Longer and more frequent marine heatwaves over the past century

Idiosyncratic patterns of chlorophyll-a anomalies in response to marine heatwaves in the Adriatic Sea (Mediterranean Sea) over the last two decades

In the open ocean, marine heatwaves (MHWs) have been associated to a decline of Chlorophyll-a (Chl-a) concentration in tropical and temperate areas while, at higher latitudes, they seem to enhance phytoplankton productivity. Currently, uncertainties remain on the outcomes of MHWs on primary production in coastal and heterogenous marine regions. We analyzed long-term modeled satellite-derived data on sea surface temperature and Chl-a concentration in the Adriatic Sea (Mediterranean Sea), a semi-enclosed basin where coastal and open-sea environmental conditions co-occur, to explore Chl-a responses to MHWs. We found that both low and high Chl-a anomalies were strictly dependent on MHWs, although following direct or inverse relationships in different areas, as a consequence of regional-scale heterogeneities in nutrient availability, riverine inputs, circulation and geomorphology. Along the west coast and shallow areas of the North and Central Adriatic, high MHWs frequency, duration and intensity corresponded to high frequency of Chl-a peaks and/or increased intensity and duration of low Chl-a anomalies, suggesting pronounced fluctuations with intense phytoplankton blooms alternating to extremely low production events. Conversely, in offshore and deeper areas, especially in the South Adriatic, MHWs frequency, duration and intensity inversely correlated with Chl-a anomalies, indicating a possible reduction of phytoplankton biomass and a decline of organic matter flow towards the sea floor. Prolonged MHWs may therefore drive shifts in primary production with possible ecosystem-wide effects in both coastal and pelagic areas. These multifaceted MHW-Chl-a interactions observed in the Adriatic Sea emphasize the need for context-specific assessments in environmentally complex marine regions to develop management strategies addressing ecological and socioeconomic issues arising from the unrelenting increase of temperature anomalies.

Laboratory experiments revealed different bleaching susceptibilities to heat stress in eight species of subtropical urban corals

Understanding species' susceptibility to environmental stressors is crucial for conservation planning, but such data are unavailable for many subtropical corals. We therefore conducted 1-month laboratory experiments to determine the heat stress susceptibility of eight species from subtropical areas by exposing them to 32 °C (treatment) or 25 °C (control). Four species (Dipsastraea rotumana, Echinophyllia aspera, Pavona decussata, and Platygyra carnosa) survived the whole experiment, although bleaching occurred after one to two weeks of exposure. The heat exposure caused total mortality in the other four species: on day 2 in Acropora solitaryensis, day 7 in Acropora digitifera, day 9 in Acropora pruinosa, and day 17 in Montipora peltiformis. These results suggest that repeated heatwaves may cause changes in coral communities by causing disproportionally high mortality of heat-sensitive species. Coral species tested in this study, collected from subtropical reefs previously thought to be refuges for coral reefs under global warming, demonstrated greater susceptibility to heat stress than their tropical counterparts. This raises concerns about the persistence of coral reefs as sea surface temperatures continue to rise.

Coastal seawater turbidity and thermal stress control growth of reef-building Porites spp. corals in Fiji

Nearshore reefs, at the interface of land-sea interactions, provide essential ecosystem services, but are susceptible to multiple global and local stressors. These stressors can detrimentally impact coral growth and the continuity of the reef framework. Here, we analyse coral growth records (1998 - 2016) of massive Porites spp. colonies from nearshore reefs in Fiji. Our aim is to assess the role of thermal stress and turbidity on coral growth across a range of environments. Our findings reveal a negative linear relationship between linear extension and seawater turbidity across locations (GLM, R2 = 0.42, p < 0.001), indicating that average coral growth is significantly influenced by local environmental conditions. On interannual timescales, all locations experienced a 14% to 30% decrease in linear extension in response to acute thermal stress during the 2013 - 2016 period. This finding highlights the existence of compounding effects between water quality and thermal stress. We suggest that inshore, long-lived massive hard corals in areas of high turbidity are more vulnerable to increasing SSTs due to an already reduced mean growth. Integrated management strategies in these regions that considers managing for multiple, interacting local stressors are warranted to enhance resilience.

Synthesis of transcriptomic studies reveals a core response to heat stress in abalone (genus Haliotis)

BACKGROUND

As climate change causes marine heat waves to become more intense and frequent, marine species increasingly suffer from heat stress. This stress can result in reduced growth, disrupted breeding cycles, vulnerability to diseases and pathogens, and increased mortality rates. Abalone (genus Haliotis) are an ecologically significant group of marine gastropods and are among the most highly valued seafood products. However, heat stress events have had devastating impacts on both farmed and wild populations. Members of this genus are among the most susceptible marine species to climate change impacts, with over 40% of all abalone species listed as threatened with extinction. This has motivated researchers to explore the genetics linked to heat stress in abalone. A substantial portion of publicly available studies has employed transcriptomic approaches to investigate abalone genetic response to heat stress. However, to date, no meta-analysis has been conducted to determine the common response to heat stress (i.e. the core response) across the genus. This study uses a standardized bioinformatic pipeline to reanalyze and compare publicly available RNA-seq datasets from different heat stress studies on abalone.

RESULTS

Nine publicly available RNA-seq datasets from nine different heat-stress studies on abalone from seven different abalone species and three hybrids were included in the meta-analysis. We identified a core set of 74 differentially expressed genes (DEGs) in response to heat stress in at least seven out of nine studies. This core set of DEGs mainly included genes associated with alternative splicing, heat shock proteins (HSPs), Ubiquitin-Proteasome System (UPS), and other protein folding and protein processing pathways.

CONCLUSIONS

The detection of a consistent set of genes that respond to heat stress across various studies, despite differences in experimental design (e.g. stress intensity, species studied-geographical distribution, preferred temperature range, etc.), strengthens our proposal that these genes are key elements of the heat stress response in abalone. The identification of the core response to heat stress in abalone lays an important foundation for future research. Ultimately, this study will aid conservation efforts and aquaculture through the identification of resilient populations, genetic-based breeding programs, possible manipulations such as early exposure to stress, gene editing and the use of immunostimulants to enhance thermal tolerance.

Growth disturbances in bivalve shell: Implications for past and future intra-annual scale climate change

Growth disturbances in bivalve shells are widely observed in both fossil and modern species, yet the conditions and mechanisms driving their formation remain unclear. Through controlled experiments on Mimachlamys nobilis, we demonstrate that abrupt intra-annual temperature fluctuations (≥5 °C) induce shell growth disturbances by altering the energy budget, diverting resources from shell growth to stress responses. Under ≥5 °C fluctuations (Groups C and D), 67 % of the individuals exhibited shell thinning, reduced growth rates, and formed growth disturbance lines (Groups C and D), whereas <5 °C fluctuations (Groups A and B) caused disturbances in only 12 % of the cases. These bivalve growth disturbances serve as a sensitive bio-indicator of short-term temperature fluctuations, providing a novel tool for reconstructing intra-annual level marine climatic fluctuations in deep time.

Short-term heat shock exposure affects the productivity of two habitat-forming NE Atlantic kelp species differently

Understanding the mechanisms by which key macroalgal species respond to temperature variation is critical to understanding changes in ecosystem functioning in rapidly warming oceans. This study tests experimentally for effects of short-term sublethal heat shock (20 °C vs. ambient 10 °C) on the health (growth and productivity), physiological performance (photosynthetic variables) and potential for compensatory mechanisms (phenolic content) of two kelp species, Saccharina latissima and Laminaria digitata, in the Northeast Atlantic. We assessed the effect of heat shock duration over five time points (0, 6, 24, 48, 72 h). Growth of S. latissima increased by ∼56 % under heat shock, while L. digitata growth was unaffected by temperature. Gross primary productivity (GPP) of S. latissima was initially greater in heat shock treatments (after 6 h) but declined after 48 h, while L. digitata GPP remained unaffected by heat shock regardless of exposure time. The relative maximum electron transport rate (rETRmax) of S. latissima differed depending on exposure duration, whereas the rETRmax of L. digitata increased with heat shock and over exposure time. Minimum saturating irradiance (Ek) did not differ for S. latissima, but varied for L. digitata over exposure time. Neither species showed changes in phenolic content after 72 h. Our results suggest that both kelp species exhibit the potential for short-term acclimation to sublethal heat shock but that responses are species-specific. These findings highlight the ability of these species to cope with short-term temperature stress, which may provide resistance to extreme temperature events, such as marine heat waves, in the future.

El Niño was a key driver of anomalous ocean warming in Southeast Asia in 2023

In 2023, global ocean heat content reached unprecedented values since records began in 1960. The translation of global ocean heat into regional and local-scale ocean warming remains poorly understood because of limited observational data, particularly within Southeast Asia. Here, we investigate the 2023 ocean warming event in Southeast Asia using near-continuous 41-month in-situ ocean temperature observations from the Singapore Strait, satellite sea surface temperature (SST) measurements, and high-resolution reanalysis products. We document anomalous ocean warming across the Singapore Strait and surrounding South China Sea and Indonesian Seas to depths of at least 40 m. Peak SSTs of 1.8 °C above the climatological mean were recorded in the central Sunda Shelf in November 2023 for the first time in > 40 years. Concurrent anomalous freshening of the Singapore Strait was observed, with average salinity below the climatological mean from October to December. We identify a southward migration of warm temperature anomalies beginning with the onset of the El Niño in July 2023 near the Luzon Strait. This occurred alongside southward shifts in mean sea-level pressure and near-surface ocean currents in the region. We attribute these observations to the southward shift of the North Equatorial Current bifurcation latitude, which permitted the intrusion of Pacific western boundary currents into the South China Sea and Indonesian seas. Compared to the oceanic drivers, atmospheric forcings played a limited role in driving the ocean warming in 2023. Our study highlights El Niño as the key driver of the ocean warming in Southeast Asia in 2023, and emphasises the need for expanded continuous, in-situ ocean temperature monitoring to enhance understanding of evolving ocean-atmosphere dynamics and impacts in Southeast Asia under a warming climate.

Heatwave duration, intensity and timing as drivers of performance in larvae of a marine invertebrate

In marine ecosystems, crustaceans face an alarming threat from the increasing frequency and intensity of marine heatwaves as their early planktonic stages are particularly temperature sensitive. While the impact of heatwaves on adult crustaceans is well-studied, their effects on larvae remain underexplored. This study focuses on heatwave effects on larvae of the European shore crab, Carcinus maenas. Through a factorial experiment, larvae were exposed to different heatwaves of varying onset timings, durations, and intensities. Survival, development duration, and dry mass decreased under intense heatwaves, with more severe effects observed when heatwaves occurred later in development, highlighting a stage-specific sensitivity to heatwave. We also identified a "region of existence" beyond which larval performance was compromised compared to baseline temperatures. This region defines the heatwave components considered "extreme" for the organism, as well as those inducing neutral or positive effects on performance. Additionally, we distinguished heatwave effects (characterised by their components) from those attributed to the average temperature experienced during the experiments. Our findings demonstrated that larval performance was lower during intense heatwaves compared to the performance expected under a constant average temperature. These findings emphasize the importance of considering heatwave timing relative to the life cycle for predicting marine population responses to climate change.

Effects of Bacterial Infections under Heatwaves on Chinese Soft-Shelled Turtles and Their Single-Cell Transcriptomic Landscapes

The intensification of global warming could precipitate the widespread dissemination of opportunistic pathogens, exerting a bidirectional strain on wildlife populations and potentially hastening the process of species extinction. In this study, we integrated indicators from peripheral blood single-cell transcriptome, behavior, and physiological indices in Chinese soft-shelled turtles (Pelodiscus sinensis) to explore the impact of dual stress caused by bacterial infections and/or heatwaves on the turtles. Turtles were randomly divided into four groups based on constant temperature at 28 °C and heatwave exposure, as well as whether they were infected with bacteria (Bacillus cereus). Principal component analysis-based cell clustering revealed that the 14 cell clusters were classified into seven distinct cell types: erythrocytes, monocytes, thrombocytes, T cells, B cells, basophils, and heterophils. All cell types participated in the host immune response to heatwaves and bacterial infection, but these cells exhibited significant group-specific differences in their gene expression patterns. Bacterial infections and heatwaves altered turtle behavior and physiology indexes. The dual stresses inhibited the expression of antioxidant enzymes and immune genes, potentially jeopardizing turtle survival. Overall, this study provides valuable insights into peripheral blood cell profiles of Chinese soft-shelled turtles under different environmental conditions, enhancing the understanding of their immune responses and potential stressors.

Exposure of larval pinto abalone to ocean acidification and warming negatively impacts survival, settlement, and size

Pinto abalone (Haliotis kamtschatkana), the only abalone species native to Washington, declined by 97% in the state from 1992 to 2017. Their decline is a loss for indigenous tribes, recreational divers, and the health of subtidal rocky reefs and kelp beds. Current restoration actions are facing threats of ocean acidification and warming in the northeast Pacific. This research aims to deepen our understanding of the tolerance and physiological flexibility of early life history stages of pinto abalone and inform hatchery practices under future climate change scenarios. We conducted an experiment to test how seawater pH and temperature stress impact abalone larvae. We exposed abalone post-fertilization to elevated temperature and reduced seawater pH for ten days spanning their larval development period: (1) 7.95pH/14°C (ambient), (2) 7.60pH/14°C, (3) 7.95pH/18°C, and (4) 7.60pH/18°C. Abalone in the ambient treatment had the best survival, those in the 7.60pH/18°C treatment had the worst survival, and those in the two single-stressor treatments had survival in between. Among the surviving larvae, pH was the dominant stressor influencing settlement success, with higher settlement rates under ambient pH treatments at both temperatures. pH also had a stronger effect than temperature on shell length. The information gleaned from this study is essential for optimizing future restoration aquaculture for pinto abalone and determining their ideal habitat and potential geographic range.

Ex Situ Thermal Preconditioning Modulates Coral Physiology and Enhances Heat Tolerance: A Multispecies Perspective for Active Restoration

Global warming threatens reef-building corals by challenging their adaptive capacity. Therefore, interventions such as stress-hardening by thermal preconditioning could become crucial for their survival. This study aimed to systematically assess the effects of distinct thermal preconditioning regimes (stable-high at 29 °C, variable-high at 29 ± 1.5 °C, and stable-ambient control at 26 °C) on the baseline physiology and thermal tolerance of six stony coral species (Galaxea fascicularis, Porites rus, Acropora muricata, Montipora digitata, Pocillopora verrucosa, and Stylophora pistillata) to determine commonalities in the stress-hardening responses that transcend species-specific signatures. For this, we quantified changes in photosynthetic efficiency and bleaching intensity before and after a short-term heat stress assay and up to 30 days later. Stress-hardening was successful in all preconditioned corals, with the variable-high regime slightly outperforming the stable-high regime. Preconditioning reduced the heat stress response by up to 90%, yet species differed in receptiveness. It also improved resilience (survival and recovery), and corals with high inherent thermal tolerance recovered better than susceptible species. Notably, both preconditioning regimes affected baseline physiology, exclusively of the branching species, causing tissue paling and decreased photosynthetic efficiency. We conclude that implementing thermal stress-hardening protocols requires consideration of the species-specific receptiveness and potential physiological trade-offs.

Functional diversity shapes the stability of reef fish biomass under global change

Understanding how environmental and human pressures impact the temporal stability of fish community biomass on shallow reefs is essential for effective conservation and management. These pressures influence community stability directly, by affecting species' stability and asynchrony in species' fluctuations. However, their effects may also indirectly depend on the functional traits of the species composing the community, which remains poorly understood. Here, we examine both direct and indirect, trait-mediated effects of environmental variability and human impacts on species' biomass stability and asynchrony in 215 Australian shallow reefs. These communities span a 10-degree sea surface temperature (SST) gradient and have been monitored over 14 years. Our results indicate higher asynchrony in tropical reefs owing to higher trait diversity and trait redundancy and higher species' stability in colder, temperate communities owing to higher mean trophic level. Human impacts, through their negative effects on species' stability and trait diversity, were the main destabilizing factor of fish community biomass. Temporal change in SST destabilized species' biomass while increasing mean trophic level in fish communities. Overall, our findings show that a comprehensive analysis of the multiple facets of functional diversity is crucial to better understand and forecast the long-term stability of marine ecosystems under global change.

Extreme marine heatwave linked to mass fish kill in the Red Sea

Anthropogenic climate change has precipitated an increase in marine heatwaves (MHWs) that have significant and multifaceted impacts on marine ecosystems. In late August 2023, an intense heatwave coincided with a mass fish kill event on the Saudi Arabian coast of the central Red Sea. Here, we compile MHW metrics from satellite data to illustrate the mortality event was linked with the most intense period of rapid heating in the central Red Sea in recent history. Using field surveys, we quantified the impact of the event on the fish community and found that nearly 1000 fish washed ashore along a 60 km stretch of coastline. Representatives of 54 species were detected, which illustrates the impact of the MHW event on a broad range of fishes. The exact cause of mortality during the event is unknown, but likely related to temperature-induced physiological stress and associated factors. Sparse coastal monitoring limited our ability to rapidly respond to the event and document the proximate cause of mortality. This study not only sheds light on the immediate impacts of a MHW on components of a coral reef ecosystem, but also emphasizes the broader ecological consequences. Mass fish kills may have cascading impacts on ecosystem functioning by causing shifts in community structure and a decrease in biodiversity, which can undermine both the ecological functioning and economic stability of marine-dependent regions. This may be especially true for reefs already occupying a thermal niche that approaches the upper limits of many species, such as those in the Red Sea. Our study highlights the critical need for enhanced reporting mechanisms and forecasting tools to effectively document and help mitigate further impacts linked to MHW-induced mass marine die-offs.

Understanding Dynamic Molecular Responses Is Key to Designing Environmental Stress Experiments: A Review of Gene and Protein Expression in Cnidaria Under Stress

Gene and protein expression analyses are powerful tools to investigate the responses of cnidarians to stress, providing information on both genetic and functional variation and capturing dynamic shifts in organismal physiology. As the use of high throughput sequencing to understand responses of cnidarians to stressors is still relatively new, standard experimental protocols have not yet been established, which limits the ability to compare studies. We (1) systematically reviewed the literature of cnidarian gene and protein expression studies related to environmental stressors to determine how the laboratory experiments were designed and (2) investigated the consistency in responses of genes commonly used as biomarkers within stress experiments conducted on the five most-studied cnidarian genera. Duration of exposure to the stressor, acclimation period and intensity of stress varied greatly among experiments, and most studies did not sample during acclimation and recovery. Before designing experiments that aim to characterise molecular responses to a specific environmental stress, research efforts need to focus on understanding the plasticity of whole transcriptome responses, as gene expression can vary under different stress intensities and durations of exposure. Additionally, only seven genes that were tested in at least two different genera showed a consistent response under heat stress (CuZn-SOD, c-type lectin, FGFR1, MMP, Zn-MP, NF-κB and SLC26). These genes have the potential to standardise evaluations of temperature stress across experiments on cnidarians, and we suggest exploring their use as general cnidarian biomarkers of temperature stress (cBATS).

Climate change and multiple sclerosis: Clinical challenges and strategies

Climate change poses significant challenges for people with multiple sclerosis (PwMS), exacerbating symptoms such as heat sensitivity. Increasing levels of air pollution contribute to neuroinflammation and has been associated with symptom flares, and mobility impairments complicate resource access for PwMS during climate-related weather emergencies. This paper explores the broad implications of climate change on multiple sclerosis (MS) and offers strategies for clinicians to address these emerging challenges, as understanding the broad impacts of climate change on MS is crucial to provide effective care in a changing world.

Environmental DNA reveals how the absolute dominant zooplankton species affects the community

Zooplankton play a crucial role in marine ecosystems. In recent years, the East China Sea (ECS) has recorded the highest levels of pollution among China's marine areas, leading to a trend toward homogenization of the dominant zooplankton species. Environmental degradation has triggered various ecological responses, but how these responses further impact community structure requires more investigation. In this study, we employed a combined approach of morphological identification and environmental DNA (eDNA) metabarcoding to analyze the effects of seasonal variation and dominant species on zooplankton communities. The results indicated that an absolute dominant species, Calanus sinicus, emerged in the ECS, significantly occupying ecological resources. As seasons changed, C. sinicus showed a trend of migrating from offshore to coastal areas. The high abundance of C. sinicus in the sub-regions led to a regional decline in zooplankton community diversity and interspecies cooperation, while the community also experienced stronger dispersal limitations. Furthermore, the spring zooplankton community exhibited higher alpha and beta diversity, as well as a more stable co-occurrence network compared to summer. However, the level of interspecies cooperation in spring was lower than in summer. In conclusion, the high abundance of C. sinicus has a significant impact on zooplankton communities, and this impact shows a lag effect.

Cell death and antioxidant responses in Mytilus galloprovincialis under heat stress: Evidence of genetic loci potentially associated with thermal resilience

The global seawater temperature is expected to further rise in the following years. While species have historically adapted to climatic variations, the current pace of climate change may exceed their ability to adapt. The abnormally increased seawater temperatures occasionally lead to high mortalities of marine bivalve mollusks, threatening the productivity of aquaculture and the sustainability of wild populations. This study investigates the antioxidant and cell death mechanisms of the Mediterranean mussel Mytilus galloprovincialis during a 25-day exposure to temperatures of 24°C, 26°C, and 28°C, by analyzing the transcription of key genes and assessing the oxidative damage on days 1, 3, 12, and 25. In addition, individuals resilient (survived at 28°C until day 30) and susceptible (died early at 26°C and 28°C) to thermal stress were collected to investigate potential polymorphisms in associated genes. The results showed increased transcription of antioxidant genes at higher temperatures. Elevated pro-apoptotic indices were initially observed at 26°C and a higher mortality than at 28°C. However, final mortality was much higher at 28°C. At 26°C, mussels exhibited the highest oxidative damage and pro-apoptotic indices after 25 days. At 28°C, although oxidative damage occurred after 24 hours, survivors maintained a prolonged activated antioxidant defense and increased lc3b transcription, which likely contributed to the observed reduction of pro-apoptotic and oxidative damage metrics on day 25, compared to 26°C. Further, the coding sequences of catalase, intracellular Cu-Zn superoxide dismutase (Cu-Zn sod), and fas-associated protein with death domain (fadd) from heat-resilient and heat-susceptible mussels were analyzed. Based on statistical correlation of nucleotide and genotype frequencies with resilience phenotypes, two novel single nucleotide polymorphisms (SNPs) in Cu-Zn sod and one in fadd were detected, potentially correlating with thermal stress resilience. These findings offer valuable insights into the physiological and genetic adaptations of M. galloprovincialis to rising temperatures and highlight loci potentially linking to thermal resilience.

Global warming drives a threefold increase in persistence and 1 °C rise in intensity of marine heatwaves

Marine heatwaves are extreme climatic events consisting of persistent periods of warm ocean waters that have profound impacts on marine life. These episodes are becoming more intense, longer, and more frequent in response to anthropogenic global warming. Here, we provide a comprehensive and quantitative assessment on the role of global warming on marine heatwaves. To do so, we construct a counterfactual version of observed global sea surface temperatures since 1940, corresponding to a stationary climate without the effect of long-term increasing global temperatures, and use it to calculate the contribution of global air temperature rise on the intensity and persistence of marine heatwaves. We determine that global warming is responsible for nearly half of these extreme events and that, on a global average, it has led to a three-fold increase in the number of days per year that the oceans experience extreme surface heat conditions. We also show that global warming is responsible for an increase of 1 °C in the maximum intensity of the events. Our findings highlight the detrimental role that human-induced global warming plays on marine heatwaves. This study supports the need for mitigation and adaptation strategies to address these threats to marine ecosystems.

Extreme compound events in the equatorial and South Atlantic

The impacts of marine heatwaves (MHWs) on marine ecosystems can be amplified when combined with other extreme events. Here, we investigate the spatiotemporal evolution of compound events of MHW, high acidity and low chlorophyll in the equatorial and South Atlantic, using observation-based datasets and reanalysis products. We show that the frequency and intensity of these triple compound events under a fixed baseline have increased dramatically over the past two decades, peaking in the most recent years. We analyse the drivers of triple compound events for six regions and find that, for the Angola Front and Brazil-Malvinas Confluence regions, these events are associated with a poleward shift of the fronts. In the Agulhas Leakage region, an increase in warmer waters entering from the Indian Ocean leads to compound extremes. In the western equatorial and subtropical Atlantic, they are caused by changes in the air-sea heat fluxes, while in the eastern equatorial by a weakening of upwelling. In addition, triple compound events are widespread over the South Atlantic during El Niño events, which is important because MHWs can be predicted when associated with ENSO.

A photophysiological model of coral bleaching under light and temperature stress: experimental assessment

Marine heatwaves occurring against the backdrop of rising global sea surface temperatures have triggered mass coral bleaching and mortality. Irradiance is critical to coral growth but is also an implicating factor in photodamage, leading to the expulsion of symbiotic algae under increased temperatures. Numerical modelling is a valuable tool that can provide insight into the state of the symbiont photochemistry during coral bleaching events. However, very few numerical physiological models combine the influence of light and temperature for simulating coral bleaching. The coral bleaching model used was derived from the coral bleaching representation in the eReefs configuration of the CSIRO Environmental Modelling Suite, with the most significant change being the equation for the rate of detoxification of reactive oxygen species. Simulated physiological bleaching outcomes from the model were compared to photochemical bleaching proxies measured during an ex situ moderate degree-heating week (up to 4.4) experiment. The bleaching response of Acropora divaricata was assessed in an unshaded and 30% shade treatment. The model-simulated timing for the onset of bleaching under elevated temperatures closely corresponded with an initial photochemical decline as observed in the experiment. Increased bleaching severity under elevated temperature and unshaded light was also simulated by the model, an outcome confirmed in the experiment. This is the first experimental validation of a temperature-mediated, light-driven model of coral bleaching from the perspective of the symbiont. When forced by realistic environmental conditions, process-based mechanistic modelling could improve accuracy in predicting heterogeneous bleaching outcomes during contemporary marine heatwave events and future climate change scenarios. Mechanistic modelling will be invaluable in evaluating management interventions for deployment in coral reef environments.

Frequent land-ocean transboundary migration of tropical heatwaves under climate change

Anthropogenic warming has exacerbated atmospheric heatwaves globally, yet the transboundary migration of heatwaves between land and ocean, along with the anthropogenic influence on this process, remain unclear. Here, we employ a Lagrangian tracking approach to identify and track spatiotemporally contiguous warm-season heatwaves in both reanalyses and simulations. This way, we show that land-ocean transboundary heatwaves, especially in the tropics, exhibit longer persistence, wider areal extent, and greater intensity than those confined to land or ocean. These transboundary migrations are primarily driven by the movement of high-pressure systems (such as the westward extension of subtropical highs) and the propagation of Rossby waves. Associated with increasing greenhouse gas concentrations, the frequency of tropical heatwave migrations has increased over the past four decades, and is projected to accelerate further in the twenty-first century under the high-emissions scenario. Anthropogenically-driven landward migrations are amplified by stronger landward winds that drive heat advection, while oceanward processes are likely intensified by increased land-ocean temperature gradient. These intensified transboundary heatwaves not only accentuate humid heat risks for humans but also threaten ecosystems.

Trends and variability of marine heatwaves in Portuguese coastal waters

Marine heatwaves (MHWs) represent a significant threat to marine ecosystems, particularly in vital fisheries regions such as the Portuguese coast. Understanding MHW dynamics at a finer spatial scale is essential for comprehending their impacts on the ecosystems. This study addresses this gap by analyzing historical MHW events from 1982 to 2023 and projecting future scenarios based on different greenhouse gas emission pathways. The primary objective is to characterize the spatiotemporal patterns of MHWs along the Portuguese coast and understand their temporal evolution. Using historical sea surface temperature (SST) data, MHW metrics were analyzed across various coastal regions. Future projections, utilizing Shared Socioeconomic Pathways (SSP) 245 and SSP 585, assessed changes in MHW frequency, duration, and intensity in the near future (2024-2060) and far future (2061-2100). Results indicate no significant spatial differences in average annual MHW metrics among study areas, but significant temporal variations were observed. The progression of MHW metrics is often more than twice as fast in the slopes and southern areas compared to the west shelf. Record-breaking MHW events in 2023 were consistent with global trends, highlighting a positive linear correlation between historical warming and increased MHW metrics. Projections under SSP 585 indicate a 2.5 to 4.5-fold increase in the annual number of MHW events, lasting 1 to 3 months, with nearly year-round MHW conditions by 2100, reaching peak intensities of 10.7 °C. These findings highlight the need for climate action to mitigate the impacts of intensified MHWs on coastal ecosystems. This research provides foundational insights into MHW dynamics along the Portuguese coast, laying the groundwork for future studies and highlighting the implications of climate change on marine environments.

Subsurface heatwaves in lakes

Lake heatwaves (extreme hot water events) can substantially disrupt aquatic ecosystems. Although surface heatwaves are well studied, their vertical structures within lakes remain largely unexplored. Here we analyse the characteristics of subsurface lake heatwaves (extreme hot events occurring below the surface) using a spatiotemporal modelling framework. Our findings reveal that subsurface heatwaves are frequent, often longer lasting but less intense than surface events. Deep-water heatwaves (bottom heatwaves) have increased in frequency (7.2 days decade-1), duration (2.1 days decade-1) and intensity (0.2 °C days decade-1) over the past 40 years. Moreover, vertically compounding heatwaves, where extreme heat occurs simultaneously at the surface and bottom, have risen by 3.3 days decade-1. By the end of the century, changes in heatwave patterns, particularly under high emissions, are projected to intensify. These findings highlight the need for subsurface monitoring to fully understand and predict the ecological impacts of lake heatwaves.

Eat more, often: The capacity of piscivores to meet increased energy demands in warming oceans

Marine heatwaves (MHWs) profoundly disturb tropical coral reefs, imperilling species fitness and survival. Ectothermic piscivorous reef fishes are particularly vulnerable to MHWs since all aspects of their survival are dictated by ambient temperature. Severe +4 °C MHWs are projected to escalate daily energy demands by ~32-55 %, compelling piscivores to pursue larger or more frequent prey to survive. However, the feasibility of these responses have been questioned, as evolved predation and digestive strategies are constrained to specific prey types and sizes to safeguard residual aerobic scope (AS) during digestion for other vital processes. Instead, prevailing theory proposes appetite reductions at temperatures above optimal, preserving AS at the expense of growth and/or fitness. We investigated this dichotomy in the thermal foraging responses of Arc-eye hawkfish (Paracirrhites arcatus) and blacktail snapper (Lutjanus fulvus), evaluating energetic demand (standard metabolic rate, SMR), AS, appetite (meal mass intake), and capacity for digestion (specific dynamic action, SDA). Spanning a thermal gradient encompassing present-day winter (24.0 ± 0.1 °C), summer (27.5 ± 0.1 °C), and MHW (31.0 ± 0.1 °C), we show that SMR increased by ~65 % from winter to MHW for both species, while AS increased by ~31-67 %. Contrary to predictions of reduced appetite, both species consumed ~106 % larger meals, yielding a ~ 35-105 % greater SDA magnitude. Surprisingly, increased appetite did not encroach on residual AS as both species maintained the physiological flexibility to process larger meals while retaining ~45-60 % of AS at the post-prandial peak. Although larger meals take longer to digest, both species exhibited faster digestion with rising temperatures resulting in a maintained or shortened SDA duration during MHWs, simultaneously enabling increased feeding rates while preserving aerobic reserves to support heightened predation. Our findings underscore the physiological feasibility of increasing appetite for some piscivores, while highlighting the ecological challenge of increasing prey numbers and sizes amid declining prey densities and prey size-reductions caused by ocean warming.

Alternative splicing in a coral during heat stress acclimation and recovery

Climate change has caused drastic declines in corals. As sessile organisms, corals acclimate to environmental shifts through genome-wide changes in gene expression, epigenetic modifications, and alterations in microbiome composition. However, alternative splicing (AS), a conserved mechanism of stress response in many organisms, has been under-explored in corals. Using short-term acute thermal stress assays, we investigated patterns of AS in the scleractinian coral Acropora cervicornis during response to low (33°C), medium (35°C), and high (37°C) heat stress and subsequent overnight recovery. Our findings demonstrate reproducible dynamic shifts in AS of at least 40 percent of all genes during response to heat treatment and the recovery phase. The relative proportion of AS increased in response to heat stress and was primarily dominated by intron retention in specific classes of transcripts, including those related to splicing regulation itself. While AS returned to baseline levels post-exposure to low heat, AS persisted even after reprieve from higher levels of heat stress, which was associated with irreversible loss of photosynthetic efficiency of the symbiont. Our findings demonstrate that, although animals, corals are more plant-like in their likely usage of AS for regulating thermal stress response and recovery.

The Thermal Stress History of South Atlantic Reefs Reveals Increasing Intensity, Duration, Frequency, and Likely Undocumented Bleaching Episodes

The primary consequence of global warming for reefs is coral bleaching, often leading to extensive coral mortality. Although bleaching is well-documented globally, the thermal stress and bleaching experienced by the unique South Atlantic reefs remain largely unknown due to insufficient monitoring on both spatial and temporal scales. Therefore, this work aimed to reconstruct past thermal stress episodes across South Atlantic reefs, and assessed whether episodes are becoming more intense, longer-lasting, and more frequent. We retrieved daily 5 km-resolution Degree Heating Week (DHW) data from the U.S. National Oceanic and Atmospheric Administration Coral Reef Watch server for 33 reef sites spanning the last 40 years. For each thermal stress episode, we assessed the intensity (maximum DHW value), duration (number of continuous days under stress), and frequency (number of days between episodes). Generalized linear models were fitted to intensity, duration, and frequency data to evaluate the influence of latitude and the time x region interaction as predictors. We recorded multiple thermal stress episodes, increasing from 2010 onwards, ranging from 10 episodes between 1985-89 and 75 between 2020-24. Intensity and duration increased over time across the entire South Atlantic. Frequency also increased across the Southwestern Atlantic coast and oceanic islands, but not for Africa. Episodes at higher latitudes were more intense, prolonged, and frequent. The validity of the thermal stress history reconstruction was groundtruthed using information from the Abrolhos Bank, the only consistently monitored reef site in the South Atlantic-DHW data accurately matched the observed bleaching episodes at this site. With this, our dataset shows that multiple bleaching episodes likely occurred in the South Atlantic, but went undocumented in the field. Therefore, the information currently available for the South Atlantic likely underestimates the extent of bleaching occurring in the area, which is experiencing increases in intensity, duration, and frequency of thermal stress.

Understanding spatiotemporal variation of heatwave projections across US cities

Heatwaves are one of the deadliest meteorological hazards, posing a substantial risk to human health, environment, and the economy. The frequency and intensity of heatwaves have substantially escalated throughout the United States (US), as evidenced by a noticeable contrast between the average occurrence of two heatwaves per year during the 1960s and the surge to six per year during the 2010s. The primary objective of this study is to assess the impacts of past heatwave events in major US cities, project future scenarios, and analyze the relationship between heatwaves and their associated health and environmental consequences. First, we investigate the spatio-temporal variations in the intensity, frequency, and duration of past heatwaves, along with anticipated changes under various shared socioeconomic pathways till the end of 2100. Secondly, we examine the adverse heatwave impacts on human health and well-being, considering both current demographics and future projections. Our results highlight significant projected increases in heatwave frequency, intensity, and duration across all US regions, with the most dramatic escalations under high-emission scenarios. By the late twenty-first century, cities could experience up to 2-4 times the frequency and duration of heatwave days annually compared to the baseline period (1985-2014), with night-time heatwave durations potentially covering over half the year in some regions. Furthermore, analysis using composite heatwave indices reveals severe heat stresses, particularly in southwestern cities like Las Vegas and Yuma, and southern cities like Miami, indicating a rising trend of heatwave susceptibility due to both climatic and demographic shifts. This study contributes to the growing body of research advocating proactive measures to address the escalating threat of heatwaves. By integrating projected climate indices and demographic shift, it provides a nuanced assessment of urban heatwave vulnerability, with a specific focus on densely populated cities and high-risk regions.

Application of genomic tools to study and potentially improve the upper thermal tolerance of farmed Atlantic salmon (Salmo salar)

BACKGROUND

The Atlantic salmon (Salmo salar) aquaculture industry must mitigate the impacts of rising ocean temperatures and the increased prevalence/severity of marine heat waves. Therefore, we investigated the genetic architecture and gene expression (transcriptomics) responsible for determining a salmon's upper thermal tolerance.

RESULTS

A genome-wide association study (GWAS) was conducted using fin clips of salmon from a previous incremental thermal maximum (ITMax) challenge (n = 251) and the North American 50 K SNP chip. ITMax was a highly polygenic trait with low/moderate heritability (mean SNP-based h2 = 0.20 and pedigree-based h2 = 0.25). Using data from the same fish, a separate GWAS assessed thermal-unit growth coefficient (TGC). Five significant SNPs were detected on chromosomes three and five, and high heritability estimates were calculated for TGC measured as fish grew from 12 to 20 °C (mean SNP-based h2 = 0.62 and pedigree-based h2 = 0.64). RNA-seq analyses of liver samples (n = 5-6 family-1 temperature-1) collected from the four most and four least tolerant families at 10 and 20 °C were also used to provide insights into potential mechanisms modulating this species' thermal tolerance. Between the top and bottom families, 347 and 175 differentially expressed transcripts (FDR-adjusted p < 0.01; fold-change ≥|2.0|) were identified at 10 and 20 °C, respectively. GO term enrichment analysis revealed unique responses to elevated temperature between family rankings (e.g., 'blood coagulation', 'sterol metabolic process' and 'synaptic growth at neuromuscular junction'). qPCR analyses further confirmed differences pertaining to cholesterol metabolism (lpl), inflammation (epx, elf3, ccl20), apoptosis (htra1b, htra2, anxa5b), angiogenesis (angl4, pdgfa), nervous system processes (insyn2a, kcnj11l) and heat stress (serpinh1b-1, serpinh1b-2). Three differentially expressed transcripts (i.e., ppp1r9a, gal3st1a, f5) were located in close proximity (± 120 kbp) to near-significant SNPs from the GWAS. Interestingly, ppp1r9a and gal3st1a have putative neurological functions, while f5 regulates blood coagulation.

CONCLUSIONS

These analyses provide several putative biomarkers of upper thermal tolerance in salmon that could prove valuable in helping the industry develop more temperature-tolerant fish. Further, our study supports previous reports that ITMax has low/moderate heritability in this species, and suggests that TGC at elevated temperatures is highly heritable.

Laboratory-simulated marine heatwave enhances physiological damage to mussels exposed to titanium dioxide nanoparticles by disrupting the gut-hepatopancreas axis

The aggregation state of nano-TiO2 in the environment is altered under marine heatwaves (MHWs), thus affecting its bioavailability and toxicity to the marine organisms. Here, we investigated the toxic mechanisms and effects of nano-TiO2 on gut-hepatopancreas axis health of Mytilus coruscus exposed to 25 and 250 μg/L of nano-TiO2 under laboratory-simulated MHW. Compared with the control conditions or post-MHW cooling phase, prolonged MHW exposure significantly inhibited digestive function, decreased immune-related enzymes activities, and caused neurotoxicity in the mussels. 16S rRNA analysis demonstrated that high concentration nano-TiO2 and combined exposures decreased the abundance of Bacteroidota while increased the Proteobacteria. Additionally, the elevated pro-inflammatory bacteria released endotoxin lipopolysaccharide (LPS), which activated Toll-like receptor 4 (TLR-4) in the hepatopancreas and induced hepatopancreatic inflammation by downregulating nuclear factor-kappa B (NF-κB) signaling pathway and detoxification-related genes. Furthermore, nano-TiO2 and MHW exposure dysregulated the glutathione system, decreased the levels of antioxidation-related genes, and induced the accumulation of ROS and lipid peroxide (LPO) contents, thus causing severe oxidative damage and hepatopancreatic cell apoptosis. These findings demonstrate that nano-TiO2 and MHW induce hepatopancreatic inflammation and cell damage, which are strongly associated with the gut lesions and disrupted gut-hepatopancreas axis homeostasis.

Mass stranding of common (weedy) seadragons (Phyllopteryx taeniolatus) in Sydney: impacts and implications

In April 2022, mass stranding of weedy (common) seadragons occurred, with a total of over 200 individuals washed ashore on beaches in the Sydney, Australia region, recorded by citizens. Causes of the stranding, which is unprecedented, were likely related to a series of east coast low storm events, leading to record wave heights, record coastal rainfall, and potential loss of critical food sources (schooling mysid crustaceans). A significant proportion of the local population was likely lost in this series of events, indicating a future threat to seadragons, with east coast low intensity predicted to increase under human-caused climate change.

Disconnect between settlement and fishery recruitment driven by decadal changes in nearshore habitats

Submerged vegetation is critical to marine ecosystems and can function as recruitment habitats for commercially targeted species, such as the highly valuable Western Rock Lobster Panulirus cygnus. The development of vegetation indices for marine remote sensing has made tracking the extent and change of submerged vegetation in space and time possible. Vegetation changes may directly or indirectly affect the recruitment and population dynamics of animals that depend on these habitats. Previous studies have found that extreme climate events, such as marine heatwaves, can cause declines in submerged vegetation extent, but these studies have been limited spatially and temporally. Here, we present multidecadal extents of submerged vegetation and settlement indices for five coastal locations throughout the range of Western Rock Lobster and explore how these vegetation trends relate to an index of recruitment. We found that the correlations of vegetative extent, climate and undersize lobster catch varied significantly between the monitored locations. For some locations, particularly those with a high composition of preferential recruitment habitat (i.e., seagrass), vegetation extent in the previous two years significantly explained variation in undersize catch rates. Regions with a time series of undersize lobster and settlement data combined with consistent remotely sensed imagery allowed for the disentanglement of the influence of habitat change and post-settlement recruitment. Whereas, at locations with poor quality historical data, often due to the combined effect of turbidity and a relatively steep coastal shelf or limited catch data, the recruitment index was not improved by information on submerged vegetation. We have found that decadal changes in nearshore habitats at representative locations have driven the disconnect between settlement and fishery recruitment. We suggest that monitoring marine habitats can complement long-term fishery data collection and coastal management.

Climate change and variability drive increasing exposure of marine heatwaves across US estuaries

Marine heatwaves (MHWs) are among the greatest threats to marine ecosystems, and while substantial advances have been made in oceanic MHWs, little is known about estuarine MHWs. Utilizing a temperature dataset spanning over two decades and 54 stations distributed across 20 estuaries in the United States National Estuarine Research Reserve System, we present a comprehensive analysis of estuarine MHW characteristics and trends. Long-term climate-change-driven warming is driving more frequent MHWs along the East Coast, and if trends continue, this region will be in a MHW state for ~ 1/3 of the year by the end of the century. In contrast, the vast majority of the West Coast showed no trends, highlighting the potential for future thermal refugia. The West Coast was more strongly influenced by climate variability through the enhancement/suppression of MHWs during different phases of climate modes, suggesting long-term predictability potential. These results can provide guidance for management actions and planning in these critical environments.

Molecular plasticity to ocean warming and habitat loss in a coral reef fish

Sea surface temperatures are rising at unprecedented rates, leading to a progressive degradation of complex habitats formed by coral reefs. In parallel, acute thermal stress can lead to physiological challenges for ectotherms that inhabit coral reefs, including fishes. Warming and habitat simplification could push marine fishes beyond their physiological limits in the near future. Specifically, questions remain on how warming and habitat structure influence the brains of marine fishes. Here we evaluated how thermal stress and habitat loss are acting independently and synergistically as stressors in a damselfish of the Western Atlantic, Abudefduf saxatilis. For this experiment, 40 individuals were exposed to different combinations of temperature (27 °C or 31 °C) and habitat complexity (complex vs. simple) for 10 days, and changes in brain gene expression and oxidative stress of liver and muscle were evaluated. The results indicate that warming resulted in increased oxidative damage in the liver (P = 0.007) and changes in gene expression of the brain including genes associated with neurotransmission, immune function, and tissue repair. Individuals from simplified habitats showed higher numbers of differentially expressed genes and changes for genes associated with synaptic plasticity and spatial memory. In addition, a reference transcriptome of A. saxatilis is presented here for the first time, serving as a resource for future molecular studies. This project enhances our understanding of how fishes are responding to the combination of coral reef degradation and thermal stress while elucidating the plastic mechanisms that will enable generalists to persist in a changing world.

Physiological and molecular responses of tropical Seagrass Enhalus acoroides exposed to simultaneous high temperature and hypoxia stress

High temperature and hypoxia pose significant threats to coastal ecosystems, often co-occurring and intensifying damage to seagrass meadows. While the independent effects of these stresses have been extensively documented, their combined impact on seagrasses remains underexplored. This study investigates the effects of high temperature and hypoxia on Enhalus acoroides, a dominant tropical seagrass. Results indicated that E. acoroides could tolerate high temperature (35 °C) and hypoxia (2.65 mg L-1) individually for 24 h. However, exposure to both stresses simultaneously led to severe, irreversible physiological damage, highlighting a synergistic effect that surpassed the additive impact of each stressor alone. Combined stresses markedly impaired PSII, reduced photosynthetic rate and chlorophyll content, alongside elevated oxidative stress. Transcriptomic analysis indicated that high temperature intensified metabolic stress, while oxygen deficiency forced a shift from aerobic to anaerobic respiration, resulting in energy deficits. Furthermore, the lack of oxygen caused the accumulation of electrons, which triggered excessive production of reactive oxygen species. Despite the antioxidant enzyme system's active response, it was unable to mitigate the overwhelming oxidative stress, leading to irreversible oxidative damage. The above results suggested that in the context of global warming and eutrophication, the combined effects of high temperature and hypoxia may accelerate the degradation of seagrass meadows at a far greater rate than previously anticipated.

Climate change will amplify the impacts of harmful microorganisms in aquatic ecosystems

More than 70% of the human population lives within five kilometres of a natural water feature. These aquatic ecosystems are heavily used for resource provision and recreation, and represent the interface between human populations and aquatic microbiomes, which can sometimes negatively impact human health. Diverse species of endemic aquatic microorganisms, including toxic microalgae and pathogenic bacteria, can be harmful to humans. Aquatic ecosystems are also subject to intrusions of allochthonous pathogenic microorganisms through pollution and runoff. Notably, environmental processes that amplify the abundance and impact of harmful aquatic microorganisms are occurring with increasing frequency owing to climate change. For instance, increases in water temperature stimulate outbreaks of pathogenic and toxic species, whereas more intense precipitation events escalate microbial contamination from stormwater discharge. In this Perspective we discuss the influence of aquatic microbiomes on the health and economies of human populations and examine how climate change is increasing these impacts.

Interactive effects of elevated temperature and venlafaxine on mitochondrial respiration and enzymatic capacity in Nile tilapia (Oreochromis niloticus)

Warming events are becoming more frequent and extreme in aquatic environments worldwide. Concurrently, many environments are polluted with biologically active compounds such as pharmaceuticals. Understanding how these challenges interact is critical for understanding the climate crisis, as contaminants may modulate how ectotherms respond to heat stress or vice versa. One potential site for these heat × contaminant interactions is the mitochondrion, which is central to metabolism, implicated in thermal tolerance, and evolutionarily conserved. Using high-resolution respirometry, we investigated how acute warming (to 35 °C, 40 °C, or 45 °C from 25 °C) impacted the respiration, coupling, and metabolic capacity of liver mitochondria isolated from Nile tilapia, and how exposure to environmentally relevant levels of the ubiquitous antidepressant venlafaxine modulated those effects. Mitochondria exposed to hotter temperatures had higher respiration rates and decreased respiratory control ratio compared to mitochondria exposed to cooler temperatures. The depressive effects of venlafaxine on respiration rates through complex I and II or complex II only (State 3 and State 4), as well as complex IV-linked respiration, were mild except in mitochondria exposed to high temperatures, suggesting an interactive effect of warming and contaminant exposure. Finally, we found that the maximal enzyme activity of intact mitochondria (represented by mitochondrial respiration) showed a different pattern of response to warming and venlafaxine compared to its underlying components (as reflected by the activity of succinate dehydrogenase [complex II] and cytochrome c oxidase [complex IV]), demonstrating the value of incorporating both interactive and reductive approaches in understanding how mitochondria cope with anthropogenic changes in the environment.

Interdecadal modulation of Ningaloo Niño/Niña strength in the Southeast Indian Ocean by the Atlantic Multidecadal Oscillation

The Southeast Indian Ocean is a global hotspot for marine heatwaves. In that region, marine heatwaves/cold-spells are known as Ningaloo Niño/Niña events, and have substantial impacts on regional climate anomalies and unique marine ecosystems. However, the strength of Ningaloo Niño/Niña events is nonstationary and varies considerably at multidecadal timescales. Here we find that the interdecadal fluctuations in Ningaloo Niño/Niña strength are modulated by the Atlantic Multidecadal Oscillation (AMO), with strengthened (weakened) Ningaloo Niño/Niña corresponding to a positive (negative) AMO phase. During the positive AMO phase, the Atlantic warm sea surface temperature (SST) anomalies drive a series of climate mean-state changes in the Indo-Pacific region through tropics-wide teleconnections, including SST cooling over the central Pacific and SST warming in the tropical eastern Indian Ocean. Those mean-state changes tend to enhance El Niño-Southern Oscillation (ENSO)-related atmospheric and oceanic teleconnections to the Southeast Indian Ocean, and increase local Indian Ocean ocean-atmosphere coupling, promoting the Ningaloo Niño/Niña growth. Our findings highlight the critical role of the remote influence of AMO in understanding the Southeast Indian Ocean marine heatwaves/cold-spells and associated climatic and socioeconomic impacts.

Vertical structure of subsurface marine heatwaves in a shallow nearshore upwelling system

Marine heatwaves (MHWs) are increasing in frequency and intensity globally and are among the greatest threats to marine ecosystems. However, limited studies have characterized subsurface MHWs, particularly in shallow waters. We utilized nearly two decades of full water-column (~ 10 m) observations from a unique automated profiler in central California to characterize, for the first time, the vertical structure of MHWs in a shallow nearshore upwelling system. We found MHWs have similar average durations and intensities across all depths, but there were ~ 17% more bottom MHW days than surface MHW days. Nearly one third of bottom MHWs occurred independently of surface MHWs, indicating that satellites miss a significant fraction of events. MHWs showed distinct seasonality with more frequent and intense events during the fall/winter when weak stratification allowed for MHWs to occupy a larger portion of the water column and persist longer. During summer, strong stratification limited the vertical extent of MHWs, leading to surface- and bottom-trapped events with shorter durations and intensities. Additionally, MHW initiation and termination across depths was consistently linked to anomalously low and high coastal upwelling, respectively. This study highlights the need for expansion of subsurface monitoring of MHWs globally amid a warming planet.

Vulnerability of benthic trait diversity across the Mediterranean Sea following mass mortality events

Unraveling the functional future of marine ecosystems amid global change poses a pressing challenge. This is particularly critical in the Mediterranean Sea, which is highly impacted by global and local drivers. Utilizing extensive mass mortality events (MMEs) datasets spanning from 1986 to 2020 across the Mediterranean Sea, we investigated the trait vulnerability of benthic species that suffered from MMEs induced by nine distinct mortality drivers. By analyzing changes in ten ecological traits across 389 benthic species-constituting an extensive compendium of Mediterranean ecological traits to date-we identified 228 functional entities (FEs), defined as groups of species sharing the same trait values. Our findings indicate that of these 55 FEs were impacted by MMEs, accentuating a heightened vulnerability within specific trait categories. Notably, more than half of the mortality records showed severe impacts on calcifying and larger species with slower growth which mostly account for tree-like and massive forms. Altogether, we highlight that 29 FEs suffered extreme mortality, leading to a maximum increase of 19.1% of the global trait volume vulnerability over 35 years. We also reveal that 10.8% of the trait volume may have been temporarily lost over the last five years, emphasizing the risk of a rapid ecological transformation in the Mediterranean Sea.

Connectivity enhances resilience of marine forests after an extreme event

The resilience of populations to extreme climatic events comprises the resistance to withstand and the ability to recover, which depends on factors such as remaining genetic diversity and population connectivity. In 2011, a MHW caused a 100 km range contraction of kelp (Ecklonia radiata) off Western Australia, but recently recovering kelp forests were discovered. To understand mechanisms of recovery and determine if recovering populations are survivors or immigrants, we used genotyping-by-sequencing to assess patterns of genetic diversity and connectivity. We found that two of the three recovering kelp forests (PG1 and 2) were likely survivors whereas a third smaller population (PGCr 1) was likely produced through re-colonisation from nearby surviving forests. Connectivity was high among populations and migration analysis identified one population (Horrocks) as the most important source for the recovering kelps. All recovering populations had higher neutral genetic diversity, and similar putative adaptive diversity to surrounding surviving populations, suggesting local adaptation. Our results elucidate how mixed processes can contribute to kelp forest resilience following MHWs but cryptic survival and maintenance of population connectivity is key to recovery.

Response of Red Sea phytoplankton biomass to marine heatwaves and cold-spells

In tropical oceans, phytoplankton experience significant alterations during marine heatwaves (MHWs), yet the consequences of reduced or absent marine cold-spells (MCSs) on these microscopic algae are currently overlooked. Synergistically combining in situ measurements, Argo-float data, remotely-sensed observations, and hydrodynamic model outputs, we explore such relationships in the Red Sea. Results show a long-term (1982 to 2018) gradual increase in MHW days (5-20 days/decade) and a clear decrease in MCS days (10-30 days/decade). Compound extreme temperature and chlorophyll-a events (Chl-a - an index of phytoplankton biomass) exhibit consistently lower Chl-a concentrations during MHWs and higher ones during MCSs, particularly in the northern and southern Red Sea. In these regions, during the main phytoplankton-growth period, the presence of MHWs/MCSs leads to respective Chl-a anomalies in 94% of the cases. Yet, phytoplankton responses in the central Red Sea are more complex, most likely linked to the region's highly dynamic circulation (e.g., mesoscale anti-cyclonic eddies), and multiple nutrient sources. In the naturally warm and stratified ecosystem of the Red Sea, where deeper mixed layers enhance the transfer of nutrient-rich waters to the lit zone, the substantial reduction of MCSs could be more impactful for phytoplankton than the gradual rise of MHWs.

Heatwaves increase the polystyrene nanoplastic-induced toxicity to marine diatoms through interfacial interaction regulation

Marine heatwaves, prolonged high-temperature extreme events in the ocean, have increased worldwide in recent decades. Plastic pollution is widespread in the ocean, and the continuous weathering of plastics leads to a substantial release of nanoplastics (NPs). However, the interactive impacts and in-depth mechanisms of heatwaves and NPs on diatoms are largely unknown. Here, we show that a heatwave intensity of 4 °C amplified the toxicity of polystyrene NPs to the globally important diatom Chaetoceros gracilis (C. gracilis), with reductions of 5.62 % and 9.46 % in growth rate and photosynthesis, respectively. Notably, NPs significantly inhibited the cell-specific C assimilation rate by 18.28 % under heatwave conditions. The enhanced NP-induced toxicity to C. gracilis was attributed to decreased mechanical strength and increased NP adsorption under heatwave conditions, which increased membrane damage and oxidative stress. Transcriptomic analysis demonstrated that NPs disturbed redox homeostasis and caused mechanical stress to C. gracilis under heatwave conditions. Moreover, NP treatment downregulated genes (psbA and rbcL) encoding photosynthesis core proteins and the pivotal carbon-fixing enzyme RubisCo under heatwave conditions, resulting in decreased growth and C fixation rates. These findings demonstrate that heatwaves render C. gracilis susceptible to NPs and emphasize the reduced primary productivity caused by NPs under global warming.

The role of lag phases between real-term marine heatwaves in the trait responses of two macrophyte species

Coastal marine macrophytes are critical ecosystem engineers providing valuable ecosystem services. However, they experience detrimental impacts from climate change-induced stresses such as marine heatwaves (MHW), which are becoming more intense and frequent. This study investigated trait responses in real-term heatwaves, Continuous (1MHW) and Consecutive (with a 4-day lag phase, 2MHW), on two key macrophytes, the seaweed Fucus vesiculosus and seagrass Zostera marina. Our results showed very few negative effects on traits from both temperature treatments. Physiological traits indicated that both macrophytes were not stressed by the treatments. Fucus vesiculosus showed little response to changes in temperature and the 2MHW treatment, which considered the lag phase, showed larger changes in frond area compared to the 1MHW treatment. In Z. marina, leaves presented statistically significant higher carbon content in the 1MHW treatment than in the control. Significantly higher leaf elongation rates and leaf width were also observed in Z. marina for the 2MHW treatment in comparison to the control. Fucus vesiculosus showed high acclimatization to changes in temperature, likely because it is a species adapted to grow in dynamic intertidal habitats. Contrary, Z. marina appeared to be more sensitive to the 1MHW treatment, as more significant changes were observed, however, the lag phase seemed not to be important in Z. marina as there was no change in trait response. Exploring the role of lag phases of different duration in the context of real-term MHW predictions is an important research direction and has relevance for ecosystem resistance that will ultimately affect the resilience of marine macrophyte populations.

Spatiotemporal variability of air-sea CO2 fluxes in response to El Niño-related marine heatwaves in the tropical Pacific Ocean

The tropical Pacific is the largest oceanic source of carbon dioxide (CO2) emissions, where persistent marine heatwaves (MHWs) frequently occur. During persistent MHW events which are associated with strong El Niño events, CO2 outgassing is notably reduced, however, its detailed spatiotemporal response to MHWs has not been fully characterized. In this study, we showed a high degree of consistency between CO2 source regions in the central and eastern tropical Pacific Ocean and the occurrence regions with average annual MHW days exceeding 45 days (co-occurring area covers 80% of the area where MHWs occur). The spatiotemporal variability of the air-sea CO2 flux on interannual and longer timescales can be reconstructed from annual MHW days and occurrence frequency, respectively, in the central and eastern Pacific Ocean of the co-occurring region. In this region, El Niño-related MHWs reduce the air-sea CO2 flux density up to 0.4-0.8 molC/m2/yr per 100 MHW days, corresponding to a reduction of CO2 emissions by approximately 0.1 PgC per 100 MHW days. This is a 10%-40% reduction in CO2 emissions during MHW periods, with the strongest impact (30%-40% CO2 emission reduction) in the equatorial Pacific (5°S-5°N) of the central and eastern Pacific Ocean. In contrast, air-sea CO2 flux variations in coastal eastern upwelling region of the co-occurring region are mainly subjected to seasonal mixed layer variations, and thus not notably affected by El Niño-related MHWs on interannual timescales. By establishing the reproducibility between MHWs and air-sea CO2 flux variations, our results pave a way for detailed future spatiotemporal evolutions of MHW-induced changes in air-sea CO2 flux in the tropical Pacific Ocean.

Mass Die-Off Events in Swarming Hyperiid Amphipods: Potential Drivers

Beach mass stranding events of marine organisms, widely documented worldwide, are triggered by a range of biotic and abiotic environmental factors, often unexplained. Such occurrences among pelagic crustaceans are less frequent, yet not uncommon. Here we studied mass mortality events of hyperiid amphipods-abundant members of pelagic zooplankton, commonly associated with gelatinous organisms. Our study examined consecutive mass die-off and stranding events of free-living hyperiids in the Red Sea during 2023 and 2024. We investigated three potential causes: semelparous reproduction, thermal stress, and physical oceanographic conditions. To place our findings in a broader context, we further performed a global review of hyperiid swarming and mass mortality events from scientific literature and a citizen science repository. Morphological and molecular analyses confirmed that the hyperiid species in the die-off events at the Red Sea was Anchylomera blossevillei (Phrosinidae). The balanced male: female sex ratio (0.99), combined with the absence of gravid or brooding females, led to the rejection of semelparity as a driving factor. The environmental data did not indicate thermally stressful conditions, and no evidence of parasitic infection was found. Nonetheless, previous studies have shown that under weak wind conditions, as measured during the stranding events, coherent cyclonic eddies with diameters of 5-6 km are developed in the northern Gulf of Aqaba, persisting for about a day. These eddies can exceed velocities of 100 cm s-1 and may have facilitated the hyperiid stranding events. Future research should unveil the impacts of such events on marine ecosystems.

More frequent, longer, and hotter consecutive marine and terrestrial heatwaves in China's coastal regions

Marine heatwave (MHW) can increase heat exchange between the land and the ocean, which may further develop into a consecutive marine and terrestrial heatwave (CMTHW). Despite their significance, the feedback mechanisms underlying these compound events remain inadequately understood. This study provides a comprehensive analysis of the interactions between terrestrial and marine heatwaves across China's coastal regions, leveraging multiple temperature datasets. Our findings reveal a marked increase in both the frequency and spatial extent of CMTHWs over the past four decades. Notably, longer lasting and more intense MHWs are more likely to trigger subsequent terrestrial heatwaves (THWs), indicating that CMTHWs are associated with more severe and prolonged MHWs compared to standalone MHWs. Atmospheric processes cause additional land surface warming relative to the ocean. Specifically, during CMTHWs, land surface latent heat flux anomalies are significantly larger than those over the ocean, highlighting the critical role of atmospheric feedback. These findings underscore the need for further investigation into the mechanisms linking marine and terrestrial heatwaves and the broader implications for coastal climate dynamics and ecosystem resilience.

Distinct interspecies thermal resistance strategies exhibited by euplanktonic, tychoplanktonic and benthic diatoms under marine heatwaves

Extreme climate events, such as marine heatwaves (MHWs), are expected to occur more frequently and intensely in the future, resulting in a substantial impact on marine life. The way that diatoms respond to MHWs may have crucial effects on global primary production and biogeochemical cycles. Euplanktonic diatoms appear to benefit from MHWs directly, but this phenomenon needs an explanation. As concerns tychoplanktonic and benthic diatoms, no studies have been addressed on their thermal response strategies. To address this, we investigated the responses and underlying mechanisms of three typical growth forms of diatoms, Pseudo-nitzschia multiseries (euplanktonic), Paralia guyana (tychoplanktonic) and Navicula avium (benthic), under heat stress by combining a growth experiment with transcriptomic analysis. Our results showed that the physiological responses of diatoms to MHWs and underlying molecular mechanisms are largely related to their growth forms. The euplanktonic diatom was first depressed, but then had a distinct increase in the growth rate accompanied by inducing zeatin and unsaturated fatty acid biosynthesis and repressing substance assimilation and energy metabolism. Contrarily, the benthic diatom showed elevated substance and energy demands for macromolecules accumulation by reducing cell division and increasing photosynthesis and nitrogen assimilation. The tychoplanktonic diatom exhibited higher physiological plasticity to maintain growth and cellular homeostasis. Our results indicate the increased rate of cell division in euplanktonic diatoms under heat stress is likely an emergency response strategy promoting diatom dispersal for survival, but at the cost of disturbances of metabolic balance.

Ecophysiological responses to heat waves in the marine intertidal zone

One notable consequence of climate change is an increase in the frequency, scale and severity of heat waves. Heat waves in terrestrial habitats (atmospheric heat waves, AHW) and marine habitats (marine heat waves, MHW) have received considerable attention as environmental forces that impact organisms, populations and whole ecosystems. Only one ecosystem, the intertidal zone, experiences both MHWs and AHWs. In this Review, we outline the range of responses that intertidal zone organisms exhibit in response to heat waves. We begin by examining the drivers of thermal maxima in intertidal zone ecosystems. We develop a simple model of intertidal zone daily maximum temperatures based on publicly available tide and solar radiation models, and compare it with logged, under-rock temperature data at an intertidal site. We then summarize experimental and ecological studies of how intertidal zone ecosystems and organisms respond to heat waves across dimensions of biotic response. Additional attention is paid to the impacts of extreme heat on cellular physiology, including oxidative stress responses to thermally induced mitochondrial overdrive and dysfunction. We examine the energetic consequences of these mechanisms and how they shift organismal traits, including growth, reproduction and immune function. We conclude by considering important future directions for improving studies of the impacts of heat waves on intertidal zone organisms.

Marine Heatwaves Exacerbate the Toxic Effects of Tire Particle Leachate on Microalgae

Additives leached from tire particles (TPs) after entering the marine environment inevitably interact with marine life. Marine heatwaves (MHWs) would play a more destructive role than ocean warming during the interaction of pollutants and marine life. To evaluate the potential risks of TPs leachate under MHWs, the physiological and nutrient metabolic endpoints of microalgae Isochrysis galbana were observed for 7 days while being exposed to TPs leachate at current or predicted concentrations under MHWs. TPs leachate mainly contained Zn and 6-PPD, which could be absorbed by microalgae mostly, especially under MHWs. Additionally, TPs leachate increased the reactive oxygen species content, activated the antioxidant system, impaired photosynthesis and glycolysis, and decreased sugar and protein content. 10 mg/L TPs leachate increased the lipid content and saturation. Meanwhile, microalgae under such TPs leachate were biased toward the synthesis of long-chain fatty acids and Δ8 desaturation pathway. MHWs promoted the positive effects of TPs leachate on microalgae growth at the current concentration but exacerbated the negative effects at the predicted concentration. Our study emphasizes the potential risks of TPs leachate to marine primary production systems, especially if accompanied by the increasing intensity and frequency of extreme climate events.