Submerged vegetated ecosystems like seagrass meadows and kelp forests capture and turnover substantial amounts of carbon and are increasingly being considered for global carbon budgets and offset schemes. Yet these ecosystems are vulnerable to climate-driven extreme events such as marine heatwaves (MHWs), leading to uncertainties in the reliability of carbon abatement benefits from restoration and protection. We quantified blue carbon loss (CO 2 equivalent release) and subsequent recovery of carbon stocks associated with reported MHWs over the past two decades. These events damaged 228,432 ha of kelp forest and seagrass meadows and resulted in a lost carbon abatement of 13 million tonnes of CO 2 (±2 SE) due to release of carbon stocks and diminished carbon sequestration capacity after the MHW. Recovery was often slow or nonexistent, with areal recovery rates of 7% y -1 (±4 SE) for kelp and 11% y -1 (±5 SE) for seagrass, leaving 91% of the impacted area unrecovered. Together the lost carbon abatement and area from MHWs was 1-2 orders of magnitude higher than current abatement from restoration. Our findings indicate that increasing MHW frequency could exacerbate emissions through the release of stored blue carbon and compromise the effectiveness of these nature carbon sinks.

Kelp forests, formed by large brown algae in the orders Laminariales and Fucales, are among the most widespread marine ecosystems. Despite their scale and importance to people, kelp forests remain under-recognized by the public and underrepresented in global policy and conservation agendas. We quantify the global spatial overlap between the kelp biome and human populations, economic activity, anthropogenic pressures, and projected ocean warming to assess the extent and geography of the coupling between kelp and people. We synthesised distributions of laminarian and fucoid kelp forests, explicitly distinguishing between 'major' kelp forests and 'minor' kelp forests. These distributions were intersected with global datasets on population, Gross Domestic Product (GDP), cumulative human impact, and projected sea surface temperature (SST) change to 2100. Analyses focused on populations and economic activity within 50 km of major kelp habitats. Approximately 959 million people live within 50 km of kelp forests globally (12% of global population), including 781 million adjacent to major kelp forests. These regions account for $27.6 trillion or ∼18% of global GDP and include major metropolitan centres such as Tokyo, Seoul, Los Angeles, Sydney, and several Mediterranean cities. Cumulative human pressures on kelp-associated coastlines are highest in densely populated temperate regions, particularly the Mediterranean, northwest Atlantic, and East Asia. Warming by 2100 is greatest at high latitudes, but the strongest link to people occurs in temperate regions where projected SST increases coincide with dense populations and high economic output. Our results support the idea that kelp forests are integrated into modern social-ecological systems, while overlooked in governance and funding. This global assessment identifies the tight coupling between people and kelp forests while identifying geographic risks and opportunity hotspots. Together, the work provides a foundation for communicating the role of kelp forests in society and for prioritising kelp conservation within marine conservation agendas.

Climate change is predicted to enhance the abundance and productivity of Arctic and sub-Arctic kelp forests. These gains may be constrained, however, by coastal darkening — the increased turbidity and light attenuation in nearshore waters, and sea urchin overgrazing. Here, we undertook a snapshot survey to examine how kelp productivity and sea urchin consumptive demand vary along a gradient of light availability (depth) across kelp forests in southwest Iceland. To do so, we quantified depth-dependent patterns in kelp (Laminaria hyperborea) canopy structure and per capita growth, and sea urchin (Echinus esculentus and Strongylocentrotus droebachiensis) abundance, biomass and consumptive demand. We found that kelp per capita growth and longevity declined sharply with depth, with canopies below 10 m accumulating up to 90% less biomass than their shallow (5 m) counterparts. In contrast, sea urchin populations at depth were dominated by large individuals with high metabolic demands, likely resulting in elevated consumptive demands despite lower overall densities. This mismatch between kelp production and consumption led to a greater “consumption debt” in deeper canopies, where urchin consumptive demands exceeded local kelp productivity. Our findings suggest that predicted poleward expansions of L. hyperborea could be depth-limited, with sea urchin grazing at depth likely to counteract productivity gains.

Episodic reductions in underwater light can be a key driver of marine ecosystem degradation. Yet a consistent event-based framework describing the frequency, duration and intensity of substantial but short-term reductions in underwater light does not exist. Here, we proposed marine darkwaves as a framework for quantifying these episodic reductions of underwater light at specific depths which aligns with definitions of other episodic and extreme events. The framework was applied to long-term in situ time series of underwater irradiance from California, USA (16 years, 6.3 metres) and New Zealand (10 years, at 7 and 20 metres). We showed evidence of several intense marine darkwaves across these sites, with durations up to 64 days, cumulative light deficits reaching −105.6 mol photon·m−2, and up to almost 100

Natural climate solutions (NCS) are increasingly recognized as important for combatting climate change, but ocean-based pathways remain underrepresented in national strategies for conservation and climate change mitigation. Here, we present a transferable national assessment framework for blue carbon, applied to Canada, that maps blue carbon ecosystem extents, quantifies national carbon stocks and sequestration potential, and evaluates the benefits of increasing their spatial protection. We estimated that 10,952.5 Tg C (95% CI: 7,035.7 – 16,098.7 Tg C) is stored in the top 30 cm of Canada’s marine soils and sediments, with marine macrophytes, including kelp forests, additionally sequestering 7.7 Tg C yr-1 (95% CI: 5.6 – 25.1 Tg C yr-1). Expanding protection of blue carbon ecosystems to 30% by 2030, in line with the Kunming-Montreal Post-2020 Global Biodiversity Framework, could provide a practical pathway for incorporating blue carbon into national NCS portfolios. If current rates of habitat loss and disturbances were effectively abated in these areas, this could provide 106.9 Tg CO2e (95% CI: 94.1 – 234.9 Tg CO2e) in cumulative climate mitigation benefits by 2050. Our approach provides a foundation for integrating blue carbon into national climate and conservation strategies.

1. Nature-Based Solutions, green-finance instruments and policies are now routinely constructed around carbon sequestration/storage (CSS) and nutrient bioremediation (NB). This integration builds on how Market-Based Instruments (e.g. payments-for-ecosystem-services) are regularly used in policies focused on terrestrial ecosystems. In marine and coastal systems poor understanding of CSS/NB biophysical processes and impacts of ecosystem quality/stressors, combined with methods and governance framework knowledge gaps, generate substantial uncertainty in outcomes. Reductions in output confidence preclude integration into Nature-Based Solutions, stifling market-based investment centred on conserving and restoring temperate coastal ecosystems. 2. To navigate this complex, rapidly evolving area, researchers from six continents engaged in a Priority Setting Exercise to generate 25 questions that, if answered within 10 years, will increase robustness, scalability and applicability of CSS/NB data across regions and ecosystems. We then used a modal analysis across five categories (time, geographic scale, technology complexity, cost and policy relevance) to expedite research-investment decisions. 3. Questions (numbers in brackets) were organised across six themes as follows: maps/quantitative evidence/long-term data (3), Processes/variability (6), Connectivity (2), Anthropogenic impacts (4), Methods/standards (6), Governance/conservation (4). 4. Questions under methods/standards and governance/trading schemes themes were generally identified to be the cheapest to answer and quickest to complete, whilst still having considerable geographic and policy relevance. 5. Policy implications: Identifying the enabling conditions for more efficient and successful approaches will greatly improve our understanding of ecosystem services. Together, these answers will then deliver the decision-grade data necessary to strengthen green-finance opportunities and address urgent climate and pollution (nutrient) crise

Understanding the genetic architecture of functional traits can provide key insights into the ecological dynamics and adaptive potential of species. We investigated whether genetic data can predict growth rate variation in a natural population of the widespread kelp, Ecklonia radiata. We tagged kelps and tracked their growth in situ over spring when growth is maximal. Individual kelps were then genotyped using reduced representation sequencing (ddRAD) and we employed multiple approaches to assess whether genetic variation corresponded with growth rate variation. Despite a limited sample size, we found evidence that growth rate can be strongly predicted from genetic variation, with approximately half of the variation in growth rate predicted by only 18 loci (R2 = 0.499). Leveraging published transcriptomic data, we confirm that most of these loci are expressed or are linked to expressed putative genes. However, many of these genes are of unknown function and do not match well-known gene families. These findings have important implications for understanding natural kelp forest dynamics and for applied approaches such as selective breeding and aquaculture. While our study offers an important first assessment of the possible genomic architecture underlying growth rate in E. radiata, future work is needed to confirm this apparent link between genetic and functional variation.

The prevailing narrative of seaweed carbon as a possible climate change mitigation solution overlooks the central challenge that climate change itself is adversely impacting seaweed forests worldwide, potentially shifting these ecosystems from carbon sinks to carbon sources. In this Essay, we show that climate-induced seaweed forest loss is eroding their carbon sink capacity three to four orders of magnitude (1,000–10,000×) faster than it can currently be restored or recovered through active interventions. We consider that this stark disparity calls for a fundamental shift in seaweed carbon research away from a focus on future sequestration gains and toward explicitly assessing the risk of carbon emissions from climate-driven ecosystem loss.

Ocean warming and marine heatwaves are among the greatest threats to kelp forests globally, with warm temperatures affecting kelp performance and survival. Resolving the thermal response in nutrient uptake by kelp is an important step toward modelling functional physiological changes in these taxa under future temperatures. We tested the effect of temperature on the dissolved inorganic nitrogen (DIN) uptake of 2 geographically isolated populations of golden kelp Ecklonia radiata that experience substantial differences in ocean conditions (warm-adapted Western Australia, WA; and cool-adapted Tasmania, TAS). DIN uptake from 2 sources (nitrate, NO3-; and ammonium, NH4+) was measured at 3 concentrations (2, 5 or 20 mu M) across 8 temperature incubations (4-30 degrees C) over 2 h. Based on these treatments (population x DIN concentration x DIN source), we modelled thermal performance curves (TPCs) to quantify changes in thermal optima (Topt) and maximum uptake rates (Rmax). Most treatments followed TPCs, except NO3- uptake by the TAS population, which showed no temperature dependence. As predicted, Topt values for NH4+ uptake by the warmer WA population (24.3-30.0 degrees C) were consistently higher than the cooler TAS population (17.7-23.1 degrees C) across all concentrations. Except for 1 treatment (TAS 20 mu M), Rmax values for NH4+ uptake were also higher in the WA population. Our results indicate that DIN uptake by kelp occurs across a broad range of conditions and that the impacts of temperature depend strongly on the availability of DIN and population-level differences.

Large brown algae known as kelp cover extensive areas of Arctic coastlines and can form underwater forests that support diverse faunal communities. In many ice-scoured environments, where shallow subtidal habitats are structurally simplified, kelp may act as the primary foundation species, yet their ecological role across Arctic seascapes remains poorly resolved. Using baited cameras, diver surveys, habitat mapping, and satellite remote sensing, we assessed kelp distribution and associated biodiversity in the central Kitikmeot Sea, a near-estuarine, nutrient-poor system of the Northwest Passage. Across our study area ~80% of the seascape was bare, with kelp restricted to ~9 discrete, low-canopy forests (max ~0.6 km2) located in hydrodynamically exposed areas with earlier spring ice opening. Epifaunal communities differed among habitat types, with kelp and understorey algae supporting disproportionately higher invertebrate richness and densities than bare and kelp-adjacent habitats (up to 900 ind. m-2). Faunal assemblages also differed among individual kelp forests, amplifying biodiversity at the seascape scale. Distribution of larger motile fauna (mainly fish and crabs) was driven primarily by temperature rather than habitat: Gadus ogac sightings and foraging increased above water temperatures of 2.5 °C, whereas Hyas alutaceus occurred mainly below 0 °C; fish sightings in kelp forests increased with temperature. Together, these results identify kelp forests of the Kitikmeot Sea as spatially isolated yet functionally important “islands of diversity,” highlighting the role of habitat-forming macrophytes in polar coastal seascapes with limited benthic productivity and providing a baseline for anticipating change as ice and circulation regimes evolve.

BACKGROUND:Kelp forests are among the most productive and biodiverse ecosystems on Earth, yet they are currently facing unprecedented degradation and decline. In Europe, the conservation of these key ecosystems has historically relied on a plethora of disconnected national policies and passive protection, and they have frequently been underrepresented in governance frameworks. The recent adoption of the European Union Nature Restoration Regulation (NRR) represents a transformative shift, establishing the first legally binding targets for large-scale conservation and restoration of marine ecosystems shared by European Union members. SCOPE:Here, we discuss the challenges and opportunities that the NRR presents for European kelp forests and associated ecosystem services across Europe. As the NRR sets ambitious mandates for mapping habitats, condition assessments, and restoration by 2030-2050, its success depends on overcoming existing data gaps as well as on financial and technical challenges. We highlight how the NRR provides a valuable instrument to initiate immediate action across regions, and we propose a set of priority actions to achieve the NRR targets and address current scientific gaps. CONCLUSIONS:We emphasize the need for transboundary harmonization of monitoring protocols and methods for establishing reference areas for healthy kelp forests, the development of high-resolution maps for habitat distribution and condition, the establishment of clear-cut criteria for identifying resilient restoration areas (i.e., priority sites capable of achieving and maintaining favourable conservation status under NRR targets), and the development of standardized condition indicators (e.g., canopy density, indicator species) to assess "good condition" across different ecoregions. By linking legal requirements with science-based strategies, the NRR provides an opportunity to finally integrate kelp forests into European marine governance and achieve long-term sustainability for these critical ecosystems.

Macroalgae play a pivotal role in the global ocean carbon cycle, yet the spatial distribution of their coastal carbon burial, a long-term sink relevant to national inventories, remains poorly resolved. Here, we present a high-resolution (5-arcminute, ~9.2 km at the equator) global assessment of the Macroalgal Carbon Burial Index (MCBI), a measure of how favorable coastal environments are for burying macroalgal carbon. Synthesizing the most comprehensive dataset of sediment accumulation rates (n = 1617), we identify the depositional environment as a critical constraint on burial, acting independently of macroalgal productivity. High-favorability sinks are geographically rare (<1% of global coastlines), clustering in regions such as the Korea Bay and Okhotsk-Sakhalin region. We further identify coastline complexity as an informative predictor of MCBI (R² = 0.59), enabling low-cost identification of burial hotspots. Notably, we identify a mismatch in conservation: existing marine protected areas cover 23.9% of low-burial coastlines but only 17.1% of primary burial hotspots. Our findings map the global pattern of macroalgal carbon burial and provide a spatially explicit basis for its integration into conservation and management. This study mapped global hotspots for macroalgal carbon burial, showing favorable sites for sequestration. Coastline complexity predicts these hotspots, and combining burial favorability with century-scale preservation identifies the most durable blue carbon sinks.

In 2023–2024, widespread marine heatwaves associated with record ocean temperatures impacted ocean processes, marine species, ecosystems and coastal communities, with economic consequences. Despite warnings, interventions were limited. Proactive strategies are needed for inevitable future events.

Intensifying marine heatwaves (MHWs) are pervasive and destructive manifestations of anthropogenic climate change. Over the past two decades, MHWs have driven biological, ecological and socioeconomic change in almost all oceans and seas. In this Review, we assess the impacts of MHWs on marine organisms and the benefits they provide to people, highlight knowledge gaps and consider opportunities to mitigate MHW impacts. Globally, MHWs have become increasingly intense and frequent, and result in mortality or movement of species when acute temperature thresholds are exceeded. Vulnerability and resilience to MHWs vary among species, but these mortality events have been prominent for habitat-forming foundation species such as corals, kelp and seagrass, causing many cascading indirect impacts on ecosystem functioning and biodiversity. Poleward species shifts produce novel and complex species interactions and altered ecosystem functions, which have considerable consequences for people and their livelihoods. Reducing greenhouse gas emissions remains essential and urgent to address impacts long term, but increases in MHW intensities and duration will be unavoidable and prominent for the foreseeable future. As such, closing the current knowledge gaps around MHWs and their impacts on biodiversity, as well as proactive management strategies, are urgently needed to mitigate further damage to ecosystems and people, and to build resilience into the future. Marine heatwaves (MHWs) have become more intense and widespread globally, affecting species, ecosystems and people. After summarizing how and why MHWs are changing, this Review explores these impacts and their underlying mechanisms, highlights knowledge gaps and considers opportunities to mitigate the effects of MHWs.

The Arctic and Subarctic seas are predicted to become hotspots for marine heatwaves (MHWs). High-latitude marine ecosystems face unique consequences from accelerated warming and sea ice loss, challenging species adapted to cold conditions. We review the literature on MHW characteristics and ecological impacts in the Arctic and Subarctic seas, and contrast MHW characteristics between the Bering Sea and Barents Sea. We uncover the pervasive impacts of MHWs across widely different organism groups, including benthic foundation species, phytoplankton, zooplankton, fish, seabirds, and marine mammals. MHWs in the Arctic marginal seas are especially prevalent in areas experiencing sea ice retreat, such as seasonal sea ice zones, highlighting the complex interplay between MHWs and sea ice dynamics. Overall, few studies have documented the ecological impacts of MHWs on high-latitude ecosystems, with the notable exception of the impacts from the Bering Sea and Chukchi Sea MHWs in 2017–2019. Many Arctic species, with their cold and narrow thermal preferences, appear vulnerable to MHWs, as they might not have access to cold climate refugia, while boreal species appear to benefit from Arctic and Subarctic MHWs. Sessile foundation species, such as kelp and seagrasses, are especially at risk during MHWs, although in the Arctic evidence of MHWs impacts remains limited. Reproductive failure and mass mortality events have been documented for several species in the Pacific Arctic (e.g., seabirds, fish, crabs). MHWs have been observed to have ecosystem-wide repercussions in the northern Bering Sea and Chukchi Sea with shifts in plankton communities affecting the entire food web. The ecological responses to MHWs in the Arctic and Subarctic ecosystems are still not fully understood, highlighting a need for further research to assess the direct and indirect impacts on various taxa and to improve predictive models for better management and conservation strategies. MHWs can also have large consequences for ecosystem services and socio-ecological systems, for example, closures of economically valuable and culturally important fisheries, as seen in Alaska, degradation of traditional ice-hunting practices, and compromised wellbeing of coastal communities. Large and abrupt ecosystem changes following MHWs underscore the urgent need for adaptive management strategies in the face of ongoing climate change.

Global warming is driving contraction of species’ ranges through migration and mortality at their warm edge. However, for most species more subtle, sub-lethal changes in performance will be a more ubiquitous response to the Anthropocene. It has been suggested that reduction in body size will be a universal response to warming for cold-water species. Here we tested this hypothesis for two dominant kelp species in the northern and southern hemispheres, respectively. We tested if populations from cool and warm environments would be morphologically distinct, with warm-water populations displaying structural features indicative of sub-optimal conditions (smaller sizes). We found empirical evidence consistent with size reduction of kelp stipes, blades, and biomass of associated epiphytes from cool to warm water in both hemispheres. These changes are ecologically significant because they affect how kelps engineer their local environment, the three-dimensional habitat they create, and the associated communities they support. Reduced size of cold-water habitat forming species such as kelps may be a sublethal effect of warming that could have widespread but previously overlooked effects on the structure of ecosystems and the services that they provide.

Climate-driven changes to the chemical landscape of reefs affect the recovery of kelp forests

Macroalgal forests have been suggested to export substantial amounts of carbon to deep ocean sinks and could account for 27-34% of annual blue carbon sequestered in Australia. However, a major knowledge gap concerns how carbon in the detrital tissue of the dominant seaweed species is remineralized as it is exported offshore. We quantified decomposition and carbon content in detrital tissue of dominant canopy-forming seaweeds Ecklonia radiata and Scytothalia dorycarpa at three depths (10, 20, and 50 m) in a 50 d in situ litterbag experiment in Western Australia. We then combined these rates with a particle tracking model to estimate the potential export of macroalgae detritus from our experiment sites into deeper waters. Decomposition of particulate organic carbon was fast relative to other cooler regions globally, and there were no significant differences between species and most depths. One-half of the detritus was remineralized within 12 (+/- 2) days for E. radiata and 8 (+/- 2) days for S. dorycarpa, with similar to 8% remaining for both species after 50 d in situ. Based on simulated transport times and decomposition, 10% and 11% of the E. radiata and S. dorycarpa detritus from shallow reefs (10-20 m) were exported beyond the shelf break (>= 200 m) whereas 47% and 37% were exported from deep reefs (50 m). These estimates highlight the variable but substantial carbon sequestration potential across the coastal zone.