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Abstract Marine protected areas (MPAs) are an important tool for conserving coastal marine ecosystems, with well‐documented benefits for fished species. However, their potential to benefit non‐exploited species, such as primary producers in kelp forest ecosystems, is less well understood, particularly under escalating climate change impacts.In this study, we used four decades of remote sensing to examine the effects of 54 MPAs on kelp canopy coverage and assess how these effects influence kelp resilience to marine heatwaves. We developed a method for identifying paired reference (control) sites using historical satellite data and then used Before‐After Control‐Impact Paired Series analysis to examine whether the implementation of MPAs leads to increases in kelp coverage. In addition to examining changes in kelp coverage before and after MPA implementation, we also analysed the effect of MPAs on the resistance and recovery of kelp canopy coverage to a series of severe marine heatwaves in the North Pacific between 2014 and 2016.We found that the implementation of MPAs led to a modest positive effect with an 8.5% increase in kelp coverage compared to reference areas, though effects varied across MPAs.The positive effect of MPAs became more evident following the marine heatwaves, with kelp forests in MPAs showing greater recovery than in reference sites, particularly in southern California.Synthesis and applications. Our results provide empirical evidence of the potential role of MPAs as climate adaptation tools and highlight that well‐managed MPAs can support ecosystem stability under increasing climate stress. 

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. 

Giant kelp and bull kelp forests are increasingly at risk from marine heatwave events, herbivore outbreaks, and the loss or alterations in the behavior of key herbivore predators. The dynamic floating canopy of these kelps is well-suited to study via satellite imagery, which provides high temporal and spatial resolution data of floating kelp canopy across the western United States and Mexico. However, the size and complexity of the satellite image dataset has made ecological analysis difficult for scientists and managers. To increase accessibility of this rich dataset, we created Kelpwatch, a web-based visualization and analysis tool. This tool allows researchers and managers to quantify kelp forest change in response to disturbances, assess historical trends, and allow for effective and actionable kelp forest management. Here, we demonstrate how Kelpwatch can be used to analyze long-term trends in kelp canopy across regions, quantify spatial variability in the response to and recovery from the 2014 to 2016 marine heatwave events, and provide a local analysis of kelp canopy status around the Monterey Peninsula, California. We found that 18.6% of regional sites displayed a significant trend in kelp canopy area over the past 38 years and that there was a latitudinal response to heatwave events for each kelp species. The recovery from heatwave events was more variable across space, with some local areas like Bahía Tortugas in Baja California Sur showing high recovery while kelp canopies around the Monterey Peninsula continued a slow decline and patchy recovery compared to the rest of the Central California region. Kelpwatch provides near real time spatial data and analysis support and makes complex earth observation data actionable for scientists and managers, which can help identify areas for research, monitoring, and management efforts. 

Abstract Kelp forests are complex underwater habitats that form the foundation of many nearshore marine environments and provide valuable services for coastal communities. Despite their ecological and economic importance, increasingly severe stressors have resulted in declines in kelp abundance in many regions over the past few decades, including the North Coast of California, USA. Given the significant and sustained loss of kelp in this region, management intervention is likely a necessary tool to reset the ecosystem and geospatial data on kelp dynamics are needed to strategically implement restoration projects. Because canopy‐forming kelp forests are distinguishable in aerial imagery, remote sensing is an important tool for documenting changes in canopy area and abundance to meet these data needs. We used small unoccupied aerial vehicles (UAVs) to survey emergent kelp canopy in priority sites along the North Coast in 2019 and 2020 to fill a key data gap for kelp restoration practitioners working at local scales. With over 4,300 hectares surveyed between 2019 and 2020, these surveys represent the two largest marine resource‐focused UAV surveys conducted in California to our knowledge. We present remote sensing methods using UAVs and a repeatable workflow for conducting consistent surveys, creating orthomosaics, georeferencing data, classifying emergent kelp and creating kelp canopy maps that can be used to assess trends in kelp canopy dynamics over space and time. We illustrate the impacts of spatial resolution on emergent kelp canopy classification between different sensors to help practitioners decide which data stream to select when asking restoration and management questions at varying spatial scales. Our results suggest that high spatial resolution data of emergent kelp canopy from UAVs have the potential to advance strategic kelp restoration and adaptive management. 

Satellite and aerial imagery have been used extensively for mapping the abundance and distribution of giant kelp ( Macrocystis pyrifera ) in southern California. There is now great potential for using unoccupied aerial vehicles (UAVs) to map kelp canopy at very high resolutions. However, tides and currents have been shown to affect the amount of floating kelp canopy on the water surface, and the impacts of these processes on remotely sensed kelp estimates in this region have not been fully quantified. UAVs were used to map fine-scale changes in canopy area due to tidal height and current speed at kelp forests off the coast of Palos Verdes, CA and Santa Barbara, CA. An automated method for detecting kelp canopy was developed that was 67% accurate using red-green-blue (RGB) UAV imagery and 93% accurate using multispectral UAV imagery across a range of weather, ocean, and illumination conditions. Increases in tidal height of 1 m reduced the amount of floating kelp canopy by 15% in Santa Barbara and by over 30% in Palos Verdes. The effect of current speed on visible kelp canopy was inconclusive, but there was a trend towards lower canopy area with increased current speed. Therefore, while tidal height and current speed can introduce significant variability to estimates of kelp abundance, the magnitude of this variability is site specific. Still, UAVs are a valuable tool for mapping of kelp canopy and can provide greater spatial resolution and temporal coverage than is possible from many satellite sensors. This data can provide insight into the patterns and drivers of high frequency fluctuations in kelp abundance. 

The emerging sector of offshore kelp aquaculture represents an opportunity to produce biofuel feedstock to help meet growing energy demand. Giant kelp represents an attractive aquaculture crop due to its rapid growth and production, however precision farming over large scales is required to make this crop economically viable. These demands necessitate high frequency monitoring to ensure outplant success, maximum production, and optimum quality of harvested biomass, while the long distance from shore and large necessary scales of production makes in person monitoring impractical. Remote sensing offers a practical monitoring solution and nascent imaging technologies could be leveraged to provide daily products of the kelp canopy and subsurface structures over unprecedented spatial scales. Here, we evaluate the efficacy of remote sensing from satellites and aerial and underwater autonomous vehicles as potential monitoring platforms for offshore kelp aquaculture farms. Decadal-scale analyses of the Southern California Bight showed that high offshore summertime cloud cover restricts the ability of satellite sensors to provide high frequency direct monitoring of these farms. By contrast, daily monitoring of offshore farms using sensors mounted to aerial and underwater drones seems promising. Small Unoccupied Aircraft Systems (sUAS) carrying lightweight optical sensors can provide estimates of canopy area, density, and tissue nitrogen content on the time and space scales necessary for observing changes in this highly dynamic species. Underwater color imagery can be rapidly classified using deep learning models to identify kelp outplants on a longline farm and high acoustic returns of kelp pneumatocysts from side scan sonar imagery signal an ability to monitor the subsurface development of kelp fronds. Current sensing technologies can be used to develop additional machine learning and spectral algorithms to monitor outplant health and canopy macromolecular content, however future developments in vehicle and infrastructure technologies are necessary to reduce costs and transcend operational limitations for continuous deployment in an offshore setting. 

Surface-canopy forming kelps provide the foundation for ecosystems that are ecologically, culturally, and economically important. However, these kelp forests are naturally dynamic systems that are also threatened by a range of global and local pressures. As a result, there is a need for tools that enable managers to reliably track changes in their distribution, abundance, and health in a timely manner. Remote sensing data availability has increased dramatically in recent years and this data represents a valuable tool for monitoring surface-canopy forming kelps. However, the choice of remote sensing data and analytic approach must be properly matched to management objectives and tailored to the physical and biological characteristics of the region of interest. This review identifies remote sensing datasets and analyses best suited to address different management needs and environmental settings using case studies from the west coast of North America. We highlight the importance of integrating different datasets and approaches to facilitate comparisons across regions and promote coordination of management strategies.