Global Change Biology

How ectothermic animals will cope with global warming, especially more frequent and intense heatwaves, is a critical determinant of the ecological impacts of climate change. There has been extensive study of upper thermal tolerance limits among fish species but how intraspecific variation in tolerance may be affected by habitat characteristics and evolutionary history has not been considered. Intraspecific variation is a primary determinant of species vulnerability to climate change, with implications for global patterns of impacts of ongoing warming. Using published critical thermal maximum (CTmax) data on 203 marine and freshwater fish species, we found that intraspecific vsariation in upper thermal tolerance varies according to a species latitude and evolutionary history. Notably, freshwater tropical species have lower variation in tolerance than temperate species in the northern hemisphere, which implies increased vulnerability to impacts of thermal stress. The extent of variation in CTmax among fish species has a strong phylogenetic signal, which may indicate a constraint on evolvability to rising temperatures in tropical fishes. That is, in addition to living closer to their upper thermal limits, tropical species may have higher sensitivity and lower adaptability to global warming compared to temperate counterparts. This is evidence that tropical fish communities, worldwide, are especially vulnerable to ongoing climate change.

AimGlaciation events shaped the present distribution of many plants and their biodiversity in the northern hemisphere. Glacial expansion forced many species south and came with much colder global temperatures, while glacial recession brought warmer temperatures and newly colonizable land without competition. However, the changes in plant diversity associated with glacial retreat and the ensuing climatic changes is not well understood. In this study, we quantify Late Quaternary climate-driven changes in Arctic plant diversity by integrating climatic shifts in species distributions since the last glacial maximum (20-16 kya) and mid-Holocene (5 kya) across the circumpolar arctic. LocationThe geographic arctic, 66{degrees}N. TaxonVascular plants. MethodsWe built species distribution models using phyloregion v. 1.0.9 in R using occurrence data from the Global Biodiversity Information Facility and climate rasters from Worldclim v2.1 for the present and v1.4 for the mid-Holocene and Last Glacial Maximum. Results and DiscussionWe found limited evidence for decreases in weighted endemism, and species richness along with diverging north-south shifts in the centroids of many distributions contrary to expectations of increased alpha diversity since the last glacial maximum. Decreases in species alpha diversity, while already quite low in the arctic, may be reflective of an increasingly variable arctic climate that disfavors plants with a slow dispersal ability. This is especially important given the projected increase in global temperature across many shared socioeconomic pathway scenarios and can be contrasted with our results of the Mid-Holocene, which was roughly a degree warmer than it is today. The arctic is presently warming at roughly two to five times the rate of the mid-latitudes and equator and understanding how plants have responded in the past will help inform on how they may change in the future.

The Arctic is warming rapidly, with winters warming up to seven times as fast as summers in some regions. Warm spells in winter lead to more frequent extreme rain-on-snow events that alter snowpack conditions and can encapsulate tundra vegetation in basal ice ( icing) for several months. However, tundra climate change studies have mainly focused on summer warming. Here, we investigate icing effects on vascular plant phenology, productivity, and reproduction in a pioneer field experiment in high Arctic Svalbard, simulating rain-on-snow and resultant icing in five consecutive winters, assessing vascular plant responses throughout each subsequent growing season. We also tested whether icing responses were modified by experimentally increased summer temperatures. Icing alone delayed early phenology of the dominant shrub, Salix polaris, but with evidence for a catch-up (through shortened developmental phases and increased community-level primary production) later in the growing season. This compensatory response occurred at the expense of delayed seed maturation and reduced community-level inflorescence production. Both the phenological delay and allocation trade-offs were associated with icing-induced lags in spring thawing and warming of the soil, crucial to regulating plant nutrient availability and acquisition. Experimental summer warming modified icing effects by advancing and accelerating plant phenology (leaf and seed development), thus increasing primary productivity already early in the growing season, and partially offsetting negative icing effects on reproduction. Thus, winter and summer warming must be considered simultaneously to predict tundra plant climate change responses. Our findings demonstrate that winter warm spells can shape high Arctic plant communities to a similar level as summer warming. However, the absence of accumulated effects over the years reveals an overall resistant community which contrasts with earlier studies documenting major die-off. As rain-on-snow events will be rule rather than exception in most Arctic regions, we call for similar experiments in coordinated circumpolar monitoring programmes across tundra plant communities.

AbstractMesopelagic fishes play a crucial role in the global carbon cycle through their diel vertical migration (DVM), but the impacts of neither natural nor anthropogenic climate change on DVM patterns are currently known. Studying the geological past can elucidate changes in DVM patterns under swelling climate pressure and allow estimating the impacts of the current climate crisis. We present a multi-proxy, ecosystem-level assessment of paleoenvironmental changes in the Eastern Mediterranean during the Middle Pleistocene (marine isotope stages MIS 23-18; 923-756 ka B.P.) and use the carbon and oxygen isotopic composition of fossil fish otoliths to assess the impacts of these changes on DVM and their possible implications on the biological pump. Temperature was the primary driver of ecosystem change during MIS 21 interglacial, whereas productivity became a dominant factor in MIS 19 interglacial. Responses of organisms throughout the water column varied. Our results indicate increased productivity across trophic levels during MIS 19, which affected foraminiferal biomasses, but did not inhibit fish DVM. In contrast, the early MIS 21 warming led to a reduction in DVM by mesopelagic fishes and consequently a drop in biological pump efficiency.

Marine species are widely expected to shift poleward or into deeper waters in response to rising ocean temperatures. However, our knowledge is primarily based on studies examining range shifts along single dimensions at a time (e.g., latitude or depth). Failing to address how movements along multiple dimensions interact, including associated changes in thermal exposure, may result in misleading conclusions and predictions of species distribution and community composition under global warming. To address this knowledge gap, we here develop and apply a multidimensional framework that jointly evaluates climate-driven redistribution of marine fish across latitude, longitude, depth and realized thermal niches, based on long-term scientific bottom-trawl surveys throughout the North Atlantic and Northeast Pacific. Our results show that net redistributions are generally small and highly region-specific, while the realized thermal niches of species have warmed substantially over the past three decades. These findings demonstrate that spatial redistribution is generally failing to keep pace with rising temperatures and challenge the prevailing assumption that marine species will move to escape warming. This has direct implications for biodiversity indicators that rely on distributional shifts as evidence of climate impacts, as well as climate-informed management and conservation of marine ecosystems, fisheries, and biodiversity at large.

Climate warming is altering the timing of seasonal growth in cold-temperate and boreal ecosystems, with potential consequences for forests on multiple continents. It has been hypothesized that in a warming climate, earlier growth in spring might lead to a longer growing season and thus higher annual total growth. However, whether the period of active growth is longer in a warming climate is unclear, as is whether changes in growth rate might reinforce or offset the effect of changing growing season length in modulating annual total growth. Understanding responses in growth to warming is crucial to projecting forest dynamics under future climate change. To address this, we experimentally warmed (by +1.62 and 3.26 {degrees}C on average) open-air southern boreal forest plots for 14 years (2010 to 2023) and measured juvenile height growth from spring until fall for 20 tree and shrub species, with a total of 11,565 individual-year growth trajectories. Across all species and in various canopy (open, closed) and rainfall (ambient, reduced) contexts, experimental warming consistently advanced the timing of height growth (4.32 days per 3 {degrees}C warming). However, species differed in their annual total growth responses. Evergreen gymnosperms such as spruce, fir, and pine tended to exhibit suppressed annual total growth under warming (-15.8% per 3 {degrees}C warming), driven by a shorter duration of active growth, despite an earlier start, with little change in relative growth rate. In contrast, deciduous angiosperms and non-native shrubs often show enhanced annual total growth under warming (19.3 and 37.4% per 3 {degrees}C warming), driven by faster rate, longer duration, or a combination of both. More broadly, species with a more conservative lifestyle responded less positively in the duration and rate of growth, thus placing them at a disadvantage under warming. These contrasting responses challenge assumptions that an earlier start of the growing season universally enhances growth and highlight how warming may shift both the timing and pace of growth and thus alter community composition within mixed-species forests.

Climate warming is increasing mismatches between thermal phenotypes and habitat temperatures, driving range shifts and population extirpations. While within-species variation in heat tolerance and local warming rates can predict responses to climate warming, how these factors shape differences in vulnerability among taxa and ecosystems is uncertain. Here we combine climate and thermal trait data from 69 species across four ecosystem types to examine the effects of incorporating intraspecific variation in heat tolerance and local warming rates on projected vulnerability to climate warming. Because vulnerability to warming depends on existing phenotypic variation in thermal performance and relative rates of habitat warming, we develop a new metric that integrates localized rates of warming with spatial variation in thermal tolerances, termed the minimum trait velocity. Incorporating intraspecific variation in heat tolerance lowered estimates of warming tolerance (a measure of vulnerability) across most ecosystem types, with the strongest negative impact on marine taxa. Although intraspecific variation in heat tolerance could facilitate adaptation to climate change, our results suggest such variation is generally less than the projected near future warming. This suggests that opportunities for evolutionary rescue via gene flow between locally adapted populations are limited, adding to mounting concern as the climate warms.

Empirical studies worldwide show substantial variability in plant litter decomposition responses to warming, leaving the overall impact of climate change on this process uncertain. We conducted a meta-analysis of 109 experimental warming studies across seven continents, utilizing natural and standardized plant material, to assess the overarching effect of warming on decomposition and identify potential moderating factors. Warming influences decomposition differently across macro-environmental gradients of moisture and temperature. Negative warming effects on decomposition in warmer, low-moisture areas were counterbalanced by the positive, though not significant, warming effects in colder areas, resulting in an overall non-significant effect. We determine that at least 5.2 degrees of warming is required for a significant increase in decomposition. This is particularly relevant given the past decades global warmth in higher latitudes, holding a significant proportion of terrestrial carbon. Low-quality plant litter was more sensitive to warming. Therefore, future vegetation changes toward low-quality, temperature-sensitive plants could increase carbon release and reduce the net supply of stored organic matter in the soil by increasing the decomposition of low-quality litter with warming. Our findings emphasize the connection between warming responses, macro-environment, and litter characteristics, refining predictions of climate changes consequences on key ecosystem processes and its contextual dependencies.

Soil carbon losses to the atmosphere through soil respiration are expected to rise with ongoing temperature increases, but available evidence from mesic biomes suggests that such response disappears after a few years of experimental warming. However, there is lack of empirical basis for these temporal dynamics in soil respiration responses, and of the mechanisms underlying them, in drylands, which collectively form the largest biome on Earth and store 32% of the global soil organic carbon pool. We coupled data from a ten-year warming experiment in a biocrust-dominated dryland ecosystem with laboratory incubations to confront 0-2 years (short-term hereafter) vs. 8-10 years (long-term hereafter) soil respiration responses to warming. Our results showed that increased soil respiration rates with short-term warming observed in areas with high biocrust cover returned to control levels in the long-term. Warming-induced increases in soil temperature were the main driver of the short-term soil respiration responses, whereas long-term soil respiration responses to warming were primarily driven by thermal acclimation and warming-induced reductions in biocrust cover. Our results highlight the importance of evaluating short and long-term soil respiration responses to warming as a mean to reduce the uncertainty in predicting the soil carbon - climate feedback in drylands.

Many species are responding to global warming by shifting their distributions upslope to higher elevations, but the observed rates of shifts vary considerably among studies. Here we test the hypothesis that this variation is in part explained by latitude, with tropical species being generally more responsive to warming temperatures than are temperate species. We find support for this hypothesis in each of two independent empirical datasets--shifts in species elevational ranges, and changes in composition of forest inventory tree plots. Tropical species are tracking rising temperatures 2.1-2.4 times (range shift dataset) and 10 times (tree plot dataset) better than their temperate counterparts. Models predict that for a 100 m upslope shift in temperature isotherm, species at the equator have shifted their elevational ranges 93-96 m upslope, while species at 45{degrees} latitude have shifted only 37-42 m upslope. For tree plots, models predict that a 1{degrees}C increase in temperature leads to an increase in community temperature index (CTI), a metric of the average temperature optima of tree species within a plot, of 0.56 {degrees}C at the equator but no change in CTI at 45{degrees} latitude (-0.033). This latitudinal gradient in temperature tracking suggests that tropical montane biotas may be on an "escalator to extinction" as global temperatures continue to rise.

O_LIEnvironmental change often occurs as a sequence of stressors rather than as isolated events. While the individual and combined effects of multiple stressors are well studied, the ecological consequences of sequential environmental change remain poorly understood. Such sequences, for example, a marine heatwave followed by seasonal herbicide runoff, are increasingly common under global change. C_LIO_LIWe investigated how legacy effects (the imprint of past environmental conditions) influence subsequent population performance and functional traits, and how these effects are mediated by strain-specific sensitivities. C_LIO_LIUsing a fully factorial design, we exposed six strains of the globally abundant pico-phytoplankton Synechococcus sp. to three environmental conditions: warming, herbicide exposure, and a control, under both chronic (same condition across time) and sequential (different conditions across time) regimes. We measured population performance (per-capita growth rate, maximum density) and key functional traits (cell size, chlorophyll content). C_LIO_LIPopulation responses diverged significantly between chronic and sequential exposures, revealing a strong legacy effect. Trait changes were often decoupled from growth metrics, suggesting independent response axes. Strain identity and its interaction with past conditions explained substantial variation in both growth and trait responses. C_LIO_LIWe identify and conceptualise four distinct mechanisms of legacy effects during sequential change: overcompensation, amplification, constraint and depression, each linked to strain-specific responses. Consequently, incorporating legacy effects into predictions of biodiversity dynamics and ecosystem function under global change is therefore both feasible and essential. C_LI

Extreme climate events such as droughts and heatwaves are intensifying under climate change, yet their combined effects on plant recovery remain unclear. In a two-year outdoor mesocosm experiment, we tested how grassland species with contrasting growth strategies recover from summer drought under four warming regimes: ambient, moderate warming (+2 {degrees}C), periodic heatwaves (+7 {degrees}C), and their combination. Experimental communities of native fast- and slow-growing species plus the invasive Solidago canadensis were assessed for above-ground biomass and leaf traits (SLA, LDMC, chlorophyll content, stomatal conductance) at one- and four-months post-drought. Biomass fully recovered within one month in both growth strategies, but leaf traits showed transient shifts, over-recovery in SLA and under-recovery in LDMC, likely reflecting production of new leaf tissues. These deviations generally returned to control levels by four months, regardless of warming treatments. Solidago canadensis exhibited high tolerance to heat and drought, with early biomass and trait recovery, indicating potential for dominance under climate extremes. Biomass recovery was similar across growth strategies, suggesting that growth-related differences play a minimal role in short-term recovery; however, early regrowth was characterised by contrasting trait shifts. Such lagged trait recovery, combined with rapid invasive recovery, suggests potential for longer-term shifts in grassland composition and function. We recommend that incorporating trait-based recovery dynamics is essential for predicting ecosystem stability under compound climate extremes.

Understanding the mechanisms that shape species geographic distributions is essential for predicting responses to climate change, particularly under winter conditions, which are warming more rapidly than summer conditions in many temperate ecosystems. Woodland salamanders, which constitute many of the species in the global hotspot of salamander diversity located in the southern Appalachian Mountains of North America, are thought to cope with harsh winters through dormancy and metabolic suppression. We used a field-based experiment to investigate the winter physiology and energetics of a woodland salamander (Plethodon metcalfi). Despite showing histological signs of dormancy, salamanders exhibited higher metabolic rates during winter than the active season when measured at low temperatures associated with winter, consistent with metabolic compensation. High-elevation salamanders also exhibited elevated metabolic rates at cold temperatures compared to low-elevation salamanders, indicative of countergradient metabolic compensation. When integrated into a biophysical species distribution model, depletion of energy stores estimated from winter metabolic rates under current climates accurately predicted the geographic range limits of our focal species. Under future warming scenarios, incorporating winter physiology indicated that 77% of the species range would experience local extinction due to depletion of energy stores. These findings challenge assumptions about winter energetics, reveal metabolic limits on cold-season survival, and highlight metabolic demands during dormancy as a key constraint on responses to climate change.

Increasing temperatures and extreme heat episodes have become more common with climate change. While forests are known to be buffered from increasing temperatures compared to non-forested areas, whether this buffering is maintained under extreme temperature events, how such events influence forests, and how forest organisms respond to extreme heat is relatively unknown. Here we assess the effects of an extreme heat event (the Pacific Northwest (PNW) heatdome in June 2021) on forest microclimates, forests, and the organisms living within them. We first asked how the PNW heatdome affected microclimates in forests with differing canopy cover (including non-forests) and found that the buffering capacity of forests is greater under denser canopies, even under extreme heat events. We then combined this information with organismal temperature tolerance curves for 12 relevant species and found that canopy buffering can minimize the negative impacts of even extreme heat events on understory organisms, with greater canopy density providing greater microclimate moderation. Finally, we analyzed seasonal NDVI trends in recent years, and found signs of canopy stress following the extreme 2021 heat event. In all, this suggests that although forest canopies may buffer the negative effects of extreme heat events on understory organisms, a greater frequency of extreme heat events may threaten this capacity by damaging forest canopies.

Under accelerating threats from climate change impacts, marine protected areas (MPAs) have been proposed as climate adaptation tools to enhance the resilience of marine ecosystems. Yet, debate persists as to whether and how MPAs may promote resilience to climate shocks. Here, we empirically assess whether a network of 85 temperate MPAs in coastal waters promotes resilience against marine heatwaves in Central and Southern California. We use 38 years of satellite-derived kelp cover to test whether MPAs enhance the resistance of kelp forest ecosystems to, and recovery from, the unprecedented 2014-2016 marine heatwave regime. We also leverage a 20-year time series of subtidal community surveys to understand whether protection and recovery of sea urchin predators within MPAs explain emergent patterns in kelp forest resilience through trophic cascades. We find that fully protected MPAs (i.e. no-take marine reserves) significantly enhance the resistance to and recovery of kelp forests to marine heatwaves in Southern California, but not in Central California. Differences in regional responses to the heatwaves may be partly explained by three-level trophic interactions comprising kelp, urchins, and predators of urchins. Urchin abundances in Southern California MPAs are significantly lower within fully protected MPAs during and after the heatwave, while the abundance of their predators are higher. In Central California, there is no significant difference in urchin abundances within protected areas as the current urchin predator, sea otters, are unilaterally protected. Therefore, we provide evidence that fully protected MPAs can be effective climate adaptation tools, but their ability to enhance resilience to extreme climate events depends upon region-specific environmental and ecological dynamics. As nations progress to protect 30% of the oceans by 2030 scientists and managers should consider whether protection will increase resilience to climate-change impacts given their local ecological contexts, and what additional measures may be needed.

Marine heatwaves are transforming ecosystems, yet their role in driving alternative states--and the conditions that enable these transitions--remains poorly understood. Using 30 years of satellite and underwater data, we assessed the impact of the 2014-2016 Pacific marine heatwaves on giant kelp forests (Macrocystis pyrifera) at their warm range limit in Mexico. By 2016, 88% of forests were lost, with limited recovery by 2023, including an 80 km range contraction at the southern edge. Surveys revealed three alternative states: replacement by heat-tolerant palm kelp (Eisenia arborea) in warmer regions; urchin barrens due to predator overfishing; and, unexpectedly, persistent giant kelp near the southern limit where high temperatures coincide with low human pressure. Pre-existing conditions, such as high urchin and palm kelp densities, shaped these outcomes. These findings show that responses to marine heatwaves are shaped by local ecological and human contexts, requiring tailored climate-adaptation strategies to promote resilience.

1. The effects of climate change are now more pervasive than ever. Marine ecosystems have been particularly impacted by climate change, with Marine Heat Waves (MHWs) being a strong driver of mass mortality events. Even in the most optimistic greenhouse gas emission scenarios, MHWs will continue to increase in frequency, intensity, and duration. For this reason, understanding the resilience of marine species to the increase of MHWs is crucial to predicting their viability under future climatic conditions. 2. In this study, we explored the consequences of Marine Heatwaves (MHWs) on the resilience of a Mediterranean key octocoral species, Paramuricea clavata, to further disturbances to their population structure. To quantify P. clavatas capacity to resist and recover from future disturbances, we used demographic information collected from 1999 to 2022, from two different sites in the NW Mediterranean Sea. 3. Our results showed that the differences in the dynamics of populations exposed and those not exposed to MHWs were driven mostly by differences in mean survivorship and growth. We also showed that after MHWs P. clavata populations had slower rates of recovery but did not experience changes in resistance. Populations exposed to MHWs had lower resistance elasticity to progression but higher stasis compared to unexposed populations. In contrast, the only demographic process showing some differences when comparing the speed of recovery elasticity values between populations exposed and unexposed to MHWs was stasis. Finally, under scenarios of increasing frequency of MHWs, the extinction of P. clavata populations will accelerate and their capacity to recover after further disturbances will be hampered. 4. Overall, these findings confirm that future climatic conditions will make octocoral populations even more vulnerable to further disturbances. These results highlight the importance of limiting local impacts on marine ecosystems to dampen the consequences of climate change.

Climate warming is driving changes in species distributions, although many species show a so-called climatic debt, where their range shifts lag behind the fast shift in temperature isoclines. Protected areas (PAs) may impact the rate of distribution changes both positively and negatively. At the cold edges of species distributions, PAs can facilitate species distribution changes by increasing the colonization required for distribution change. At the warm edges, PAs can mitigate the loss of species, by reducing the local extinction of vulnerable species. To assess the importance of PAs to affect species distribution change, we evaluated the changes in a non-breeding waterbird community as a response to temperature increase and PA status, using changes of species occurrence in the Western-Palearctic over 25 years (97 species, 7,071 sites, 39 countries, 1993- 2017). We used a community temperature index (CTI) framework based on species thermal affinities to investigate the species turn-over induced by temperature increase. In addition, we measured whether the thermal community adjustment was led by cold-dwelling species extinction and/or warm-dwelling species colonization, by modelling the change in standard deviation of the CTI (CTIsd). Using linear mixed-effects models, we investigated whether communities within PAs had lower climatic debt and different patterns of community change regarding the local PA surface. Thanks to the combined use of the CTI and CTIsd, we found that communities inside PAs had more species, higher colonization, lower extinction and the climatic debt was 16% lower than outside PAs. The results suggest the importance of PAs to facilitate warm-dwelling species colonization and attenuate cold-dwelling species extinction. The community adjustment was however not sufficiently fast to keep pace with the strong temperature increase in central and northeastern Western-Palearctic regions. Our study underlines the potential of the combined CTI and CTIsd metrics to understand the colonization-extinction patterns driven by climate warming.

Recent climate warming is associated with the increasing magnitude and frequency of extreme events, including heatwaves and drought periods worldwide. Such events can have major effects on the species composition of plant communities, hence on biodiversity and ecosystem functioning. Here we studied responses of Central European dry grassland plants to fluctuating temperature and precipitation over the last thirty years with monthly temporal resolution. We assessed the seasonal and annual dynamics of plant recruitment and growth based on the analysis of annual growth rings from the root collar. Although most studies so far applied such methods to trees and shrubs, we focused on typical grassland plants, two forbs and two chamaephytes. We related the recruitment and annual growth to monthly and annual precipitation, temperature and aridity between 1991 and 2019. We revealed species-specific responses, namely the (i) recruitment of deep-rooted, heavy-seeded species was positively affected by precipitation in both late winter-early spring and summer, whereas recruitment of shallow-rooted, light-seeded species was weakly influenced by climate fluctuations; (ii) growth of shallow-rooted species was more adversely affected by high summer temperature and drought than the growth of deep-rooted species. The population age structure of all the studied species was affected by the climate of the past decades. Most individuals established in the wet period of the 2000s, fewer in the precipitation-poorer 1990s, and the establishment was considerably reduced in the dry and warm period of the 2010s. Our findings indicate that the change towards warmer and drier climate has a profound effect even on drought-adapted ecosystems such as temperate dry grasslands. However, plant responses to various climatic extremes are species-specific, depending on their characteristics, such as life form or rooting depth. Consequently, the ongoing and anticipated climate warming will likely result in complex changes in species composition and other ecosystem properties of temperate grasslands

Marine heatwaves (MHWs) are intensifying under climate change, yet the physiological limits that constrain seagrass resilience remain poorly defined. We experimentally tested the responses of the surfgrass Phyllospadix scouleri, a foundation species of the Northeast Pacific coast, to simulated MHWs of contrasting intensity. In a 27-day mesocosm experiment, plants were exposed to fluctuating temperatures representing a severe MHW (23.5 {+/-} 1.5 {degrees}C) and an extreme MHW (26.5 {+/-} 1.5 {degrees}C), while photosynthetic performance, respiration, nitrogen metabolism, oxidative stress, and growth were monitored during and after warming. Phyllospadix scouleri maintained photosynthetic capacity and carbon balance under severe warming but exhibited pronounced physiological disruption at extreme temperatures, including sustained photoinhibition, reduced nitrate assimilation, elevated respiration, and negative daily productivity. These effects persisted after heat stress, leading to reduced growth and indicating incomplete recovery. Multivariate analyses revealed a distinct transition from tolerance to functional breakdown near 26.5 {degrees}C, suggesting a physiological tipping point only 5-6 {degrees}C above current summer maxima in the area of the studied population. Our findings demonstrate that intensifying MHWs may rapidly erode the thermal safety margin of temperate seagrasses, pushing foundational coastal ecosystems toward metabolic instability and potential regime shifts under continued ocean warming. HighlightExtreme marine heatwave disrupts photosynthesis, nitrogen metabolism, and carbon balance in the seagrass Phyllospadix scouleri, suggesting a narrow thermal safety margin in the face of ocean warming.