- Long-term, large-scale studies of meta-ecosystems provide critical information about how global change influences communities. In my dissertation, I analyzed data from studies encompassing 18 years (2006 – 2023) and over 1,000 km of coastline to investigate drivers of rocky intertidal community structure and dynamics. Specifically, I explored the roles of recruitment, species interactions, and oceanographic context in structuring rocky intertidal communities of the Pacific Northwest and Atlantic Canada. Spanning 24 sites in these two regions, my research utilized observational studies, manipulative experiments, remote sensing, and statistical modeling in a highly integrative approach. I investigated several research questions concerning how variation in ecosystem conditions influenced delivery of marine subsidies (plankton and detritus), community stability, and species interactions.
In Chapter 2, I compared benthic-pelagic coupling in Oregon and Nova Scotia. Coastal marine habitats are connected via flows of propagules, nutrients, and energy – collectively termed ecological subsidies. Upwelling influences the delivery of marine subsidies to the nearshore ocean, and surf zone hydrodynamics affect their transport to shore. However, the relative importance of these processes remains understudied in many regions. Consequently, I evaluated the influence of upwelling and surf zone hydrodynamics on the delivery of invertebrate larvae, phytoplankton, and particulates from the nearshore ocean to wave-exposed rocky shores on the Pacific and Atlantic Oceans. Filter-feeder larvae and their food supply of phytoplankton and detritus are crucial marine subsidies to rocky intertidal habitats. Thus, I sampled barnacle recruitment at 19 sites in both regions, and mussel recruitment at nine sites in Nova Scotia from 2014 to 2018. I used NASA satellite imagery to measure concentrations of phytoplankton and particulate organic carbon (POC) in the nearshore ocean adjacent to our sites. Finally, I measured intertidal topography, surf zone width, and upwelling magnitude and duration to characterize the oceanographic regimes. I found that upwelling was relatively more important than surf zone hydrodynamics in delivering larvae, phytoplankton, and particulate subsidies to rocky coasts. Barnacle recruitment in both regions and mussel recruitment in Nova Scotia were unrelated to either intertidal slope or surf zone width. Instead, barnacle recruitment was associated with intermittent upwelling in Oregon, but not in Nova Scotia, and mussel recruitment was associated with intermittent upwelling in Nova Scotia. Furthermore, upwelling magnitude was unimodally related to phytoplankton concentrations in Oregon, but not in Nova Scotia. POC concentrations were unrelated to upwelling in both regions. These findings indicate that although Nova Scotia and Oregon span similar temperate latitudes, the delivery mechanisms for these marine subsidies are distinct for each region.
In Chapter 3, I assessed the consequences of two systemic perturbations on rocky intertidal community structure. When environmental changes are severe, they can trigger abrupt transitions from one ecosystem type to another leading to a regime shift. From 2014 – 2016, rocky intertidal habitats in the northeast Pacific Ocean experienced extreme temperatures during a multiyear marine heatwave (MHW) and sharp population declines of the keystone predator Pisaster ochraceus due to sea star wasting disease (SSWD). In a 15-year succession experiment conducted in a rocky intertidal meta-ecosystem spanning 13 sites on four capes in Oregon and northern California, USA, I measured community structure before, during, and after the MHW onset and SSWD outbreak. Kelp abundance declined during the MHW due to extreme temperatures, while gooseneck barnacle and mussel abundances increased due to reduced predation pressure after the loss of Pisaster from SSWD. Using hidden Markov models, I detected multiple types of regime shifts from substrate or algae-dominated to invertebrate-dominated community states at several sites. Moreover, some communities did not revert to the original states after water temperatures cooled and Pisaster population densities recovered. Consequently, thermal stress and predator loss can result in regime shifts that fundamentally alter community structure even after restoration of baseline conditions.
In Chapter 4, I analyzed how variation in top-down control affects interactions between two rocky intertidal foundation species. Depending on the ecosystem context, species interactions can switch from negative (competition) to positive (facilitation) due to variation in consumer pressure, physical stress, and disturbance. Transitions between competition and facilitation are especially consequential for community structure when they affect foundation species, which provide or modify habitat for other organisms. In rocky intertidal communities of the northeast Pacific Ocean, two dominant foundation species are the California mussel (Mytilus californianus) and surfgrass (Phyllospadix spp.). I conducted an observational study to measure the co-occurrence of mussels with surfgrass and other low intertidal zone species assemblages on the coast of Oregon, USA. Outside of mussel beds, mussels occurred more frequently and at higher densities at sites with low predation pressure from the ochre sea star (Pisaster ochraceus). In a series of field experiments, I also measured the effects of mussels and surfgrass on each other in the context of variable Pisaster predation pressure. At sites with high Pisaster densities and predation rates, surfgrass facilitated mussel survival via associational defense. This interaction switched to competition for space at sites with low Pisaster densities and predation rates. I determined that site-level variation in predation pressure was the primary factor underlying this competition-facilitation shift for the effect of surfgrass on mussels, but not for the effect of mussels on surfgrass. My findings highlight the importance of ecosystem context for understanding species interactions, and the necessity of measuring interaction strengths along relevant abiotic and biotic gradients, including those in consumer pressure.
Overall, my research elucidates the roles of recruitment, species interactions, and oceanographic context in structuring rocky intertidal communities of the Pacific Northwest and Atlantic Canada. Coastal marine habitats are experiencing dramatic changes including marine heatwaves and declines of apex predators. Understanding how these drivers integrate with natural variation in oceanographic and ecological conditions will facilitate more accurate prediction of rocky intertidal community structure and dynamics, and may contribute to the holistic preservation of these incredible ecosystems.