UC Santa Barbara

Marine heatwaves are becoming increasingly prevalent and pose significant threats to kelp forest ecosystems worldwide. Community responses to these disturbances are context-dependent and influenced by a myriad of local factors. In this study, I analyzed a 20-year time series of subtidal kelp forest data to assess the impact of the 2013-2016 North Pacific marine heatwave on the long-term restructuring of macroalgal and benthic invertebrate communities along an environmental gradient in a biogeographic transition zone. To investigate pathways of community change, I employed multivariate analyses using canonical analysis of principal coordinates (CAP) techniques and assessed the trajectories of key species through generalized linear mixed models (GLMMs). I hypothesized that communities in distinct regions would be differentially impacted, exhibiting varying directions and rates of change, with the extent of marine protection influencing these effects. Results showed differing broad regional trajectories, with regions in the warmer end of the biogeographic transition zone showing significant increases in kelp alongside a decline in urchin grazers (Strongylocentrotus purpuratus and Mesocentrotus franciscanus), and the colder region exhibiting the opposite trend. Contrary to my expectations, marine protected areas (MPAs) did not uniformly yield slower rates of community change; however, the effects of protection varied across the regions studied, with some direct benefits observed in terms of targeted species abundance, increased diversity, and the behavior of key kelp grazers such as sea urchins. Additionally, the population increase of other grazers, particularly the gastropod Megastraea undosa, was associated with the marine heatwave, though the long-term implications of this trend remain uncertain. These findings highlight the unpredictable ecological responses of kelp forests to marine heatwaves, challenging the prevailing notion that warming consistently leads to kelp loss and ecosystem degradation. Instead, this research reveals a spectrum of post-disturbance pathways within a relatively small geographical area, influenced by the interplay of biogeographical gradients, marine protection, and disease dynamics.