Regime shifts in rocky intertidal communities associated with a marine heatwave and disease outbreak

Background/Question/Methods

Long-term, large-scale studies of succession provide critical evidence for understanding how species assemble into communities and how they respond to disturbance events and stressors, including keystone species loss and changing environmental conditions. When stressors or disturbances are severe enough, they can trigger critical transitions from one community state to another in a process called a regime shift. From 2014-2016, rocky intertidal ecosystems in the northeast Pacific Ocean experienced extreme temperatures during a multiyear marine heatwave (MHW; known as “the blob”) and sharp population declines of the keystone predator Pisaster ochraceus due to sea star wasting disease (SSWD). In a 16-year succession experiment conducted at 13 field sites on four capes in Oregon and northern California, we quantified community structure in fixed plots before, during, and after the MHW onset and SSWD outbreak. We asked: (Q1) Where, when, and what kind of regime shifts occurred between community states? and (Q2) Which regime shifts were associated with the MHW and SSWD events? We used cluster analysis to classify experimental plots into community states and hidden Markov models (HMMs: state-space models with unobservable states) to identify regime shifts and evaluate the extent to which they were driven by the MHW and SSWD.

Results/Conclusions

(Q1) Transitions between many types of community states occurred throughout the course of the experiment. Most plots were covered in early successional species (e.g., turfy red algae and acorn barnacles) within two years, followed by the gradual establishment and persistence of late successional species (e.g., surfgrasses and mussels). Prior to 2015, sessile invertebrates were dominant on the central Oregon cape and macrophytes (algae and surfgrasses) were dominant on the remaining three capes. However, from 2014 to 2016 community states shifted from macrophytes to sessile invertebrates at many field sites in northern Oregon, southern Oregon, and northern California. In the HMMs, these regime shifts are exemplified by high transition probabilities from kelp-dominated or surfgrass-dominated to mussel-dominated community states. (Q2) The sudden loss of macroalgae (particularly kelps) was associated with the 2014-2016 MHW. Some species of rocky intertidal kelp are susceptible to extreme temperature stress caused by marine heatwaves and El Niño. Furthermore, the increase in mussels and gooseneck barnacles from 2015 to 2017 was associated with SSWD, as the dramatic loss of large Pisaster ochraceus reduced predation pressure. Consequently, this study demonstrates how warming temperatures and trophic downgrading can result in widespread regime shifts in marine ecosystems.