Drivers of kelp forest refugia during successive disturbance events

Abstract

Abstract Increased ocean temperatures have led to large‐scale declines in many ecologically important species, including kelp forests. Spatial heterogeneity across seascapes could protect kelp individuals and small populations from thermal stress and nutrient limitation. Habitat features within upwelling regions may facilitate the transport of deep, cold water into shallow systems, but little is known about the spatiotemporal occurrence or stability of these climate refugia. Kelp in climate refugia may, however, also experience other stressors, such as overgrazing by kelp herbivores, reducing their effectiveness. Here, we use high‐resolution kelp canopy maps generated from CubeSat constellation data to characterize kelp persistence in northern California following a dramatic decline in kelp abundance due to increased temperature and nutrient limitation during a severe marine heatwave and continued intense grazing pressure by purple sea urchins. Kelp persistence was associated with local areas of relatively cool water temperature and seascape features such as shallow depths and low‐complexity bathymetry, which may have provided refuge from overgrazing. However, a very small percentage of kelp forests in the region exhibited high persistence, with many forests present in only one or two of the 9 years studied. Most kelp patches were not spatially stable over time. Initially, kelp presence aligned with climate refugia, but as overgrazing emerged as the dominant driver of kelp distributions post‐2019, kelp shifted to areas that offered protection from grazing pressure. Synthesis. Cooler areas with localized upwelling acted as climate refugia during the increased ocean temperatures from the 2014–2016 marine heatwave, supporting nutrient‐rich environments and mitigating heat stress for kelp forests. However, these temperature refugia often did not spatially overlap with areas providing protection from grazing pressure, leaving kelp forests vulnerable to future warming even within temperature refugia if grazing pressure remains high.