Ocean Acidification

Ocean acidification (OA) and microplastic (MP) pollution are widespread marine stressors, yet their interactive effects on seafood quality and molecular metabolism remain unclear. This study investigated the combined effects of OA (pH 7.7) and MPs (2 and 200 μg/L) on the Pacific oyster (Magallana gigas). OA was the primary driver of textural deterioration, significantly reducing springiness and chewiness, while combined stress synergistically depleted protein and lipid reserves. Distinct lipid remodeling strategies were identified: OA induced DHA accumulation potentially associated with membrane stabilization, whereas MPs triggered EPA and ARA upregulation associated with stress and immune responses. The flavor profile was severely compromised, characterized by depletion of umami amino acids, nucleotide redistribution, and altered succinate contribution. Transcriptomic analysis revealed that high MP exposure activated genome maintenance and DNA repair-associated pathways, including the Fanconi anemia pathway, superimposed on OA-associated metabolic suppression. Concurrent upregulation of nucleotide salvage (APRT, HPRT) and amino acid catabolic genes (GLS, GDH) suggests increased utilization of flavor metabolites for energetic demands. These findings support a bioenergetic trade-off in which oysters exposed to OA and MPs reallocate resources from nutritional and sensory quality toward cellular maintenance and stress adaptation, highlighting underrecognized consequences of climate change and plastic pollution for seafood quality.

Across the world’s oceans, lakes, and rivers, climate change is reshaping fisheries and aquaculture systems on which billions of people depend for food and millions rely for their livelihoods. This narrative review summarizes peer-reviewed and selected literature published between 2008 and 2026 on climate impacts, adaptation options, and critical knowledge gaps in marine and freshwater fisheries, with a particular focus on tropical and African inland fisheries. This review shows that warming, acidification, deoxygenation, and altered hydrology are disrupting fish physiology; shifting the distribution of marine species poleward by over 70 km per decade, at rates that vary greatly by taxon, depth, and area; and destroying critical habitats. Although this rapid poleward shift is well documented in open marine environments, it contrasts sharply with landlocked or fragmented freshwater systems, where horizontal migration is physically constrained, a contrast central to the concept of climate entrapment developed in this review. In scenarios characterized by high emissions, tropical fisheries may see a reduction of up to 40% in their maximum catch potential, a statistic relevant to specific tropical Exclusive Economic Zones based on bioclimate-envelope forecasts, which should not be interpreted as a global or universal result. This figure is a model-derived projection under a bioclimate-envelope framework rather than an observed trend, and the realized outcome will depend on future emission pathways. These biophysical changes compound already serious pressures, overfishing, habitat loss, and pollution, and the burden falls heaviest on those least responsible: small-scale fishers, women, and indigenous communities. We introduce the concept of climate entrapment to describe the distinct vulnerability of freshwater fisheries: trapped by fragmented habitats and hydrological barriers, fish stocks cannot migrate to more secure waters as conditions deteriorate. While there are promising adaptation strategies, major gaps remain in understanding what works, for whom, and under what conditions. We conclude with priority recommendations for research, policy, and practice.

This doctoral thesis presents a multi-scale and interdisciplinary investigation into the impacts of climate change and anthropogenic pressures on coastal marine ecosystems, integrating the regional dynamics of the Adriatic Sea and the broader context of the Mediterranean Sea. This research combines long-term environmental data analysis with physiological and biochemical studies on key indicator species to elucidate the complex, and often synergistic, nature of the major threats facing these vulnerable environments. The first part of the work highlights a shift in the Adriatic Sea’s physical and biogeochemical regime, from historical eutrophication toward contemporary oligotrophic conditions. The combined effects of reduced Po River runoff and climate-driven warming, particularly during summer, modify the hydrography of the entire basin. These changes have cascading effects, clearly visible in the clam Chamelea gallina. Northern populations, while showing short-term resilience (e.g., higher survival in air), exhibit a lower condition index and higher bioaccumulation of trace metals, which activate costly metabolic defense mechanisms. This energetic stress makes the populations more vulnerable to additional pressures, possibly aligning with the recurrent mass mortality events. The second part of this work was conducted at the Panarea CO2 vents, which constitute a natural laboratory for assessing future global change impacts. Through the study of the coral Balanophyllia europaea, it is found that ocean acidification (OA), especially when combined with ocean warming, significantly impairs coral tissue regeneration, with species-specific responses. A crucial finding was that OA appears to stimulate a response mediated by the coral’s microbiome, enhancing its bacterial degradation capacity and reducing the presence of PAHs in coral tissues. Collectively, this work demonstrates that synergistic stressor interactions pose the greatest threat to marine ecosystems. While evidence of resilience emerges through physiological and microbial adaptations, the findings emphasize the urgent need for integrated management strategies that address cumulative impacts across scales.

This convergent mixed methods study examined the impact of a new phenomena based (PhBL) lesson plan—grounded by social constructivism and the NGSS framework—on 1) student ability to explain and contextualize a phenomenon, and 2) assess student perceptions of PhBL compared to traditional science instruction. This addressed the shift towards science education reform that better prepares K-12 students for the STEM pipeline and creates student agency surrounding human impact on the environment. Ocean acidification was the selected phenomenon for this study as it was culturally and locally relevant to the participants—sixth-grade science students at a Pacific Island public middle school. Findings revealed apparent differences in the distribution of inductive codes, showing treatment group students acquiring more diverse knowledge of ocean acidification as assessed by graphic organizers and field notes. Findings also showed an increase in student ability to construct accurate explanations of ocean acidification and contextualizations in both treatment and control groups, as assessed by the pre/post-tests. But the treatment group students exhibited the larger increase, alluding to PhBL creating the more effective learning environment. Using the Constructivist Learning Environment Survey (CLES), the instrument revealed no significant differences in perceptions between control and treatment group students. However, treatment group students expressed overall positive experiences and opinions towards PhBL via interview questions—and provided insight that would help inform future PhBL.

Highlights

• First discovery of Pseudoalteromonas enrichment in A. japonicus gut under acidification.
• The dominant strain P. carrageenovora P1 was successfully isolated.
• P. carrageenovora P1 activates host antioxidant and immune pathways to bolster stress resilience.
• P. carrageenovora P1 significantly enhances resistance to V. splendidus 21,915 infection in A. japonicus.

Abstract

The sea cucumber (Apostichopus japonicus) is a key mariculture species of high economic value in East Asia. However, Ocean acidification, resulting from the increased uptake of anthropogenic CO₂ by seawater, severely impairs its growth, survival, and reproductive performance, thereby threatening the sustainable development of this aquaculture industry. The gut microbiota play a critical role in host adaptation to environmental stress; however, their involvement in mediating A. japonicus responses to seawater acidification remains unclear. Therefore, this study first compared the gut microbial communities of juvenile and adult A. japonicus under acidified and ambient seawater conditions. The results showed that Pseudoalteromonas abundance was significantly enriched under seawater acidification, from which the dominant strain, Pseudoalteromonas carrageenovora P1, was screened and isolated. This strain, when supplied as a dietary supplement under seawater acidification, improved A. japonicus growth performance and intestinal morphology, activated the Keap1-Nrf2-ARE and NF-κB pathways, enhanced antioxidant capacity and immune function, and increased resistance against Vibrio splendidus 21915 infection. Moreover, P. carrageenovora P1 modulated gut microbial structure by promoting beneficial bacteria such as Lutibacter while suppressing potential pathogens, including Vibrio, thereby maintaining intestinal homeostasis under acidification stress. This study reveals the important value of the gut microbiota of A. japonicus in regulating environmental tolerance of benthic invertebrates, and also provides a scientific reference for stress resistance regulation and health maintenance in A. japonicus aquaculture under seawater acidification.

Mangrove ecosystems are critical blue carbon sinks, yet the role of submarine groundwater discharge (SGD) in carbon outwelling and coastal acidification buffering remains a major knowledge gap. This study integrates radium isotopes (²²⁴Ra and ²²⁸Ra) and carbonate system parameters to quantify SGD-driven carbon fluxes and their impacts on ocean acidification buffering in Tieshan Bay, a mangrove-fringed semi-enclosed bay in Beibu Gulf, China. Field measurements revealed SGD fluxes of (3.93 ± 1.52) × 10⁷ m³/d (11.6 ± 4.5 cm/d), delivering 31-fold more dissolved inorganic carbon [DIC, (1.06 ± 0.13) × 10⁸ mol/d] and 21-fold more total alkalinity [TA, (7.25 ± 0.89) × 10⁷ mol/d] than local rivers. Notably, SGD exhibited a low TA/DIC ratio (0.68 ± 0.14), which mechanistically weakens the bay’s buffering capacity against acidification by reducing the efficiency of the carbonate system to neutralize atmospheric CO₂. Furthermore, bicarbonate dominated SGD-derived DIC (84.7% ± 14.3%), enhancing long-term carbon sequestration potential but concurrently suppressing carbonate saturation states via dilution of carbonate ions [SGD: (27.4 ± 28.1) µmol/L vs. bay: (80.3 ± 22.4) µmol/L]. These findings demonstrate SGD’s dual role as a major carbon outwelling vector and a driver of coastal acidification, redefining blue carbon budgets in mangrove ecosystems.

Marine environments that are subjected to seasonal changes are increasingly exposed to various factors, such as water pollution, climate change, coastal development, and human impacts. Therefore, this study examined seasonal changes in the main physicochemical parameters of seawater around Dangli Island, Langkawi, Malaysia to better understand the environmental conditions that affect the local marine ecosystem. This study examined seasonal variations in the main physicochemical parameters of seawater such as salinity, temperature, conductivity, pH, dissolved oxygen (DO), Total Ammoniacal Nitrogen (TAN), and Orthophosphate (PO₄³⁻) between twelve monitoring stations D1–D12 around Dangli Island during the Southwest and Northwest Monsoons. The results showed a significant difference between the Southwest Monsoon (SWM) season, which exhibited higher salinity up to 31.16 ppt, temperature up to 30.62°C, and conductivity, while the Northwest Monsoon (NEM) was recorded with cooler temperatures as low as 28.73°C and a slight increase in dissolved oxygen in some areas. This phenomenon causes the ocean to absorb CO₂ and control ocean acidification. Warmer waters during NEM can reduce CO₂ solubility, while cooler, oxygen-rich SWM conditions increase CO₂ uptake, potentially increasing acidification. By studying and understanding how these seasonal conditions affect ocean chemistry, it can help assess the long-term impacts of climate change on the marine environment of Dangli Island and allow us to prepare early conservation strategies for the future. These factors can be influenced by seasonal variations and upwelling phenomena. Two-way ANOVA results demonstrated that all physicochemical parameters have no significant difference between sampling stations (p > 0.05). In addition, there have high significant differences between monsoon seasons (p < 0.05) except for conductivity and DO level. The variations in the physicochemical properties of seawater around Dangli Island reveal the influences of rainfall, river discharges, and upwelling phenomenon on stratification shifts that show the differences in the distribution of seawater physicochemical parameters. These findings also provide valuable insights and help to conserve the marine environment, offering a basis for future research and environmental management strategies around Dangli Island.

Marine heatwaves are becoming more frequent and intense, driving mass fish mortalities and reshaping marine ecosystems, yet the capacity of fishes to tolerate heatwaves remains unclear. We quantified physiological condition and behavioural responses of juvenile warm-water (Abudefduf vaigiensis) and cool-water (Microcanthus strigatus) fishes from three natural climate analogues—an ambient temperate reef, an ocean warming analogue, and a combined warming and extreme acidification analogue—before and during the 2023 global marine heatwave, and under a + 3 °C simulated heatwave in the laboratory. In the laboratory, warm-water fish from the warm reef began the experiment in leaner condition than cool reef fish but increased their bite rates under heatwave conditions, compensating for elevated metabolic demand and maintaining body condition. Cool-water fish showed no behavioural adjustment to the heatwave, and body condition diverged by reef origin: cool reef fish lost weight across all treatments while warm- and extreme-origin fish maintained condition, suggesting tolerance was conferred by prior thermal history rather than acute behavioural responses. In the field, warm-water fish were leaner and had lower protein content during the 2023 heatwave than before it, while cool-water fish from the extreme reef had lower body condition than those from the cool reef prior to the heatwave. Our findings suggest that behavioural plasticity can facilitate warm-water fish tolerance to heatwaves at their cold leading edges, while thermal history supports heatwave tolerance in cool-water fish within their core ranges. Populations inhabiting naturally extreme climate analogues may therefore harbour pre-adapted, climate-resilient fish in a future ocean.

Early life stages play a key role in structuring marine benthic communities, yet the mechanisms controlling recruitment in confined habitats remain poorly understood. In particular, the relative importance of substrate stability and visual cues under low-exchange, low-light conditions is unresolved. We investigated zoobenthic recruitment on 8 natural rock types differing in mineral composition, grain size, and surface reflectance, in simulated interstitial sub-boulder conditions in the Ligurian Sea (NW Mediterranean). After 90 d immersion, recruitment was dominated by spirorbid and serpulid polychaetes. Settlement intensity differed markedly among lithotypes, decreasing from granitoids (239.4 ± 37.7 spirorbids and 23.9 ± 3.4 serpulids) to marbles (128.7 ± 45.5 and 13.6 ± 4.8, respectively) and highly porous travertines (55.4 ± 18.5 and 8.4 ± 2.8), in agreement with their estimated dissolution rates. These results suggest that substrate stability is a primary driver of recruitment under confined conditions, likely mediated by localized acidification and enhanced dissolution of calcareous substrates. Within lithotypes, differences in recruitment were also associated with surface reflectance: specifically, a clear preference for darker surfaces emerged within marbles, whereas the colour effect was overridden by structural or mineralogical traits within the heterogeneous matrix of granitoids. These findings indicate that both chemical stability and optical properties contribute to recruitment patterns, although their relative importance varies with environmental context. The modulation of the early colonization in these restricted sub-boulder environments provide a valuable hypothetical framework to investigate the effects of large-scale environmental stressors like future ocean acidification scenario.

Climate change exerts a strong impact on marine ecosystems, particularly through ocean acidification and rising water temperatures. Within this context, shifts in lipid composition have emerged as valuable stress indicators in many organisms, providing insights into the trophic ecology of ecosystems. The objective of this study was to assess the combined effects of warming and ocean acidification scenarios projected for the end of the century (+5 °C and pH 7.5) on the lipid profiles of the hydrocoral Millepora alcicornis, recently recorded for the first time in Tenerife (Canary Islands, Spain). After an 84-day experiment, corals exhibited significant reductions in total lipid content, key lipid classes and fatty acids, particularly triacylglycerides (TAG) and docosahexaenoic acid (DHA, 22:6n-3), whereas sterol esters (ST) and saturated fatty acids (SFAs) showed the opposite trend. By contrast, symbiont-derived glycolipids and C18 fatty acids characteristic of zooxanthellae were not significantly affected by environmental stress, suggesting relative stability of membrane lipid composition. Overall, the maintenance of membrane lipid composition in symbiotic algae together with evidence of lipid remodelling in the host suggests a capacity for short-term physiological acclimation of M. alcicornis under the simulated climate change conditions. However, reduced DHA availability may have important physiological implications for the colonies, justifying future studies on the molecular regulation of LC-PUFA biosynthesis and the long-term acclimation capacity of Millepora alcicornis under climate change.

This Collection supports and amplifies research related to SDG 13 and SDG 14.

Submission status: Open

Submission deadline: 24 June 2027

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Growing human reliance on marine resources has disrupted marine ecosystems, with ocean acidification emerging as a critical threat to many of these ecosystems and the resources they provide. As a direct result of human CO2 emissions, ocean acidification amplifies the impacts of both climate change and other human stressors, forming a “human activities-ocean acidification-climate change” negative cycle. 

As the Earth’s largest carbon sink, the ocean has absorbed massive amounts of anthropogenic CO₂ since the Industrial Revolution. As this CO₂ dissolves into the surface ocean, it lowers pH and carbonate saturation states (a chemical phenomenon termed ocean acidification) which, in turn, causes harm to many marine species and ocean processes. Such widespread damage to marine ecosystems can reduce the services they provide by triggering fishery losses, disrupting climate regulation, and driving biodiversity decline. 

Ocean acidification and climate change reinforce each other: warming seawater reduces the ocean’s ability to absorb CO₂ absorption, while acidification can increase organism sensitivity to temperature rise and other stressors. This interaction ultimately threatens marine health and ocean-dependent human societies globally. Moreover, local and regional solutions are needed; systematic research is urgent. This special collection in npj Ocean Sustainability focuses on ocean acidification, exploring its drivers and consequences, and synthesizing global advances in emission reduction, ecological restoration, and policy to break the negative cycle. 

This collection welcomes the following research directions: 

• Research on the two-way feedback mechanisms and driving factors between ocean acidification and other climate change, or human-induced, stressors. 
• Marine ecological responses and biological adaptation mechanisms under the combined effects of ocean acidification and climate change. 
• Research and practice of mitigation, adaptation, and ecological restoration technologies that build resilience against the impacts of ocean acidification and climate change. 
• Global collaborative governance, policy formulation, and socio-economic impacts in addressing ocean acidification and climate change. 

Copper contamination and rising atmospheric CO2, which lowers pH through acidification. Because copper becomes more toxic at lower pH, acidification may make copper more harmful than current water quality guidelines predict. Northern Clearwater crayfish (Faxonius propinquus) and zebrafish (Danio rerio) were exposed to a range of copper concentration under current atmospheric CO2 to identify the No Observed Effect Concentration (NOEC) and Lowest Observed Effect Concentration (LOEC) for chemosensory impairment, measured as the ability to detect a food odour. These NOEC concentrations were then combined with elevated pCO2 levels projected for the end of this century under a high-emission scenario, and Visual MINTEQ was used to model copper speciation across treatments. Under ambient CO2, crayfish were impaired at 30μg/L copper but not 20μg/L, while zebrafish were roughly six times more sensitive, impaired at 5μg/L but not 2.5μg/L. Elevated pCO2 alone did not impair crayfish, but did impair zebrafish at 1000μatm. Critically, when the previously safe copper concentration (NOEC) was paired with elevated pCO2 that caused no impairment on its own, both species lost the ability to detect food odours. MINTEQ modelling confirmed that lower pH under elevated pCO2 increased bioavailable free Cu2+ by 1.9- to 3.8 fold, offering a likely mechanism for this combined effect. These results show that freshwater acidification can lower copper toxicity thresholds, with direct consequences for how aquatic animals are protected from metal contamination under climate change. Current water quality guidelines do not account for this pH-driven increase in toxicity, representing a significant and largely overlooked gap in the literature.

The impact of ocean acidification (OA) on seawater carbonate chemistry, particularly on the aragonite saturation state (Ω_arag), is inadequately understood in the Gulf of Mannar (GoM) and Palk Bay (PB). In this study, we analyzed the spatial and temporal variability of Ω_arag to pinpoint potential ocean acidification hotspots within these two ecologically critical coastal habitats along the southeastern coast of India. A pilot study was conducted over 24 locations during 2023–2024. The findings reveal that PB had a higher mean pH (8.33 ± 0.06) than GoM (8.08 ± 0.02). In GoM, Ω_arag values were lower, at 2.82 ± 0.20, indicating that calcification conditions were not adequate. In PB, it reached 3.22 ± 0.57, which means that conditions for calcifying organisms were favourable. Seasonal variability in Ω_arag was primarily controlled by pH, carbonate ion concentration, pCO₂, and monsoonal hydrographic processes Seasonal patterns corroborated that these conditions were modulated by both anthropogenic activities and natural processes. During the Northeast and Southwest monsoons, there were significant drops in Ω_arag that matched changes in important biogeochemical indices. Overall, these results reveal that PB experiences greater variability in Seawater carbonate chemistry, while GoM exhibits relatively stable but slightly lower carbonate saturation. Pearson correlation and Structural Equation Modelling further demonstrated that pH and carbonate ion concentration positively influenced Ω_arag, whereas pCO₂ and Revelle factor showed negative relationships. The findings enunciate that GoM is more vulnerable to ocean acidification than PB and emphasize the value of Ω_arag as a sensitive indicator for detecting emerging acidification hotspots in coastal waters. It also emphasizes the importance of regular monitoring of OA parameters in these regions to protect these critical ecosystems and their associated services.

Ocean warming and acidification pose significant threats to marine biodiversity and human nutrition by fundamentally altering the biochemical composition of marine organisms. A primary concern is the potential decline in omega-3 fatty acids (FAs) that are essential to human health and primarily obtained through seafood consumption. The influence of these climate stressors on FA content across marine food webs remains poorly understood. To address this critical knowledge gap, we conducted a global meta-analysis of 489 experiments across 143 publications and 132 marine species, quantifying the effects of warming and acidification on nutritionally important FAs in marine primary producers, invertebrates and fishes. Under warmer conditions, we detected reductions of up to 34% and 50% in omega-3 FA proportions and concentrations, respectively, and up to 53% in omega-3: omega-6 ratios, together with increases of up to 22% and 17% in saturated FA proportions and concentrations, respectively across taxa groups. Critically, these effects were intensified as warming increased. The most severe reductions in omega-3 FAs were observed in primary producers, suggesting that climate-driven changes at the base of the food web could impair trophic transfer of these micronutrients, limiting nutrient availability for higher trophic levels and ultimately humans. Ocean acidification, conversely, demonstrated a minor overall effect on FA levels, although this result was based on substantially fewer studies. Furthermore, we identified species from environments with broader temperature ranges, diatoms, herbivorous invertebrates and low-resilience fish to display larger reductions in omega-3 FA proportions under warming. Our meta-analysis further highlighted the need for long-term studies under ecologically realistic conditions to improve predictions of nutritional responses to climate change. These findings are essential for understanding how the nutritional value of species change under climate change, and can inform fisheries management, aquaculture and public health policy aimed at securing the future availability of these vital micronutrients.

There is increasing demand to compare long-term biological observations with derived or measured carbonate chemistry data to characterize the biological consequences of ocean acidification (OA) in natural environments. Meeting this need calls for a standardized methodology by which to quantify and directly compare biological and carbonate chemistry change. Widdicombe et al. (2023) proposed a conceptual framework based on biological traits and paired biological-chemical rate comparisons, but its practical implementation has remained unresolved.

Here we operationalize that conceptual framework through a standardized analytical workflow that integrates heterogeneous biological observations with established OA time-series methodology. The workflow addresses five analytical challenges that hinder comparison of long-term biological and OA records: 1) fundamentally different temporal structures of biological and OA time series, 2) heterogeneity of biological observations, 3) sparse and irregular biological sampling, 4) nonlinear biological trajectories, and 5) appearance–disappearance events that invalidate conventional relative change metrics. The workflow addresses these challenges through standardized biological grouping, annual aggregation, piecewise linear segmentation, normalization by ecological boundaries, and direct pairing of biological and chemical rates of change.

We evaluated workflow performance using 36 synthetic biological time series generated by applying predefined temperature-pH-driven biological performance functions to a 108-year hydrodynamic-biogeochemical model simulation under the SSP3-7.0 emissions scenario. Biological rates estimated by the workflow closely agreed with the corresponding reference rates. Rate estimates remained similar across the two annual representations, and their relative ordering was largely preserved across contrasting site-specific temperature-pH transitions. These results demonstrate the robustness of the workflow under the tested patterns of abiotic forcing.

The analytical workflow developed here operationalizes the conceptual approach of Widdicombe et al. (2023) by standardizing the estimation and comparison of rates of change in biological and carbonate chemistry time series. It supports comparisons among compatible biological indicators while retaining the ecological context of each time series. The workflow does not establish causality; instead, it assesses the temporal coherence between long-term changes in candidate biological indicators of OA and concurrent changes in carbonate chemistry. It therefore provides a practical foundation for regional and global syntheses of biological responses to OA and other long-term abiotic stressors.

Highlights

• Acidification compresses benthic community isotopic space via carbon homogenization.
• Functional groups show differential isotopic responses consistent with bottom-up trophic reorganization.
• Arbacia lixula shifts from a sponge-dominated diet to herbivory under low pH.
• Paracentrotus lividus maintains stable trophic niche despite acidification.
• Asymmetric grazer responses drive site-specific increases in isotopic niche overlap under acidification.

Abstract

Ocean acidification is expected to alter marine food webs, yet community-level evidence from long-term, naturally acidified systems remains limited. We used stable isotope ratios (δ¹³C and δ¹⁵N) to investigate how natural acidification restructures benthic trophic interactions, from basal resources to consumer niche dynamics, a naturally acidified system (pH gradient: 8.1 to ~7.4; pHT scale) spanning present-day to end-of-century projected conditions (SSP2–4.5 to SSP5–8.5). After accounting for local differences in the isotopic baseline, we found that: (1) community isotopic space contracted under acidification, with the carbon range decreasing as primary producers converged toward ¹³C-depleted values, while the nitrogen range expanded, indicating species-specific trophic adjustments; (2) functional groups responded asymmetrically, with primary producers showing the strongest δ¹³C shifts, filter-feeders remaining relatively stable, and omnivorous consumers showing reduced δ¹³C and δ¹⁵N values; (3) the two dominant sea urchin grazers responded in opposite directions — the macrophyte-based omnivore Paracentrotus lividus maintained stable isotopic values across the pH gradient, whereas the invertebrate-based omnivore Arbacia lixula exhibited pronounced isotopic shifts — resulting in markedly increased, though site-dependent, niche overlap at the most acidified sites (up to 27%); (4) Bayesian mixing models revealed that this convergence was driven by dietary shifts in A. lixula, which transitioned from a predominantly invertebrate-based diet (70–79% animal prey) toward algal-based feeding under acidification (19–24% animal prey), while P. lividus maintained relatively consistent dietary composition (50–70% algae). Together, these results indicate that acidification-driven homogenization of primary producers, combined with asymmetric consumer responses, compresses trophic niche space, with broad implications for the resilience and functioning of benthic ecosystems under future ocean change.

Graphical abstract

Along a natural CO₂ gradient in the Canary Islands (pH 8.1–7.4, pHT scale; spanning present-day to end-of-century projected conditions, SSP2-4.5–SSP5-8.5), ocean acidification compresses community isotopic space through homogenization of primary producers. The two dominant sea urchin grazers respond asymmetrically: Paracentrotus lividus maintains a stable trophic niche, while Arbacia lixula shifts toward algal-based feeding, increasing niche overlap up to 27% at the most acidified sites.

Macroalgal-herbivore interactions are fundamental to the stability of coastal ecosystems and their vulnerability to climate-driven shifts will affect biodiversity and ecosystem functioning. However, the effects of climate change on species interactions may be context dependent and vary by seasonal oceanographic processes. We examined the effects of warming and acidification on an intertidal primary producer foundation species, Silvetia compressa, and its herbivore, Agathistoma eiseni, under differing upwelling regimes in early (low upwelling) and late spring (high upwelling). We measured metabolic rates including photosynthesis, respiration, and calcification after four weeks of exposure to a range of nine pH levels (7.2–8.0) at two temperature levels (16°C, 20°C). Rockweed responded more to acidification than to warming, but showed different seasonal responses, decreasing photosynthetic rates in early spring and increasing rates during late spring. Their snail consumer, however, responded most strongly to temperature, increasing both respiration and calcification rates under warm conditions in late spring. Our findings demonstrate that seasonal differences can modulate responses of macroalgae and herbivores to changing environmental conditions. Warming may increase top-down effects through increased snail metabolism, while acidification affects rockweed production to affect bottom-up forcing. By assessing physiological responses across different ecological regimes, our study advances understanding of how top-down and bottom-up processes interact in intertidal ecosystems under global change.

Highlights

• Impact of ocean acidification on coastal copepods is proportional to its intensity.
• Projected low pH significantly increases Pseudodiaptomus serricaudatus mortality.
• Lower pH reduces overall population density and alters life-stage composition.
• Predicted acidification delays larval development in P. serricaudatus.

Abstract

Anthropogenic ocean acidification, driven by increased atmospheric CO₂ absorption by seawater, poses significant threats to marine ecosystems, particularly in coastal environments, where pH reductions are expected to be more severe than in the open ocean. Copepods represent a critical link between primary producers and higher trophic levels in the classical marine food chain. However, their responses to the rapid decline in seawater pH expected by the century’s end remain poorly constrained—especially in dynamic coastal regions. The current study quantifies how near-future pH scenarios affect the survival and population dynamics of the Pseudodiaptomus serricaudatus, a perennial coastal and estuarine copepod of the West Coast of India. Adult copepods of this species were incubated at four target pH levels (8.10, 7.81, 7.51, 7.04) using an automated CO₂-bubbling system to assess mortality and population dynamics. Mortality was evaluated every three days up to day 10; population stage composition was recorded on day 15. Mortality remained 23% at pH 8.1 (representing open-ocean conditions) but rose to 30% at pH 7.81 (present-day coastal average), 47% at pH 7.51 (projected late-21st-century coastal value), and 77% at pH 7.04 (extreme 23rd-century scenario). Population data revealed declines across life stages and copepodite-to-nauplius (C_t/N) ratio at lower pH values, indicating impaired development. Any potential impact of ocean acidification on copepods could affect the fisheries sector, as these copepods are the chief food source for mackerel, an economically important fish in the region. The critical pH threshold for P. serricaudatus appeared to lie between 7.51 and 7.81, corresponding closely to current coastal conditions and near-future projections. These findings suggest that coastal copepod populations may already be experiencing sublethal stress, with potential demographic collapse occurring within decades rather than by the end of the century.

Highlights

• Hatching success of Artemia salina was impaired by acidified conditions.
• Development of Artemia salina pre-nauplii was negatively impacted by both species of harmful algae and low pH exposure.
• Only toxin-producing harmful algal species produced significant morphological abnormalities of pre-naupliar Artemia salina.
• No synergistic effects of harmful algae and low pH exposure was seen in this study.

Abstract

Harmful algal blooms (HABs) and ocean acidification are major stressors in coastal marine ecosystems, yet their combined effects on early zooplankton development remain poorly understood. This study tested whether HABs and low pH exposure exert synergistic effects on the embryonic development and hatching success of brine shrimp (Artemia salina) cysts. Cysts were exposed to controls, toxin-producing HAB species (i.e. Margalefidinium polykrikoides or Alexandrium catenella), or a non-toxin producing HAB control (i.e. Gymnodinium aureolum) and at either ambient (pH ∼ 8) or acidified (pH ∼ 7.2) conditions. We hypothesized that the combined exposure to a toxin-producing HAB at acidified pH conditions would reduce hatching success and alter embryonic morphology. Low pH exposure consistently reduced hatching success and slowed developmental progression across treatments, reducing nauplii production. HAB exposure alone did not affect hatching success, but exposure to toxin-producing HABs in ambient pH conditions did significantly increase the morphological abnormalities at the membrane stage. Overall, no synergistic effects of HABs and low pH exposure were found for any response variable. Instead, HABs and low pH exposure exerted distinct, stage-specific impacts that were additive rather than interactive. These findings suggest that while exposure to toxin-producing HABs primarily disrupts morphology and low pH exposure slows development and reduces hatching success, their combined effects do not exceed additive expectations. Future work should explore physiological mechanisms underlying these stress responses and assess whether similar patterns occur in other cyst-producing zooplankton.