation [4,8]. Here, we report results of a 14-year study of responses of rocky intertidal com- munities in the northern California Current Large Marine Ecosystem (CCLME, coastal ecosys- tem from Baja California, Mexico, to British Columbia, Canada) to fluctuations in large-scale ocean climate and environmental patterns. These include the Pacific Decadal Oscillation (PDO), North Pacific Gyre Oscillation (NPGO), El Niño-Southern Oscillation (ENSO), as well as upwelling regime and variation (intermittency) in upwelling, maximum daily intertidal air temperatures, and average daily intertidal sea surface temperatures. We also tested the effects of the sea star wasting disease epidemic, which caused severe coast-wide declines in the key- stone predator Pisaster ochraceus in 2013 and 2014 [9,10]. Recent evidence of increasing instability

Our recent work shows some worrisome ‘early warning signals’ of impending change in this rocky intertidal system [11,12]. For example, annually repeated disturbance experiments to track recovery rates in the low zone at several sites in Oregon revealed increasing community instability (more variance in disturbed and undisturbed plots over time) and diminishing resil- ience (slower recovery from disturbance over time [11]). Further, overall declines and/or increasing variability in several key ecological metrics have occurred, including ecological sub- sidies to the nearshore from offshore (i.e. phytoplankton and invertebrate recruitment), mussel performance (body condition and reproductive output), and sea star performance (reproduc- tive output and predation rates [12]. Climate fluctuations may provide insight into drivers of community instability Oscillating oceanic phenomena like the PDO (20–30 yr cycle), NPGO (7–15 yr cycle), and ENSO (3–7 year cycle), and environmental conditions like upwelling and air or sea tempera- tures have well-known impacts on some marine communities [13–18]. Higher PDO indices generally indicate warmer sea surface temperatures [13]. NPGO is orthogonal to PDO and higher indices generally indicate windier conditions and higher primary productivity [19,20]. In the CCLME, ENSO results in alteration between warmer, less productive El Niño years and cooler, more productive La Niña years [15,21,22]. Upwelling is caused by alongshore wind stress [23] that draws deep, nutrient-rich water up to the surface, and is usually strongest in

June and July northern California and Oregon [24]. Variability in upwelling is also important, where intermittently strong upwelling (typical of central Oregon) provides both enough nutri- ents and enough wind relaxation for phytoplankton blooms to develop nearshore, while persis- tently strong upwelling winds (typical of southern Oregon and northern California) transports the blooms offshore [16,25]. Further, these environmental patterns and atmospheric climate are intricately linked, so global climate change is expected to alter them [17,26–29]. For exam- ple, the intensity and relative ecological dominance of NPGO versus PDO may be increasing [30,31], extreme El Niño and La Niña events are expected to increase [32]. On the other hand, PLOS ONE Resistance of rocky intertidal communities PLOS ONE | https://doi.org/10.1371/journal.pone.0297697 May 29, 2024 2 / 34 Foundation, the Kingfisher Foundation, and the NSF Biological Oceanography Program (OCE07-26983, OCE10-61233, OCE14-48913, and OCE17-35911). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist.