Meta-analysis reveals complex marine biological responses to the interactive effects of ocean acidification and warming

Influence of salinity on the thermal tolerance of aquatic organisms

Aquatic organisms are challenged by changes in their external environment, such as temperature and salinity fluctuations. If these variables interacted with each other, the response of organisms to temperature changes would be modified by salinity and vice versa. We tested for potential interaction between temperature and salinity effects on freshwater, brackish, and marine organisms, including algae, macrophytes, heterotrophic protists, parasites, invertebrates, and fish. We performed a meta-analysis that compared the thermal tolerance (characterised by the temperature optimum, lower and upper temperature limits, and thermal breadth) at various salinities. The meta-analysis was based on 90 articles (algae: 15; heterotrophic protists: 1; invertebrates: 43; and fish: 31). Studies on macrophytes and parasites were lacking. We found that decreasing salinity significantly increased and decreased the lower and upper temperature limits, respectively, in all groups. Thus, a lowered salinity increased the thermal sensitivity of organisms. These findings mainly reflect the response of brackish and marine organisms to salinity changes, which dominated our database. The few studies on freshwater species showed that their lower thermal limits increased and the upper thermal limits decreased with increasing salinity, albeit statistically nonsignificant. Although non-significant, the response of thermal tolerance to salinity changes differed between various organism groups. It generally decreased in the order of: algae > invertebrates > fish. Overall, our findings indicate adverse effects of salinity changes on the temperature tolerance of aquatic organisms. For freshwater species, studies are comparatively scarce and further studies on their thermal performance at various salinity gradients are required to obtain more robust evidence for interactions between salinity and temperature tolerance. Considering test conditions such as acclimation temperature and potential infection with parasites in future studies may decrease the variability in the relationship between salinity and thermal tolerance.

Seawater warming rather than acidification profoundly affects coastal geochemical cycling mediated by marine microbiome

The most concerning consequences of climate change include ocean acidification and warming, which can affect microbial communities and thus the biogeochemical cycling they mediate. Therefore, it is urgent to study the impact of ocean acidification and warming on microbial communities. In the current study, metagenomics was utilized to reveal how the structure and function of marine microorganisms respond to ocean warming and acidification. In terms of community structure, Non-metric Multidimensional Scaling analysis visualized the similarity or difference between the control and the warming or acidification treatments, but the inter-group differences were not significant. In terms of gene functionality, warming treatments showed greater effects on microbial communities than acidification. After treatment with warming, the relative abundance of genes associated with denitrification increased, suggesting that ocean nitrogen loss can increase with increased temperature. Conversely, acidification treatments apparently inhibited denitrification. Warming treatment also greatly affected sulfur-related microorganisms, increasing the relative abundance of certain sulfate-reducing prokaryote, and enriched microbial carbon-fixation pathways. These results provide information on the response strategies of coastal microorganisms in the changing marine environments.

Could future ocean acidification be affecting the energy budgets of marine fish?

With the unprecedented environmental changes caused by climate change including ocean acidification, it has become crucial to understand the responses and adaptive capacity of fish to better predict directional changes in the ecological landscape of the future. We conducted a systematic literature review to examine if simulated ocean acidification (sOA) could influence growth and reproduction in fish within the dynamic energy budget theory framework. As such, we chose to examine metabolic rate, locomotion, food assimilation and growth in early life stages (i.e. larvae and juvenile) and adults. Our goal was to evaluate if acclimatization to sOA has any directional changes in these traits and to explore potential implications for energetic trade-offs in these for growth and reproduction. We found that sOA had negligible effects on energetic expenditure for maintenance and aerobic metabolism due to the robust physiological capacity regulating acid-base and ion perturbations but substantive effects on locomotion, food assimilation and growth. We demonstrated evidence that sOA significantly reduced growth performance of fish in early life stages, which may have resulted from reduced food intake and digestion efficiency. Also, our results showed that sOA may enhance reproduction with increased numbers of offspring although this may come at the cost of altered reproductive behaviours or offspring fitness. While these results indicate evidence for changes in energy budgets because of physiological acclimatization to sOA, the heterogeneity of results in the literature suggests that physiological and neural mechanisms need to be clearly elucidated in future studies. Lastly, most studies on sOA have been conducted on early life stages, which necessitates that more studies should be conducted on adults to understand reproductive success and thus better predict cohort and population dynamics under ongoing climate change.

Temperature-dependent responses and trophic interaction strengths of a predatory marine gastropod and rock oyster under ocean warming

Predator-prey interactions are important in shaping ecosystem structure. Consequently, impacts of accelerating global warming on predators will have notable implications. Effects are likely to be particularly marked for tropical organisms which are anticipated to be sensitive to further thermal stress. Here, we investigated effects of future ocean warming on the predatory dogwhelk Reishia clavigera and its predation of Saccostrea cucullata. Mortality of the predators rapidly increased under the extreme elevated temperature, while those exposed to moderate elevated temperature displayed similar mortality as the ambient. Predators that survived moderate temperature increases altered their oxygen consumption patterns, increased average feeding rates, and functional responses, although condition index and energy reserves were unchanged. Overall, we show extreme ocean warming scenarios can remove predators and their consumption of prey from an ecosystem, whereas moderate warming can intensify predator-prey interactions. Such temperature-dependent alterations to predator-prey interactions would lead to fundamental changes of ecosystem structure as the ocean warms.

Water temperature affects somatic growth, body condition and oxygen and carbon otolith isotopes of stout whiting (Sillago robusta)

Ocean warming will continue to affect the growth, body condition and geographic distributions of marine fishes and understanding these effects is an urgent challenge for fisheries research and management. Determining how temperature is recorded in fish otolith carbonate, provides an additional chronological tool to investigate thermal histories, preferences and patterns of movement throughout an individual's life history. The influence of three water temperature treatments (22°C, 25°C, and 28°C) on hatchery-reared juvenile stout whiting, Sillago robusta, was tested using a controlled outdoor mesocosm system. Fish were measured for change in length and weight, and body condition was determined using bioelectrical impedance analysis. Sagittal otoliths were analysed for stable oxygen (δ18Ootolith) and carbon (δ13Cotolith) isotopes via isotope ratio mass spectrometry. Whiting kept at 22°C were significantly smaller and had diminished body condition compared to fish in 25°C and 28°C, which did not significantly differ from each other. The δ18O otolith values of stout whiting demonstrated a negative temperature-dependent fractionation relationship which was similar in slope but had a different intercept to the relationships reported for inorganic aragonite and other marine fish species. The δ13C otolith values also showed a negative relationship with water temperature, and the calculated proportion of metabolic carbon M in otoliths differed between fish reared in the coolest (22°C) and warmest (28°C) temperature treatments. Overall, the results suggest that stout whiting may have reached an upper growth threshold between 25°C and 28°C, and that growth and body condition may be optimised during warmer seasons and toward the northerly regions of their distribution. Otolith oxygen thermometry shows promise as a natural tracer of thermal life history, and species-specific fractionation equations should be utilised when possible to prevent errors in temperature reconstructions of wild-caught fish.

Global changes and their environmental stressors have a significant impact on soil biodiversity-A meta-analysis

Identifying the main threats to soil biodiversity is crucial as soils harbor ∼60% of global biodiversity. Many previous meta-analyses investigating the impact of different global changes (GCs) on biodiversity have omitted soil fauna or are limited by the GCs studied. We conducted a broad-scale meta-analysis focused on soil fauna communities, analyzing 3,161 effect sizes from 624 publications studying climate change, land-use intensification, pollution, nutrient enrichment, invasive species and habitat fragmentation. Land-use intensification resulted in large reductions in soil fauna communities, especially for the larger-bodied groups. Unexpectedly, pollution caused the largest negative impact on soil biodiversity - particularly worrying due to continually increasing levels of pollution and poor mechanistic understanding of impacts relative to other GCs. Not all GCs and stressors were detrimental; organic-based nutrient enrichment often resulted in positive responses. Including soil biodiversity in large-scale analyses is vital to fully understand the impact of GCs across the different realms.

Weathered polyethylene microplastics induced immunomodulation in zebrafish

Microplastics are pollutants of emerging concern and the aquatic biota consumes microplastics (MPs), which has a range of toxicological and environmental effects on aquatic organisms that are not the intended targets. The current study looked into how weathered polyethylene (wPE) MPs affected Danio albolineatus immunological and haematological markers. In this experiment, fish of both sexes were placed in control and exposure groups, and they were exposed for 40 d at the sublethal level (1 μg L-1) of fragmented wPE, which contained 1074 ± 52 MPs per litre. Similarly, fish exposed to wPE MPs showed significant modifications in lysozyme, antimicrobial, and antiprotease activity, as well as differential counts. Results of the present study show that the male fish were more susceptible than female fish after 40 d of chronic exposure. Further studies are needed to ascertain how the innate and humoral immune systems of the fish respond to MPs exposure.

Ocean warming and acidification adjust inter- and intra-specific variability in the functional trait expression of polar invertebrates

Climate change is known to affect the distribution and composition of species, but concomitant alterations to functionally important aspects of behaviour and species-environment relations are poorly constrained. Here, we examine the ecosystem ramifications of changes in sediment-dwelling invertebrate bioturbation behaviour-a key process mediating nutrient cycling-associated with near-future environmental conditions (+ 1.5 °C, 550 ppm [pCO2]) for species from polar regions experiencing rapid rates of climate change. We find that responses to warming and acidification vary between species and lead to a reduction in intra-specific variability in behavioural trait expression that adjusts the magnitude and direction of nutrient concentrations. Our analyses also indicate that species behaviour is not predetermined, but can be dependent on local variations in environmental history that set population capacities for phenotypic plasticity. We provide evidence that certain, but subtle, aspects of inter- and intra-specific variation in behavioural trait expression, rather than the presence or proportional representation of species per se, is an important and under-appreciated determinant of benthic biogeochemical responses to climate change. Such changes in species behaviour may act as an early warning for impending ecological transitions associated with progressive climate forcing.

Differential intestinal effects of water and foodborne exposures of nano-TiO2 in the mussel Mytilus coruscus under elevated temperature

With the wide use of nanomaterials in daily life, nano-titanium dioxide (nano-TiO2) presents potential ecological risks to marine ecosystems, which can be exacerbated by ocean warming (OW). However, most previous studies have only centered around waterborne exposure, while there is a scarcity of studies concentrating on the impact of trophic transfer exposure on organisms. We investigated the differences in toxic effects of 100 μg/L nano-TiO2 on mussels via two pathways (waterborne and foodborne) under normal (24 °C) and warming (28 °C) conditions. Single nano-TiO2 exposure (waterborne and foodborne) elevated the superoxide dismutase (SOD) and catalase (CAT) activities as well as the content of glutathione (GSH), indicating activated antioxidatant response in the intestine. However, depressed antioxidant enzymes and accumulated peroxide products (LPO and protein carbonyl content, PCC) demonstrated that warming in combination with nano-TiO2 broke the prooxidant-antioxidant homeostasis of mussels. Our findings also indicated that nano-TiO2 and high temperature exhibited adverse impacts on amylase (AMS), trypsin (PS), and trehalase (THL). Additionally, activated immune function (lysozyme) comes at the cost of energy expenditure of protein (decreased protein concentration). The hydrodynamic diameter of nano-TiO2 at 24 °C (1693-2261 nm) was lower than that at 28 °C (2666-3086 nm). Bioaccumulation results (range from 0.022 to 0.432 μg/g) suggested that foodborne induced higher Ti contents in intestine than waterborne. In general, the combined effects of nano-TiO2 and warming demonstrated a more pronounced extent of interactive effects and severe damage to antioxidant, digestive, and immune parameters in mussel intestine. The toxicological impact of nano-TiO2 was intensified through trophic transfer. The toxic effects of nano-TiO2 are non-negligible and can be exerted together through both water- and foodborne exposure routes, which deserves further investigation.

Disparate response of decapods to low pH: A meta-analysis of life history, physiology and behavior traits across life stages and environments

We employed a meta-analysis to determine if the presumed resilience of decapods to ocean acidification extends to all biological aspects, environments, and life stages. Most response categories appeared unaffected by acidification. However, certain fitness-related traits (growth, survival, and, to some extent, calcification) were impacted. Acid-base balance and stress response scaled positively with reductions in pH, which maintains homeostasis, possibly at the cost of other processes. Juveniles were the only stage impacted by acidification, which is believed to reduce recruitment. We observed few differences in responses to acidification among decapods inhabiting contrasting environments. Our meta-analysis shows decapods as a group slightly to moderately sensitive to low pH, with impacts on some biological aspects rather than on all specific life stages or habitats. Although extreme pH scenarios may not occur in the open ocean, coastal and estuarine areas might experience lower pH levels in the near to medium future, posing potential challenges for decapods.

Responses of marine trophic levels to the combined effects of ocean acidification and warming

Marine organisms are simultaneously exposed to anthropogenic stressors associated with ocean acidification and ocean warming, with expected interactive effects. Species from different trophic levels with dissimilar characteristics and evolutionary histories are likely to respond differently. Here, we perform a meta-analysis of controlled experiments including both ocean acidification and ocean warming factors to investigate single and interactive effects of these stressors on marine species. Contrary to expectations, we find that synergistic interactions are less common (16%) than additive (40%) and antagonistic (44%) interactions overall and their proportion decreases with increasing trophic level. Predators are the most tolerant trophic level to both individual and combined effects. For interactive effects, calcifying and non-calcifying species show similar patterns. We also identify climate region-specific patterns, with interactive effects ranging from synergistic in temperate regions to compensatory in subtropical regions, to positive in tropical regions. Our findings improve understanding of how ocean warming, and acidification affect marine trophic levels and highlight the need for deeper consideration of multiple stressors in conservation efforts.

Hidden impacts of ocean warming and acidification on biological responses of marine animals revealed through meta-analysis

Conflicting results remain on the impacts of climate change on marine organisms, hindering our capacity to predict the future state of marine ecosystems. To account for species-specific responses and for the ambiguous relation of most metrics to fitness, we develop a meta-analytical approach based on the deviation of responses from reference values (absolute change) to complement meta-analyses of directional (relative) changes in responses. Using this approach, we evaluate responses of fish and invertebrates to warming and acidification. We find that climate drivers induce directional changes in calcification, survival, and metabolism, and significant deviations in twice as many biological responses, including physiology, reproduction, behavior, and development. Widespread deviations of responses are detected even under moderate intensity levels of warming and acidification, while directional changes are mostly limited to more severe intensity levels. Because such deviations may result in ecological shifts impacting ecosystem structures and processes, our results suggest that climate change will likely have stronger impacts than those previously predicted based on directional changes alone.

The effects of temperature and sulfamethoxazole on the growth and photosynthetic characteristics of Phaeodactylum tricornutum

The misuse of antibiotics has brought potential ecological risks to marine ecosystems, especially under a changing climate. Laboratory experiments were conducted to understand the impact of rising temperatures and antibiotic sulfamethoxazole (SMX) abuse on marine diatom Phaeodactylum tricornutum. Temperatures of 21 and 24 °C were optimal for the growth and photosynthetic characteristics of P. tricornutum. When exposed to higher temperatures (≥27 °C), the growth and photosynthesis were inhibited. High concentrations of SMX (≥100 mg/L) caused rapid and acute toxicological effects on the phytoplankton. In contrast, low concentrations of SMX (1 mg/L) exhibited hormesis. When P. tricornutum was exposed to SMX at high temperatures, the stress on the phytoplankton was even more pronounced. This suggests that the combination of rising temperatures and antibiotic pollution may have a more significant negative impact on marine phytoplankton than either stressor alone. Neglecting the interaction between these stressors may lead to underestimating their combined effects on marine ecosystems.

Comparative physiology reveals heat stress disrupts acid-base homeostasis independent of symbiotic state in the model cnidarian Exaiptasia diaphana

Climate change threatens the survival of symbiotic cnidarians by causing photosymbiosis breakdown in a process known as bleaching. Direct effects of temperature on cnidarian host physiology remain difficult to describe because heatwaves depress symbiont performance, leading to host stress and starvation. The symbiotic sea anemone Exaiptasia diaphana provides an opportune system to disentangle direct versus indirect heat effects on the host, as it can survive indefinitely without symbionts. We tested the hypothesis that heat directly impairs cnidarian physiology by comparing symbiotic and aposymbiotic individuals of two laboratory subpopulations of a commonly used clonal strain of E. diaphana, CC7. We exposed anemones to a range of temperatures (ambient, +2°C, +4°C and +6°C) for 15-18 days, then measured their symbiont population densities, autotrophic carbon assimilation and translocation, photosynthesis, respiration and host intracellular pH (pHi). Symbiotic anemones from the two subpopulations differed in size and symbiont density and exhibited distinct heat stress responses, highlighting the importance of acclimation to different laboratory conditions. Specifically, the cohort with higher initial symbiont densities experienced dose-dependent symbiont loss with increasing temperature and a corresponding decline in host photosynthate accumulation. In contrast, the cohort with lower initial symbiont densities did not lose symbionts or assimilate less photosynthate when heated, similar to the response of aposymbiotic anemones. However, anemone pHi decreased at higher temperatures regardless of cohort, symbiont presence or photosynthate translocation, indicating that heat consistently disrupts cnidarian acid-base homeostasis independent of symbiotic status or mutualism breakdown. Thus, pH regulation may be a critical vulnerability for cnidarians in a changing climate.

Structurally stable but functionally disrupted marine microbial communities under a future climate change scenario: Potential importance for nitrous oxide emissions

The blue mussel Mytilus edulis is a widespread and abundant bivalve species along the North Sea with high economic and ecological importance as an engineer species. The shell of mussels is intensively colonized by microbial organisms that can produce significant quantities of nitrous oxide (N2O), a potent greenhouse gas. To characterize the impacts of climate change on the composition, structure and functioning of microbial biofilms on the shell surface of M. edulis, we experimentally exposed them to orthogonal combinations of increased seawater temperature (20 vs. 23 °C) and decreased pH (8.0 vs. 7.7) for six weeks. We used amplicon sequencing of the 16S rRNA gene to characterize the alpha and beta diversity of microbial communities on the mussel shell. The functioning of microbial biofilms was assessed by measuring aerobic respiration and nitrogen emission rates. We did not report any significant impacts of climate change treatments on the diversity of mussel microbiomes nor on the structure of these communities. Lowered pH and increased temperature had antagonistic effects on the functioning of microbial communities with decreased aerobic respiration and N2O emission rates of microbial biofilms in acidified seawater compared to increased rates in warmer conditions. An overriding impact of acidification over warming was finally observed on N2O emissions when the two factors were combined. Although acidification and warming in combination significantly reduced N2O biofilm emissions, the promotion of aquaculture activities in coastal waters where shellfish do not normally occur at high biomass and density could nonetheless result in unwanted emissions of this greenhouse gas in a near future.

Meta-analysis reveals responses of coccolithophores and diatoms to warming

A meta-analysis was conducted to explore the effects of warming on the physiological processes of coccolithophores and diatoms by synthesizing a large number of published literatures. A total of 154 studies consisting 301 experiments were synthesized in this study. Under a projected temperature increase of 3-5 °C by IPCC AR6 at the end of this century, our results suggest that the growth and photosynthetic rate of coccolithophores were significantly enhanced by the rising temperature, while the calcification of coccolithophores was only slightly promoted. Warming also had significantly positive effects on the growth but not photosynthesis of diatoms. In comparison, the effect size of warming on the growth rate of coccolithophores was larger than that of diatoms. However, there was no significant effect of warming on either the ratio of particulate inorganic carbon to particulate organic carbon (PIC:POC) of coccolithophores or the ratio of biogenic silica to carbon (BSi:C) of diatoms. Furthermore, the results reveal latitudinal and size-specific patterns of the effect sizes of warming. For diatoms, the effects of warming on growth were more prominent in high latitudes, specifically for the Southern Hemisphere species. In addition, the effect size of warming on the small-sized diatoms was larger than that of the large-sized diatoms. For coccolithophores, the growth of the Southern Hemisphere temperate strains was significantly promoted by warming. Overall, the results based on the meta-analysis indicate that the projected warming of the end of this century will be more favor to the growth of coccolithophores than that of diatoms, thus potentially affect the competitive advantages of coccolithophores over diatoms; while the mid-to high latitude species/strains of both coccolithophores and diatoms will benefit more than their counterparts in the lower latitudes. Therefore, this study offers novel insights into predicting both the inter- and intra-group competitive advantages of diatoms and coccolithophores under the future warming climate change scenario.

Rapid climate change alters the environment and biological production of the Indian Ocean

We synthesize and review the impacts of climate change on the physical, chemical, and biological environments of the Indian Ocean and discuss mitigating actions and knowledge gaps. The most recent climate scenarios identify with high certainty that the Indian Ocean (IO) is experiencing one of the fastest surface warming among the world's oceans. The area of surface waters of >28 °C (IO Warm Pool) has significantly increased during 1982-2021 by expanding into the northern-central basins. A significant decrease in pH and aragonite (building blocks of calcified organisms) levels in the IO was observed from 1981-2020 due to an increase in atmospheric CO2 concentrations. There are also signals of decreasing trends in primary productivity in the north, likely related to enhanced stratification and nutrient depletion. Further, the rapid warming of the IO will manifest more extreme weather conditions along its adjacent continents and oceans, including marine heat waves that are likely to reshape biodiversity. However, the impact of climate change beyond the unprecedented warming, increase in marine heat waves, expansion of the IO Warm Pool, and decrease in pH, remains uncertain for many other key variables in the IO including changes in salinity, oxygen, and net primary production. Understanding the response of these physical, chemical, and biological variables to climate change is vital to project future changes in regional fisheries and identify mitigation actions. We accordingly conclude by identifying knowledge gaps and recommending directions for sustainable fisheries and climate impact studies.

Using meta-analysis to explore the roles of global upwelling exposure and experimental design in bivalve responses to low pH

Low pH conditions, associated with ocean acidification, represent threats to many commercially and ecologically important organisms, including bivalves. However, there are knowledge gaps regarding factors explaining observed differences in biological responses to low pH in laboratory experiments. Specific sources of local adaptation such as upwelling exposure and the role of experimental design, such as carbonate chemistry parameter changes, should be considered. Linking upwelling exposure, as an individual oceanographic phenomenon, to responses measured in laboratory experiments may further our understanding of local adaptation to global change. Here, meta-analysis is used to test the hypotheses that upwelling exposure and experimental design affect outcomes of individual, laboratory-based studies that assess bivalve metabolic (clearance and respiration rate) responses to low pH. Results show that while bivalves generally decrease metabolic activity in response to low pH, upwelling exposure and experimental design can significantly impact outcomes. Bivalves from downwelling or weak upwelling areas decrease metabolic activity in response to low pH, but bivalves from strong upwelling areas increase or do not change metabolic activity in response to low pH. Furthermore, experimental temperature, exposure time and magnitude of the change in carbonate chemistry parameters all significantly affect outcomes. These results suggest that bivalves from strong upwelling areas may be less sensitive to low pH. This furthers our understanding of local adaptation to global change by demonstrating that upwelling alone can explain up to 49 % of the variability associated with bivalve metabolic responses to low pH. Furthermore, when interpreting outcomes of individual, laboratory experiments, scientists should be aware that higher temperatures, shorter exposure times and larger changes in carbonate chemistry parameters may increase the chance of suppressed metabolic activity.

Same species, different population dynamics: Spatio-temporal differences of Undaria pinnatifida (Ochrophyta, Phaeophyceae) in the intertidal of North Patagonia, Argentina

Population dynamics can be influenced by physical and biological factors, particularly in stressful environments. Introduced species usually have great physiological plasticity, resulting in populations with different traits. Undaria pinnatifida, a macroalga originally described from northeast Asia, was introduced in Northern Patagonia, Argentina (San Matías Gulf) around 2010. To describe the spatio-temporal variability in population structure and morphometry of U. pinnatifida, we conducted monthly field samplings for 2 years at the intertidal area of two contrasting sites in the San Matías Gulf. Individuals of U. pinnatifida were classified by developmental stage, and their morpho-gravimetric variables were measured. In both intertidal sites juveniles were found in higher proportion during austral autumn and grew and matured during the autumn-winter months (from May onwards), and individuals senesced during early austral summer (December and January). Conversely, density and biomass were largely different between sites, and individuals showed slight morphological variability between sites. Environmental (e.g., nutrient concentration, available substrate) and biological factors (e.g., facilitation, competition) may explain the observed differences. Since there is not a macroalga with U. pinnatifida morphometrical characteristics in the intertidal environments of San Matías Gulf, studying this recent introduction gives us a better understanding of its potential ecological effects.

Northerly range expansion and first confirmed records of the smalltooth sand tiger shark, Odontaspis ferox, in the United Kingdom and Ireland

Three Odontaspis ferox (confirmed by mtDNA barcoding) were found in the English Channel and Celtic Sea in 2023 at Lepe, UK (50.7846, -1.3508), Kilmore Quay, Ireland (52.1714, -6.5937), and Lyme Bay, UK (50.6448, -2.9302). These are the first records of O. ferox in either country, and extend the species' range by over three degrees of latitude, to >52° N. They were ~275 (female), 433 (female), and 293 cm (male) total length, respectively. These continue a series of new records, possibly indicative of a climate change-induced shift in the species' range.

Winter is coming: Interactions of multiple stressors in winter and implications for the natural world

Winter is a key driver of ecological processes in freshwater, marine and terrestrial ecosystems, particularly in higher latitudes. Species have evolved various adaptive strategies to cope with food limitations and the cold and dark wintertime. However, human-induced climate change and other anthropogenic stressors are impacting organisms in winter in unpredictable ways. In this paper, we show that global change experiments investigating multiple stressors have predominantly been conducted during summer months. However, effects of anthropogenic stressors sometimes differ between winter and other seasons, necessitating comprehensive investigations. Here, we outline a framework for understanding the different effects of anthropogenic stressors in winter compared to other seasons and discuss the primary mechanisms that will alter ecological responses of organisms (microbes, animals and plants). For instance, while the magnitude of some anthropogenic stressors can be greater in winter than in other seasons (e.g. some pollutants), others may alleviate natural winter stress (e.g. warmer temperatures). These changes can have immediate, delayed or carry-over effects on organisms during winter or later seasons. Interactions between stressors may also vary with season. We call for a renewed research direction focusing on multiple stressor effects on winter ecology and evolution to fully understand, and predict, how ecosystems will fare under changing winters. We also argue the importance of incorporating the interactions of anthropogenic stressors with winter into ecological risk assessments, management and conservation efforts.

Effect of elevated temperature, sea water acidification, and phenanthrene on the expression of genes involved in the shell and pearl formation of economic pearl oyster (Pinctada radiata)

Our study aims to examine the effect of some stressors on the gene expression levels of shell matrix proteins in a pearl oyster. Oysters were exposed to the different combinations of the temperature, pH, and phenanthrene concentration is currently measured in the Persian Gulf and the predicted ocean warming and acidification for 28 days. The expression of all the studied genes was significantly downregulated. Time and temperature had the greatest effects on the decreases in n19 and n16 genes expression, respectively. Aspein and msi60 genes expression were highly influenced by pH. Pearlin was affected by double interaction temperature and phenanthrene. Moreover, a correlation was observed among the expression levels of studied genes. This study represents basic data on the relationship between mRNA transcription genes involved in the shell and pearl formation and climate changes in pollutant presence conditions and acclimatizing mechanism of the oyster to the future scenario as well.

Seagrass Thalassia hemprichii and associated bacteria co-response to the synergistic stress of ocean warming and ocean acidification

Seagrass meadows play vital ecological roles in the marine ecosystem. Global climate change poses considerable threats to seagrass survival. However, it is unclear how seagrass and its associated bacteria will respond under future complex climate change scenarios. This study explored the effects of ocean warming (+2 °C) and ocean acidification (-0.4 units) on seagrass physiological indexes and bacterial communities (sediment and rhizosphere bacteria) of the seagrass Thalassia hemprichii during an experimental exposure of 30 days. Results demonstrated that the synergistic effect of ocean warming and ocean acidification differed from that of one single factor on seagrass and the associated bacterial community. The seagrass showed a weak resistance to ocean warming and ocean acidification, which manifested through the increase in the activity of typical oxidoreductase enzymes. Moreover, the synergistic effect of ocean warming and ocean acidification caused a significant decrease in seagrass's chlorophyll content. Although the bacterial community diversity exhibited higher resistance to ocean warming and ocean acidification, further bacterial functional analysis revealed the synergistic effect of ocean warming and ocean acidification led to significant increases in SOX-related genes abundance which potentially supported the seagrass in resisting climate stress by producing sulfates and oxidizing hydrogen sulfide. More stable bacterial communities were detected in the seagrass rhizosphere under combined ocean warming and ocean acidification. While for one single environmental stress, simpler networks were detected in the rhizosphere. In addition, the observed significant correlations between several modules of the bacterial community and the physiological indexes of the seagrass indicate the possible intimate interaction between seagrass and bacteria under ocean warming and ocean acidification. This study extends our understanding regarding the role of seagrass associated bacterial communities and sheds light on both the prediction and preservation of the seagrass meadow ecosystems in response to global climate change.

Examining the reproductive success of bull kelp (Nereocystis luetkeana, Phaeophyceae, Laminariales) in climate change conditions

Climate change is affecting marine ecosystems in many ways, including raising temperatures and leading to ocean acidification. From 2014 to 2016, an extensive marine heat wave extended along the west coast of North America and had devastating effects on numerous species, including bull kelp (Nereocystis luetkeana). Bull kelp is an important foundation species in coastal ecosystems and can be affected by marine heat waves and ocean acidification; however, the impacts have not been investigated on sensitive early life stages. To determine the effects of changing temperatures and carbonate levels on Northern California's bull kelp populations, we collected sporophylls from mature bull kelp individuals in Point Arena, CA. At the Bodega Marine Laboratory, we released spores from field-collected bull kelp, and cultured microscopic gametophytes in a common garden experiment with a fully factorial design crossing modern conditions (11.63 ± 0.54°C and pH 7.93 ± 0.26) with observed extreme climate conditions (15.56 ± 0.83°C and 7.64 ± 0.32 pH). Our results indicated that both increased temperature and decreased pH influenced growth and egg production of bull kelp microscopic stages. Increased temperature resulted in decreased gametophyte survival and offspring production. In contrast, decreased pH had less of an effect but resulted in increased gametophyte survival and offspring production. Additionally, increased temperature significantly impacted reproductive timing by causing female gametophytes to produce offspring earlier than under ambient temperature conditions. Our findings can inform better predictions of the impacts of climate change on coastal ecosystems and provide key insights into environmental dynamics regulating the bull kelp lifecycle.

Simultaneous warming and acidification limit population fitness and reveal phenotype costs for a marine copepod

Phenotypic plasticity and evolutionary adaptation allow populations to cope with global change, but limits and costs to adaptation under multiple stressors are insufficiently understood. We reared a foundational copepod species, Acartia hudsonica, under ambient (AM), ocean warming (OW), ocean acidification (OA), and combined ocean warming and acidification (OWA) conditions for 11 generations (approx. 1 year) and measured population fitness (net reproductive rate) derived from six life-history traits (egg production, hatching success, survival, development time, body size and sex ratio). Copepods under OW and OWA exhibited an initial approximately 40% fitness decline relative to AM, but fully recovered within four generations, consistent with an adaptive response and demonstrating synergy between stressors. At generation 11, however, fitness was approximately 24% lower for OWA compared with the AM lineage, consistent with the cost of producing OWA-adapted phenotypes. Fitness of the OWA lineage was not affected by reversal to AM or low food environments, indicating sustained phenotypic plasticity. These results mimic those of a congener, Acartia tonsa, while additionally suggesting that synergistic effects of simultaneous stressors exert costs that limit fitness recovery but can sustain plasticity. Thus, even when closely related species experience similar stressors, species-specific costs shape their unique adaptive responses.

Impacts of ocean warming on echinoderms: A meta-analysis

Rising ocean temperatures are threatening marine species and populations worldwide, and ectothermic taxa are particularly vulnerable. Echinoderms are an ecologically important phylum of marine ectotherms and shifts in their population dynamics can have profound impacts on the marine environment. The effects of warming on echinoderms are highly variable across controlled laboratory-based studies. Accordingly, synthesis of these studies will facilitate the better understanding of broad patterns in responses of echinoderms to ocean warming. Herein, a meta-analysis incorporating the results of 85 studies (710 individual responses) is presented, exploring the effects of warming on various performance predictors. The mean responses of echinoderms to all magnitudes of warming were compared across multiple biological responses, ontogenetic life stages, taxonomic classes, and regions, facilitated by multivariate linear mixed effects models. Further models were conducted, which only incorporated responses to warming greater than the projected end-of-century mean annual temperatures at the collection sites. This meta-analysis provides evidence that ocean warming will generally accelerate metabolic rate (+32%) and reduce survival (-35%) in echinoderms, and echinoderms from subtropical (-9%) and tropical (-8%) regions will be the most vulnerable. The relatively high vulnerability of echinoderm larvae to warming (-20%) indicates that this life stage may be a significant developmental bottleneck in the near-future, likely reducing successful recruitment into populations. Furthermore, asteroids appear to be the class of echinoderms that are most negatively affected by elevated temperature (-30%). When considering only responses to magnitudes of warming representative of end-of-century climate change projections, the negative impacts on asteroids, tropical species and juveniles were exacerbated (-51%, -34% and -40% respectively). The results of these analyses enable better predictions of how keystone and invasive echinoderm species may perform in a warmer ocean, and the possible consequences for populations, communities and ecosystems.

Interaction matters: Bottom-up driver interdependencies alter the projected response of phytoplankton communities to climate change

Phytoplankton growth is controlled by multiple environmental drivers, which are all modified by climate change. While numerous experimental studies identify interactive effects between drivers, large-scale ocean biogeochemistry models mostly account for growth responses to each driver separately and leave the results of these experimental multiple-driver studies largely unused. Here, we amend phytoplankton growth functions in a biogeochemical model by dual-driver interactions (CO2 and temperature, CO2 and light), based on data of a published meta-analysis on multiple-driver laboratory experiments. The effect of this parametrization on phytoplankton biomass and community composition is tested using present-day and future high-emission (SSP5-8.5) climate forcing. While the projected decrease in future total global phytoplankton biomass in simulations with driver interactions is similar to that in control simulations without driver interactions (5%-6%), interactive driver effects are group-specific. Globally, diatom biomass decreases more with interactive effects compared with the control simulation (-8.1% with interactions vs. no change without interactions). Small-phytoplankton biomass, by contrast, decreases less with on-going climate change when the model accounts for driver interactions (-5.0% vs. -9.0%). The response of global coccolithophore biomass to future climate conditions is even reversed when interactions are considered (+33.2% instead of -10.8%). Regionally, the largest difference in the future phytoplankton community composition between the simulations with and without driver interactions is detected in the Southern Ocean, where diatom biomass decreases (-7.5%) instead of increases (+14.5%), raising the share of small phytoplankton and coccolithophores of total phytoplankton biomass. Hence, interactive effects impact the phytoplankton community structure and related biogeochemical fluxes in a future ocean. Our approach is a first step to integrate the mechanistic understanding of interacting driver effects on phytoplankton growth gained by numerous laboratory experiments into a global ocean biogeochemistry model, aiming toward more realistic future projections of phytoplankton biomass and community composition.

Dietary exposure to nTiO2 reduces byssus performance of mussels under ocean warming

Nano‑titanium dioxide (nTiO₂) is a widely used nanomaterial posing potential ecological risk for marine ecosystems that might be enhanced by elevated temperatures such as expected during climate change. nTiO₂ may affect benthic filter feeders like mussels through waterborne exposures and via food chain due to the adsorption on/in algae. Mussel byssus are proteinaceous fibers secreted by byssal glands of the mussels for attachment. Byssus production and mechanical properties are sensitive to environmental stressors but the combined effects of warming and nTiO₂ on byssus performance of mussels are unclear hampering our understanding of the predation and dislodgement risk of mussels under the multiple stressor scenarios. We explored the effects of a short-term (14-day) single and combined exposures to warming (28 °C) and 100 μg L^-1 nTiO₂ (including food co-exposure) on the byssus performance of the thick shell mussel Mytilus coruscus. The mechanical strength (measured as the breaking force) of the byssal threads was impaired by warming and nTiO₂ (including food co-exposure), but the number and length of the byssal threads were increased. The mRNA expression levels of mussel foot proteins (mfp-3, mfp-5) and pre-collagens (preCOL-D, preCOL-P, preCOL-NG) were up-regulated to varying degrees, with the strongest effects induced by warming. This indicates that the physiological and molecular mechanisms of byssus secretion are plastic. However, downregulation of the mRNA expression of preCOL-D and preCOL-P under the combined warming and nTiO₂ exposures indicate the limits of these plasticity mechanisms and suggest that the attachment ability and survival of the mussels may be impaired if the pollution or temperature conditions further deteriorate.

Ocean acidification alters foraging behaviour in Dungeness crab through impairment of the olfactory pathway

Crustacean olfaction is fundamental to most aspects of living and communicating in aquatic environments and more broadly, for individual- and population-level success. Accelerated ocean acidification from elevated CO2 threatens the ability of crabs to detect and respond to important olfactory-related cues. Here, we demonstrate that the ecologically and economically important Dungeness crab (Metacarcinus magister) exhibits reduced olfactory-related antennular flicking responses to a food cue when exposed to near-future CO2 levels, adding to the growing body of evidence of impaired crab behaviour. Underlying this altered behaviour, we find that crabs have lower olfactory nerve sensitivities (twofold reduction in antennular nerve activity) in response to a food cue when exposed to elevated CO2 . This suggests that near-future CO2 levels will impact the threshold of detection of food by crabs. We also show that lower olfactory nerve sensitivity in elevated CO2 is accompanied by a decrease in the olfactory sensory neuron (OSN) expression of a principal chemosensory receptor protein, ionotropic receptor 25a (IR25a) which is fundamental for odorant coding and olfactory signalling cascades. The OSNs also exhibit morphological changes in the form of decreased surface areas of their somata. This study provides the first evidence of the effects of high CO2 levels at multiple levels of biological organization in marine crabs, linking physiological and cellular changes with whole animal behavioural responses.

Reorganization of seagrass communities in a changing climate

Although climate change projections indicate significant threats to terrestrial biodiversity, the effects are much more profound and striking in the marine environment. Here we explore how different facets of locally distinctive α- and β-diversity (changes in spatial composition) of seagrasses will respond to future climate change scenarios across the globe and compare their coverage with the existing network of marine protected areas. By using species distribution modelling and a dated phylogeny, we predict widespread reductions in species' range sizes that will result in increases in seagrass weighted and phylogenetic endemism. These projected increases of endemism will result in divergent shifts in the spatial composition of β-diversity leading to differentiation in some areas and the homogenization of seagrass communities in other regions. Regardless of the climate scenario, the potential hotspots of these projected shifts in seagrass α- and β-diversity are predicted to occur outside the current network of marine protected areas, providing new priority areas for future conservation planning that incorporate seagrasses. Our findings report responses of species to future climate for a group that is currently under represented in climate change assessments yet crucial in maintaining marine food chains and providing habitat for a wide range of marine biodiversity.

A metric-based framework for climate-smart conservation planning

Climate change is already having profound effects on biodiversity, but climate change adaptation has yet to be fully incorporated into area-based management tools used to conserve biodiversity, such as protected areas. One main obstacle is the lack of consensus regarding how impacts of climate change can be included in spatial conservation plans. We propose a climate-smart framework that prioritizes the protection of climate refugia-areas of low climate exposure and high biodiversity retention-using climate metrics. We explore four aspects of climate-smart conservation planning: (1) climate model ensembles; (2) multiple emission scenarios; (3) climate metrics; and (4) approaches to identifying climate refugia. We illustrate this framework in the Western Pacific Ocean, but it is equally applicable to terrestrial systems. We found that all aspects of climate-smart conservation planning considered affected the configuration of spatial plans. The choice of climate metrics and approaches to identifying refugia have large effects in the resulting climate-smart spatial plans, whereas the choice of climate models and emission scenarios have smaller effects. As the configuration of spatial plans depended on climate metrics used, a spatial plan based on a single measure of climate change (e.g., warming) will not necessarily be robust against other measures of climate change (e.g., ocean acidification). We therefore recommend using climate metrics most relevant for the biodiversity and region considered based on a single or multiple climate drivers. To include the uncertainty associated with different climate futures, we recommend using multiple climate models (i.e., an ensemble) and emission scenarios. Finally, we show that the approaches we used to identify climate refugia feature trade-offs between: (1) the degree to which they are climate-smart, and (2) their efficiency in meeting conservation targets. Hence, the choice of approach will depend on the relative value that stakeholders place on climate adaptation. By using this framework, protected areas can be designed with improved longevity and thus safeguard biodiversity against current and future climate change. We hope that the proposed climate-smart framework helps transition conservation planning toward climate-smart approaches.

Separate and combined effects of elevated pCO2 and temperature on the branching reef corals Acropora digitifera and Montipora digitata

Ocean acidification (OA) and warming (OW) are major global threats to coral reef ecosystems; however, studies on their combined effects (OA + OW) are scarce. Therefore, we evaluated the effects of OA, OW, and OA + OW in the branching reef corals Acropora digitifera and Montipora digitata, which have been found to respond differently to environmental changes. Our results indicate that OW has a greater impact on A. digitifera and M. digitata than OA and that the former species is more vulnerable to OW than the latter. OW was the main stressor for increased mortality and decreased calcification in the OA + OW group, and the effect of OA + OW was additive in both species. Our findings suggest that the relative abundance and cover of M. digitata are expected to increase whereas those of A. digitifera may decrease in the near future in Okinawa.

Embryonic encapsulated development of the gastropod Acanthina monodon is impacted by future environmental changes of temperature and pCO2

Egg capsules of the gastropod Acanthina monodon were maintained during the entire period of encapsulated development at three temperatures (10, 15, 20 °C) and two pCO₂ levels (400, 1200 μatm). Embryos per capsule, size at hatching, time to hatching, embryonic metabolic rates, and the resistance of juveniles to shell breakage were quantified. No embryos maintained at 20 °C developed to hatching. The combination of temperature and pCO₂ levels had synergistic effects on hatching time and developmental success, antagonistic effects on number of hatchlings per capsule, resistance to juvenile shell cracking and metabolism, and additive effect on hatching size. Juveniles hatched significantly sooner at 15 °C, independent of the pCO₂ level that they had been exposed to, while individuals hatched at significantly smaller sizes if they had been held under 15 °C/1200 μatm rather than at 10 °C/low pCO₂. Embryos held at the higher pCO₂ had a significantly greater percentage of abnormalities. For capsules maintained at low pCO₂ and 15 °C, emerging juveniles had less resistance to shell breakage. Embryonic metabolism was significantly higher at 15 °C than at 10 °C, independent of pCO₂ level. The lower metabolism occurred in embryos maintained at the higher pCO₂ level. Thus, in this study, temperature was the factor that had the greatest effect on the encapsulated development of A. monodon, increasing the metabolism of the embryos and consequently accelerating development, which was expressed in a shorter intracapsular development time, but with smaller individuals at hatching and a lower resistance of their shells to breakage. On the other hand, the high pCO₂ level suppressed metabolism, prolonged intracapsular development, and promoted more incomplete development of the embryos. However, the combination of the two factors can mitigate--to some extent--the adverse effects of both incomplete development and lower resistance to shell breakage.

Predicting effects of multiple interacting global change drivers across trophic levels

Global change encompasses many co-occurring anthropogenic drivers, which can act synergistically or antagonistically on ecological systems. Predicting how different global change drivers simultaneously contribute to observed biodiversity change is a key challenge for ecology and conservation. However, we lack the mechanistic understanding of how multiple global change drivers influence the vital rates of multiple interacting species. We propose that reaction norms, the relationships between a driver and vital rates like growth, mortality, and consumption, provide insights to the underlying mechanisms of community responses to multiple drivers. Understanding how multiple drivers interact to affect demographic rates using a reaction-norm perspective can improve our ability to make predictions of interactions at higher levels of organization-that is, community and food web. Building on the framework of consumer-resource interactions and widely studied thermal performance curves, we illustrate how joint driver impacts can be scaled up from the population to the community level. A simple proof-of-concept model demonstrates how reaction norms of vital rates predict the prevalence of driver interactions at the community level. A literature search suggests that our proposed approach is not yet used in multiple driver research. We outline how realistic response surfaces (i.e., multidimensional reaction norms) can be inferred by parametric and nonparametric approaches. Response surfaces have the potential to strengthen our understanding of how multiple drivers affect communities as well as improve our ability to predict when interactive effects emerge, two of the major challenges of ecology today.

High temperatures enhance the strength of multiple predator effects in a typical crab-clam system

Although marine heatwaves pose urgent threats to marine life, our understanding of how these events influence interactions between key species in marine ecosystems is still inadequate. Herein, we examined the behavioral mechanisms by which heat regulates multiple predator effects in different foraging systems that include Asian paddle crabs (Charybdis japonica) and swimming crabs (Portunus trituberculatus) by quantifying their predation and competition at two temperatures. Our results show that non-independent multiple predator effects occurred in the conspecific treatment of Asian paddle crabs and in the interspecific treatment, whereas independent multiple predator effects occurred in the conspecific treatment of swimming crabs. Asymmetrical behavior responses of these crabs to competition and heat triggered divergences in multiple predator effects. High temperatures increased the strength of multiple predator effects but did not alter their types. The reason is that heat negatively impacts predation by enhancing aggressive interactions, outweighing its direct positive effects on predation.

Limited trait responses of a tropical seagrass to the combination of increasing pCO2 and warming

Understanding species-specific trait responses under future global change scenarios is of importance for conservation efforts and to make informed decisions within management projects. The combined and single effects of seawater acidification and warmer average temperature were investigated by means of the trait responses of Cymodocea serrulata, a tropical seagrass, under experimental conditions. After a 35 d exposure period, biochemical, morphological, and photo-physiological trait responses were measured. Overall, biochemical traits mildly responded under the individual exposure to high temperature and increasing pCO2 values. The response of C. serrulata was limited to a decrease in %C and an increase in the sucrose content in the rhizome under the high temperature treatment, 32 °C. This suggests that this temperature was lower than the maximum tolerance limit for this species. Increasing pCO2 levels increased %C in the rhizome, and also showed a significant increase in leaf δ13C values. The effects of all treatments were sublethal; however, small changes in their traits could affect the ecosystem services they provide. In particular, changes in tissue carbon concentrations may affect carbon storage capacity, one key ecosystem service. The simultaneous study of different types of trait responses contributes to establish a holistic framework of seagrass ecosystem health under climate change.

Interaction effects of crude oil and nutrient exposure on settlement of coral reef benthos

Anthropogenic stressors increasingly cause ecosystem-level changes to sensitive marine habitats such as coral reefs. Intensification of coastal development and shipping traffic can increase nutrient and oil pollution on coral reefs, yet these two stressors have not been studied in conjunction. Here, we simulate a disturbance scenario exposing carbonate settlement tiles to nutrient and oil pollution in a full-factorial design with four treatments: control, nutrients, oil, and combination to examine community structure and net primary productivity (NPP) of pioneer communities throughout 28 weeks. Compared to the control treatment oil pollution decreased overall settlement and NPP, while nutrients increased turf algae and NPP. However, the combination of these two stressors resulted in similar community composition and NPP as the control. These results indicate that pioneer communities may experience shifts due to nutrient enrichment, and/or oil pollution. However, the timing and duration of an event will influence recovery trajectories requiring further study.

Unraveling cellular and molecular mechanisms of acid stress tolerance and resistance in marine species: New frontiers in the study of adaptation to ocean acidification

Since the industrial revolution, fossil fuel combustion has led to a 30 %-increase of the atmospheric CO₂ concentration, also increasing the ocean partial CO₂ pressure. The consequent lowered surface seawater pH is termed ocean acidification (OA) and severely affects marine life on a global scale. Cellular and molecular responses of marine species to lowered seawater pH have been studied but information on the mechanisms driving the tolerance of adapted species to comparatively low seawater pH is limited. Such information may be obtained from species inhabiting sites with naturally low water pH that have evolved remarkable abilities to tolerate such conditions. This review gathers information on current knowledge about species naturally facing low water pH conditions and on cellular and molecular adaptive mechanisms enabling the species to survive under, and even benefit from, adverse pH conditions. Evidences derived from case studies on naturally acidified systems and on resistance mechanisms will guide predictions on the consequences of future adverse OA scenarios for marine biodiversity.

Seasonal nearshore ocean acidification and deoxygenation in the Southern California Bight

The California Current System experiences seasonal ocean acidification and hypoxia (OAH) owing to wind-driven upwelling, but little is known about the intensity, frequency, and depth distribution of OAH in the shallow nearshore environment. Here we present observations of OAH and dissolved inorganic carbon and nutrient parameters based on monthly transects from March 2017 to September 2018 extending from the surf zone to the ~ 40 m depth contour in La Jolla, California. Biologically concerning OAH conditions were observed at depths as shallow as 10 m and as close as 700 m to the shoreline. Below 20 m depth, 8% of observations were undersaturated with respect to aragonite, 28% of observations had a pH_T less than 7.85, and 19% of observations were below the sublethal oxygen threshold of 157 µmol kg^-1. These observations raise important questions about the impacts of OAH on coastal organisms and ecosystems and how future intensified upwelling may exacerbate these conditions.

Experimental evolution reveals the synergistic genomic mechanisms of adaptation to ocean warming and acidification in a marine copepod

Metazoan adaptation to global change relies on selection of standing genetic variation. Determining the extent to which this variation exists in natural populations, particularly for responses to simultaneous stressors, is essential to make accurate predictions for persistence in future conditions. Here, we identified the genetic variation enabling the copepod Acartia tonsa to adapt to experimental ocean warming, acidification, and combined ocean warming and acidification (OWA) over 25 generations of continual selection. Replicate populations showed a consistent polygenic response to each condition, targeting an array of adaptive mechanisms including cellular homeostasis, development, and stress response. We used a genome-wide covariance approach to partition the allelic changes into three categories: selection, drift and replicate-specific selection, and laboratory adaptation responses. The majority of allele frequency change in warming (57%) and OWA (63%) was driven by shared selection pressures across replicates, but this effect was weaker under acidification alone (20%). OWA and warming shared 37% of their response to selection but OWA and acidification shared just 1%, indicating that warming is the dominant driver of selection in OWA. Despite the dominance of warming, the interaction with acidification was still critical as the OWA selection response was highly synergistic with 47% of the allelic selection response unique from either individual treatment. These results disentangle how genomic targets of selection differ between single and multiple stressors and demonstrate the complexity that nonadditive multiple stressors will contribute to predictions of adaptation to complex environmental shifts caused by global change.

Is Ocean Acidification Really a Threat to Marine Calcifiers? A Systematic Review and Meta-Analysis of 980+ Studies Spanning Two Decades

Ocean acidification is considered detrimental to marine calcifiers, but mounting contradictory evidence suggests a need to revisit this concept. This systematic review and meta-analysis aim to critically re-evaluate the prevailing paradigm of negative effects of ocean acidification on calcifiers. Based on 5153 observations from 985 studies, many calcifiers (e.g., echinoderms, crustaceans, and cephalopods) are found to be tolerant to near-future ocean acidification (pH ≈ 7.8 by the year 2100), but coccolithophores, calcifying algae, and corals appear to be sensitive. Calcifiers are generally more sensitive at the larval stage than adult stage. Over 70% of the observations in growth and calcification are non-negative, implying the acclimation capacity of many calcifiers to ocean acidification. This capacity can be mediated by phenotypic plasticity (e.g., physiological, mineralogical, structural, and molecular adjustments), transgenerational plasticity, increased food availability, or species interactions. The results suggest that the impacts of ocean acidification on calcifiers are less deleterious than initially thought as their adaptability has been underestimated. Therefore, in the forthcoming era of ocean acidification research, it is advocated that studying how marine organisms persist is as important as studying how they perish, and that future hypotheses and experimental designs are not constrained within the paradigm of negative effects.

Effects of Ultraviolet Radiation on Sediment Burial Parameters and Photo-Oxidative Response of the Intertidal Anemone Anthopleura hermaphroditica

Anthopleura hermaphroditica is an intertidal anemone that lives semi-buried in soft sediments of estuaries and releases its brooded embryos directly to the benthos, being exposed to potentially detrimental ultraviolet radiation (UVR) levels. In this study, we investigated how experimental radiation (PAR: photosynthetically active radiation; UVA: ultraviolet A radiation; and UVB: ultraviolet B radiation) influences burrowing (time, depth and speed) in adults and juveniles when they were exposed to PAR (P, 400–700 nm), PAR + UVA (PA, 315–700 nm) and PAR + UVA + UVB (PAB, 280–700 nm) experimental treatments. The role of sediment as a physical shield was also assessed by exposing anemones to these radiation treatments with and without sediment, after which lipid peroxidation, protein carbonyls and total antioxidant capacity were quantified. Our results indicate that PAB can induce a faster burial response compared to those anemones exposed only to P. PAB increased oxidative damage, especially in juveniles where oxidative damage levels were several times higher than in adults. Sediment offers protection to adults against P, PA and PAB, as significant differences in their total antioxidant capacity were observed compared to those anemones without sediment. Conversely, the presence or absence of sediment did not influence total antioxidant capacity in juveniles, which may reflect that those anemones have sufficient antioxidant defenses to minimize photooxidative damage due to their reduced tolerance to experimental radiation. Burrowing behavior is a key survival skill for juveniles after they have been released after brooding.

Compound marine heatwaves and ocean acidity extremes

Compound MHW-OAX events, during which marine heatwaves (MHWs) co-occur with ocean acidity extreme (OAX) events, can have larger impacts on marine ecosystems than the individual extremes. Using monthly open-ocean observations over the period 1982–2019, we show that globally 1.8 in 100 months (or about one out of five present-day MHW months) are compound MHW-OAX event months under a present-day baseline, almost twice as many as expected for 90th percentile extreme event exceedances if MHWs and OAX events were statistically independent. Compound MHW-OAX events are most likely in the subtropics (2.7 in 100 months; 10°−40° latitude) and less likely in the equatorial Pacific and the mid-to-high latitudes (0.7 in 100 months; >40° latitude). The likelihood pattern results from opposing effects of temperature and dissolved inorganic carbon on [H⁺]. The likelihood is higher where the positive effect on [H⁺] from increased temperatures during MHWs outweighs the negative effect on [H⁺] from co-occurring decreases in dissolved inorganic carbon. Daily model output from a large-ensemble simulation of an Earth system model is analyzed to assess changes in the MHW-OAX likelihood under climate change. The projected long-term mean warming and acidification trends have the largest effect on the number of MHW-OAX days per year, increasing it from 12 to 265 days per year at 2 °C global warming relative to a fixed pre-industrial baseline. Even when long-term trends are removed, an increase in [H⁺] variability leads to a 60% increase in the number of MHW-OAX days under 2 °C global warming. These projected increases may cause severe impacts on marine ecosystems.

Compound extreme events in two or more oceanic ecosystem stressors are increasingly considered as a major concern for marine life. Here the authors present a first global analysis on compound marine heatwave and ocean acidity extreme events, identifying hotspots, drivers, and projecting future changes.

Ocean Warming Amplifies the Effects of Ocean Acidification on Skeletal Mineralogy and Microstructure in the Asterinid Starfish Aquilonastra yairi

Ocean acidification and ocean warming compromise the capacity of calcifying marine organisms to generate and maintain their skeletons. While many marine calcifying organisms precipitate low-Mg calcite or aragonite, the skeleton of echinoderms consists of more soluble Mg-calcite. To assess the impact of exposure to elevated temperature and increased pCO2 on the skeleton of echinoderms, in particular the mineralogy and microstructure, the starfish Aquilonastra yairi (Echinodermata: Asteroidea) was exposed for 90 days to simulated ocean warming (27 °C and 32 °C) and ocean acidification (455 µatm, 1052 µatm, 2066 µatm) conditions. The results indicate that temperature is the major factor controlling the skeletal Mg (Mg/Ca ratio and Mgnorm ratio), but not for skeletal Sr (Sr/Ca ratio and Srnorm ratio) and skeletal Ca (Canorm ratio) in A. yairi. Nevertheless, inter-individual variability in skeletal Sr and Ca ratios increased with higher temperature. Elevated pCO2 did not induce any statistically significant element alterations of the skeleton in all treatments over the incubation time, but increased pCO2 concentrations might possess an indirect effect on skeletal mineral ratio alteration. The influence of increased pCO2 was more relevant than that of increased temperature on skeletal microstructures. pCO2 as a sole stressor caused alterations on stereom structure and degradation on the skeletal structure of A. yairi, whereas temperature did not; however, skeletons exposed to elevated pCO2 and high temperature show a strongly altered skeleton structure compared to ambient temperature. These results indicate that ocean warming might exacerbate the skeletal maintaining mechanisms of the starfish in a high pCO2 environment and could potentially modify the morphology and functions of the starfish skeleton.

Consequences of Warming and Acidification for the Temperate Articulated Coralline Alga, Calliarthron Tuberculosum (Florideophyceae, Rhodophyta)

Global climate changes, such as warming and ocean acidification (OA), are likely to negatively impact calcifying marine taxa. Abundant and ecologically important coralline algae may be particularly susceptible to OA; however, multi-stressor studies and those on articulated morphotypes are lacking. Here, we use field observations and laboratory experiments to elucidate the impacts of warming and acidification on growth, calcification, mineralogy, and photophysiology of the temperate articulated coralline alga, Calliarthron tuberculosum. We conducted a 4-week fully factorial mesocosm experiment exposing individuals from a southern CA kelp forest to current and future temperature and pH/pCO₂ conditions (+2°C, -0.5 pH units). Calcification was reduced under warming (70%) and further reduced by high pCO₂ or high pCO₂ x warming (~150%). Growth (change in linear extension and surface area) was reduced by warming (40% and 50%, respectively), high pCO₂ (20% and 40%, respectively), and high pCO₂ x warming (50% and 75%, respectively). The maximum photosynthetic rate (P_max ) increased by 100% under high pCO₂ conditions, but we did not detect an effect of pCO₂ or warming on photosynthetic efficiency (α). We also did not detect the effect of warming or pCO₂ on mineralogy. However, variation in Mg incorporation in cell walls of different cell types (i.e., higher mol % Mg in cortical vs. medullary) was documented for the first time in this species. These results support findings from a growing body of literature suggesting that coralline algae are often more negatively impacted by warming than OA, with the potential for antagonistic effects when factors are combined.

Meta-analysis suggests negative, but pCO2-specific, effects of ocean acidification on the structural and functional properties of crustacean biomaterials

Crustaceans comprise an ecologically and morphologically diverse taxonomic group. They are typically considered resilient to many environmental perturbations found in marine and coastal environments, due to effective physiological regulation of ions and hemolymph pH, and a robust exoskeleton. Ocean acidification can affect the ability of marine calcifying organisms to build and maintain mineralized tissue and poses a threat for all marine calcifying taxa. Currently, there is no consensus on how ocean acidification will alter the ecologically relevant exoskeletal properties of crustaceans. Here, we present a systematic review and meta-analysis on the effects of ocean acidification on the crustacean exoskeleton, assessing both exoskeletal ion content (calcium and magnesium) and functional properties (biomechanical resistance and cuticle thickness). Our results suggest that the effect of ocean acidification on crustacean exoskeletal properties varies based upon seawater pCO2 and species identity, with significant levels of heterogeneity for all analyses. Calcium and magnesium content was significantly lower in animals held at pCO2 levels of 1500-1999 µatm as compared with those under ambient pCO2. At lower pCO2 levels, however, statistically significant relationships between changes in calcium and magnesium content within the same experiment were observed as follows: a negative relationship between calcium and magnesium content at pCO2 of 500-999 µatm and a positive relationship at 1000-1499 µatm. Exoskeleton biomechanics, such as resistance to deformation (microhardness) and shell strength, also significantly decreased under pCO2 regimes of 500-999 µatm and 1500-1999 µatm, indicating functional exoskeletal change coincident with decreases in calcification. Overall, these results suggest that the crustacean exoskeleton can be susceptible to ocean acidification at the biomechanical level, potentially predicated by changes in ion content, when exposed to high influxes of CO2. Future studies need to accommodate the high variability of crustacean responses to ocean acidification, and ecologically relevant ranges of pCO2 conditions, when designing experiments with conservation-level endpoints.

Meta-analysis reveals variance in tolerance to climate change across marine trophic levels

Marine ecosystems are currently facing a variety of anthropogenic perturbations, including climate change. Trophic differences in response to climate change may disrupt ecological interactions and thereby threaten marine ecosystem function. Yet, we still do not have a comprehensive understanding of how different trophic levels respond to climate change stressors in marine ecosystems. By including 1278 experiments, comprising 236 different marine species from 18 different phyla in a meta-analysis of studies measuring the direct effect of ocean acidification and ocean warming on marine organisms, we found that higher trophic level species display greater tolerance to ocean acidification but greater sensitivity to warming. In contrast, marine herbivores were the most vulnerable trophic level to both acidification and warming. Such imbalances in the community and a general reduction of biodiversity and biomass in lower trophic levels can significantly disrupt the system and could drive negative bottom-up effects. In conclusion, with ocean acidification and elevated temperatures, there is an alarming risk that trophic disparity may disrupt species interactions, and thereby drive community destabilization under ocean climate change.

Understanding the combined effects of multiple stressors: A new perspective on a longstanding challenge

Wildlife populations and their habitats are exposed to an expanding diversity and intensity of stressors caused by human activities, within the broader context of natural processes and increasing pressure from climate change. Estimating how these multiple stressors affect individuals, populations, and ecosystems is thus of growing importance. However, their combined effects often cannot be predicted reliably from the individual effects of each stressor, and we lack the mechanistic understanding and analytical tools to predict their joint outcomes. We review the science of multiple stressors and present a conceptual framework that captures and reconciles the variety of existing approaches for assessing combined effects. Specifically, we show that all approaches lie along a spectrum, reflecting increasing assumptions about the mechanisms that regulate the action of single stressors and their combined effects. An emphasis on mechanisms improves analytical precision and predictive power but could introduce bias if the underlying assumptions are incorrect. A purely empirical approach has less risk of bias but requires adequate data on the effects of the full range of anticipated combinations of stressor types and magnitudes. We illustrate how this spectrum can be formalised into specific analytical methods, using an example of North Atlantic right whales feeding on limited prey resources while simultaneously being affected by entanglement in fishing gear. In practice, case-specific management needs and data availability will guide the exploration of the stressor combinations of interest and the selection of a suitable trade-off between precision and bias. We argue that the primary goal for adaptive management should be to identify the most practical and effective ways to remove or reduce specific combinations of stressors, bringing the risk of adverse impacts on populations and ecosystems below acceptable thresholds.