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Kelp forests (Order Laminariales) form key biogenic habitats in coastal regions of temperate and Arctic seas worldwide, providing ecosystem services valued in the range of billions of dollars annually. Although local evidence suggests that kelp forests are increasingly threatened by a variety of stressors, no comprehensive global analysis of change in kelp abundances currently exists. Here, we build and analyze a global database of kelp time series spanning the past half-century to assess regional and global trends in kelp abundances. We detected a high degree of geographic variation in trends, with regional variability in the direction and magnitude of change far exceeding a small global average decline (instantaneous rate of change = −0.018 y−1). Our analysis identified declines in 38% of ecoregions for which there are data (−0.015 to −0.18 y−1), increases in 27% of ecoregions (0.015 to 0.11 y−1), and no detectable change in 35% of ecoregions. These spatially variable trajectories reflected regional differences in the drivers of change, uncertainty in some regions owing to poor spatial and temporal data coverage, and the dynamic nature of kelp populations. We conclude that although global drivers could be affecting kelp forests at multiple scales, local stressors and regional variation in the effects of these drivers dominate kelp dynamics, in contrast to many other marine and terrestrial foundation species.

Marine heatwaves threaten the persistence of kelp forests globally. However, the observed responses of kelp forests to these events have been highly variable on local scales. Here, we synthesize distribution data from an environmentally diverse region to examine spatial patterns of canopy kelp persistence through an unprecedented marine heatwave. We show that, although often overlooked, temperature variation occurring at fine spatial scales (i.e., a few kilometers or less) can be a critical driver of kelp forest persistence during these events. Specifically, though kelp forests nearly all persisted toward the cool outer coast, inshore areas were >3°C warmer at the surface and experienced extensive kelp loss. Although temperatures remained cool at depths below the thermocline, kelp persistence in these thermal refugia was strongly constrained by biotic interactions, specifically urchin populations that increased during the heatwave and drove transitions to urchin barrens in deeper rocky habitat. Urchins were, however, largely absent from mixed sand and cobble benthos, leading to an unexpected association between bottom substrate and kelp forest persistence at inshore sites with warm surface waters. Our findings demonstrate both that warm microclimates increase the risk of habitat loss during marine heatwaves and that biotic interactions modified by these events will modulate the capacity of cool microclimates to serve as thermal refugia.

There is increasing interest in mitigating the loss of kelp forests through restoration, but this has received scant attention relative to other coastal habitats. We evaluate current knowledge centered on key restoration principles to provide guidelines for best practice in kelp restoration. The cause and scale of degradation is fundamental in determining if kelp can be restored and the methods required to promote reestablishment. Removal of stressors may be adequate to achieve restoration goals where degradation is not too widespread or acute. Extensive losses of kelp forests will often require active reseeding of areas because of the low dispersal ability of many kelp species. Restoration efforts have generally taken a trial-and-error approach at experimental scales to develop techniques for establishing individuals. Furthermore, studies that inform cost–benefit analysis and the appropriate spatial scales for restoration of sustainable kelp forests are urgently needed for prioritizing and scaling up restoration efforts globally.

1. Climate change is driving a redistribution of species and the reconfiguration of ecological communities at a global scale. Persistent warming in many regions has caused species to extend their geographical ranges into new habitats, with thermally tolerant species often becoming competitively dominant over species with colder affinities. Although these climate-driven changes in species abundance and diversity are well documented, their ecosystem-level implications are poorly understood, and resolving whether reconfigured communities can maintain fundamental ecosystem functions represents a pressing challenge in an increasingly warmer world. 2. Here, we investigated how climate-driven substitutions of foundation species influence processes associated with the cycling of organic matter (biomass production, detritus flow, herbivory, decomposition) by comparing two habitat-forming kelp species with contrasting thermal affinities. We examined the wider ecosystem consequences of such shifts for the observed (and predicted) emergence of novel marine forest communities in the NE Atlantic, which are expected to become more dominated by range-expanding, warm-temperate kelp species. 3. Warm-temperate kelps both accumulated and released 80% more biomass than the cold-temperate species despite being taxonomically closely related and morphologically similar. Furthermore, the warm-temperate species accumulated biomass and released detritus year-round, whereas the cold-temperate species did so during short, discrete periods. The warm-temperate kelps supported higher densities of invertebrate grazers and were a preferred food source. Finally, their detritus decomposed 6.5 times faster, despite supporting comparable numbers of detritivores. Overall, our results indicate an important shift in organic matter circulation along large sections of NE Atlantic coastline following the climate-driven expansion of a warm-affinity kelp, with novel forests supplying large amounts of temporally continuous—yet highly labile—organic matter. 4. Synthesis. Collectively, our results show that, like species invasions, climate-driven range expansions and consequent shifts in the identity of dominant species can modify a wide range of important ecosystem processes. However, alterations in overall ecosystem functioning may be relatively limited where foundation species share similar ecological and functional traits.

Kelp forests along temperate and polar coastlines represent some of most diverse and productive habitats on the Earth. Here, we synthesize information from >60 years of research on the structure and functioning of kelp forest habitats in European waters, with particular emphasis on the coasts of UK and Ireland, which represents an important biogeographic transition zone that is subjected to multiple threats and stressors. We collated existing data on kelp distribution and abundance and reanalyzed these data to describe the structure of kelp forests along a spatial gradient spanning more than 10° of latitude. We then examined ecological goods and services provided by kelp forests, including elevated secondary production, nutrient cycling, energy capture and flow, coastal defense, direct applications, and biodiversity repositories, before discussing current and future threats posed to kelp forests and identifying key knowledge gaps. Recent evidence unequivocally demonstrates that the structure of kelp forests in the NE Atlantic is changing in response to climate‐ and non‐climate‐related stressors, which will have major implications for the structure and functioning of coastal ecosystems. However, kelp‐dominated habitats along much of the NE Atlantic coastline have been chronically understudied over recent decades in comparison with other regions such as Australasia and North America. The paucity of field‐based research currently impedes our ability to conserve and manage these important ecosystems. Targeted observational and experimental research conducted over large spatial and temporal scales is urgently needed to address these knowledge gaps. The kelp dominated ecosystems of the northeast Atlantic are unique and provide a wealth of goods and services to regional and centralized human populations. However, compared to other regions, such as North America and Australasia, NE Atlantic kelp ecosystems have been understudied in recent decades, resulting in extensive knowledge gaps and insufficient understanding of ecological responses to rapid environmental change.

Financial support was provided by the European Regional Development Fund (ERDF) through the ‘Programa Operacional Factores de Competitividade’ (POFC‐COMPETE) within the ‘Quadro de Referência Estratégico Nacional (QREN)’ and the Portuguese Fundação para a Ciência e a Tecnologia (FCT) through the projects OCEANKELP (PTDC/MAR/109954/2009), PHYSIOGRAPHY (PTDC/MAR/105147/2008) and a PhD grant (SFRH/BD/84933/2012) to J.N.F., F.A. and I.S.P. were funded by the European Regional Development Fund (ERDF) through the Programa Operacional Factores de Competitividade (POFC‐COMPETE) within the Quadro de Referência Estratégico Nacional (QREN) and PEst‐C/MAR/LA0015/2011 (by ERDF) through the COMPETE Program. This article received additional funding from the project MarInfo ‐ Integrated Platform for Marine Data Acquisition and Analysis (reference NORTE‐01‐0145‐FEDER‐000031), supported by North Portugal Regional Operational Programme (NORTE 2020), under the PORTUGAL 2020 Partnership Agreement, through the European Regional Development Fund (ERDF). We are very grateful to L. Ramos, A. Trilla, I. Jacob, R. Silva and V. Modesto for their help with experimental setup and two anonymous reviewers and the editors for constructive comments on an earlier version of the manuscript. All the authors have no conflicts of interest to declare.

Kelp forests are declining in many regions globally with climatic perturbations causing shifts to alternate communities and significant ecological and economic loss. Range edge populations are often at most risk and are often only sustained through localised areas of upwelling or on deeper reefs. Here we document the loss of kelp forests ( Ecklonia radiata ) from the Sultanate of Oman, the only confirmed northern hemisphere population of this species. Contemporary surveys failed to find any kelp in its only known historical northern hemisphere location, Sadah on the Dhofar coast. Genetic analyses of historical herbarium specimens from Oman confirmed the species to be E. radiata and revealed the lost population contained a common CO1 haplotype found across South Africa, Australia and New Zealand suggesting it once established through rapid colonisation throughout its range. However, the Omani population also contained a haplotype that is found nowhere else in the extant southern hemisphere distribution of E. radiata . The loss of the Oman population could be due to significant increases in the Arabian Sea temperature over the past 40 years punctuated by suppression of coastal upwelling. Climate-mediated warming is threatening the persistence of temperate species and precipitating loss of unique genetic diversity at lower latitudes.

Arctic West Spitsbergen in Svalbard is currently experiencing gradual warming due to climate change showing decreased landfast sea-ice and increased sedimentation. In order to document possible changes in 2012–2014, we partially repeated a quantitative diving study from 1996 to 1998 in the kelp forest at Hansneset, Kongsfjorden, along a depth gradient between 0 and 15 m. The seaweed biomass increased between 1996/1998 and 2012/2013 with peak in kelp biomass shifted to shallower depth, from 5 to 2.5 m. The kelp biomass at 2.5 m was 8.2-fold higher in 2012/2013 (14 kg fresh biomass m −2 ) than in 1996/1998 and mostly due to an increase in the kelp Laminaria digitata . This resulted in a very high density of 2- to 8-year-old kelp (70 ind. m −2 ) and a high leaf area index of nearly 10 at 2.5 m. The entire zonation seemed to have shifted upwards to shallower depth, since also the lower depth limit of most dominant brown algae was shallower as well as the biomass maximum of several taxa. The cumulated annual photosynthetic active radiation at 15 m depth (42 mol m −2  year −1 ) determined the current depth limit of kelps. Changes also resulted in an altered seaweed community pattern. The complex pattern of change was probably driven by opposing effects of co-acting environmental drivers, namely lack of ice-scouring, elongation of the open-water period and deterioration of the underwater irradiance climate. The results are interpreted as a consequence of Arctic warming probably reflecting a typical scenario for change along other Arctic shores in near future.

We combine data collected from the past 40 years to estimate the indirect effects of sea otters ( Enhydra lutris ) on ecosystem carbon (C) production and storage across their North American range, from Vancouver Island to the western edge of Alaska's Aleutian Islands. We find that sea otters, by suppressing sea urchin ( Strongylocentrotus spp) populations, allow kelp (Order Laminariales) ecosystems to develop with a net primary productivity (NPP) of 313-900 grams C per square meter per year (g C m −2 yr −1 ) and biomass density of 101-180 grams C per square meter (g C m −2 ). In the absence of sea otters, these areas would have an NPP of 25-70 g C m −2 yr −1 and biomass density of 8-14 g C m −2 . Over an ecosystem area of approximately 5.1 × 10 10 m 2 , the effect of sea otter predation on living kelp biomass alone represents a 4.4-to 8.7-teragram increase in C storage. At 2012 prices (US$47 per ton of C), this stored C would be valued at US$205 million-$408 million on the European Carbon Exchange. Although questions remain concerning the pathways and compartments of kelp C flux and storage, sea otters undoubtedly have a strong influence on these elements of the C cycle. Predator-induced trophic cascades likely influence the rates of C flux and storage in many other species and ecosystems.

Clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9-mediated genome editing has been used for reverse genetics studies in many organisms. However, application to commercially important species of seaweeds is still limited. The genetics and breeding technologies for species of the Laminariales will be advanced by developing a genome editing tool. In this study, we attempted to edit a counter-selectable marker, the adenine phosphoribosyl transferase (APT) gene using CRISPR-Cas9 ribonucleoprotein (RNP) complexes delivered by microinjection into gametophytes of Saccharina japonica. After injection of CRISPR-Cas9 RNP, 2-fluoroadenine (2-FA) was added to the medium (10–40 μM) to select APT mutants. Twenty-three female and 12 male 2-FA resistant gametophytes had mutations in their APT genes. Genome editing efficiency for the injection trials was 8.64% and 4.46% for the female and male gametophytes, respectively. The apt mutant gametophytes were able to produce sperm or eggs. Sporophytes derived by crossing sperm and eggs from apt mutant gametophytes were resistant to 40 μM 2-FA. Sporophytes derived from crosses with one wild type and one mutant parent also showed resistance to high concentrations (20–40 μM) of 2-FA, while wild-type sporophytes died in 10 μM 2-FA. This is the first report showing the validity of gene editing of S. japonica using the CRISPR-Cas9 RNP complex and microinjection method.