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 (Ct/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.
Ocean acidification and ocean warming are among the most pressing threats to marine ecosystems, with significant implications for calcifying organisms such as crustose coralline algae (CCA). These habitat-forming species play critical ecological roles in reef systems, yet they are particularly vulnerable to ocean acidification and ocean warming. While laboratory studies have documented declines in coralline algal growth and calcification under elevated CO₂ and temperature, less is known about their responses in natural settings where environmental variability and potential acclimatization may modulate stress impacts. Furthermore, the interactive effects of multiple stressors across generations remain poorly understood, limiting our ability to predict ecosystem-level consequences of climate change. This thesis explores the physiological responses of coralline algae to ocean acidification, ocean warming, and marine heatwaves through complementary field and laboratory approaches. Using natural climate analogues, I investigated how CCA communities respond to chronic exposure to low pH and elevated temperatures in situ (Chapters 2 and 3). These natural systems provide unique opportunities to assess long-term acclimatization and identify potentially resilient populations. To disentangle the mechanisms underlying stress responses and examine multi-generational effects, I conducted a multigenerational, multi-stressor laboratory experiment exposing coralline algae to factorial combinations of elevated pCO₂ and temperature over successive generations (Chapter 4). Together, these approaches enabled assessment of single-driver impacts, stressor interactions, and the capacity for phenotypic plasticity and acclimatization under both fluctuating natural conditions and controlled experimental regimes. Results from natural analogues revealed species-specific variation in stress tolerance, with some CCA populations maintaining growth and calcification despite chronic exposure to conditions projected for year 2100. The laboratory experiment revealed that growth significantly declined when exposed to ocean acidification and the combined effect of ocean acidification and ocean warming over multiple generations. This research advances our understanding of how marine calcifiers respond to climate change across ecological and temporal scales. The findings have important implications for predicting ecosystem trajectories and identifying potential climate refugia in an increasingly acidic and warming ocean.
Reef-building corals are under pressure to acclimatize and/or adapt to multiple co-occurring stressors, including climate change and declines in water quality. However, their capacity to do so is largely unknown. Yet, some corals naturally persist in habitats with multi-stressor conditions which can provide crucial insight into coral adaptive capacity under future ocean conditions. To assess the role of phenotypic plasticity and local adaptation in coral acclimatory responses to novel conditions, we reciprocally transplanted two coral species (Siderastrea siderea, branching Porites) between an inland bay characterized by multiple, co-varying stressors, including warmer seawater temperatures, higher pollution and more variable temperatures, pH and dissolved oxygen concentrations, and an environmentally less extreme, more stable fringing reef. We measured key phenotypic traits (e.g., calcification, tissue biomass, metabolic rates, chlorophyll concentrations) of all transplant groups after 0, 4 and 12 months. Reef-origin corals demonstrated high plasticity when transplanted to the bay, as both species maintained high survival and calcification, and matched bay natives’ other phenotypic traits. Yet, reef-to-bay transplants of both species had reduced photosynthesis-to-respiration ratios in the bay, highlighting the risk of metabolic trade-offs unless compensated for through increased heterotrophy. Conversely, bay-origin corals had enhanced photosynthetic performance but lower calcification on the reef. Reaction norms provided stronger evidence for environmental specialization in bay-origin than reef-origin corals, which could limit their use as stress-tolerant source corals for reef restoration. Overall, our findings suggest multi-stressor variability may promote specialized genotypes, rather than increased phenotypic plasticity, though this may also depend on predictability of environmental variability.
Anthropogenic CO2 emissions are expected to increase ocean acidity and temperature over the coming century. Under the Shared Socioeconomic Pathway (SSP) 5–8.5 scenario, mean surface ocean pH is projected to decrease by approximately 0.39 units, while sea surface temperature is expected to rise by 2–4 °C between 2010 and 2100. Sea pens (Octocorallia) are important bioindicators of soft-sediment habitat quality and associated benthic communities; however, their responses to these stressors remain considerably less studied than those of their hexacoral counterparts.
This thesis investigates the individual and combined effects of ocean acidification and warming, based on SSP5–8.5 projections, on the sea pen Pennatula phosphorea (Linnaeus, 1758), collected from the Swedish Gullmarsfjord. During winter and summer 2025, four pH treatments (8.0, control; 7.8, present-day low value; 7.4, projected low value for 2100; and 7.0, extreme scenario) and four temperature treatments (7.7 °C, winter control; 14 °C, summer control; 17 °C, present-day high value; and 20 °C, projected high value for 2100) were assessed. Response variables included behavioural traits (colony burrowing, colony inflation, polyp opening and bioluminescence emission), survival, and bioluminescence substrate levels (coelenterazine concentration and maximum light intensity, Lmax). Three hypotheses were tested: (1) ocean acidification and warming would negatively affect all response variables, with stronger effects under combined exposure; (2) prolonged exposure would result in either progressive deterioration or acclimation of behavioural traits; and (3) larger individuals would be less affected by stressors than smaller individuals.
The effects of acidification and warming varied among response variables, seasons, and whether stressors were applied individually or in combination. Overall, pH 7.8 produced few significant effects, except on burrowing behaviour, whereas pH 7.4 and 7.0 generated effects ranging from negligible (p > 0.1) to highly significant (p < 0.01), generally with greater impacts at pH 7.0. Similarly, temperatures of 17 and 20 °C elicited responses ranging from negligible to highly significant, with the strongest effects observed at 20 °C for quantitative measures of bioluminescence substrates. Combined exposure generally amplified the observed effects, suggesting that tolerance to one stressor may be compromised by the presence of another. No consistent evidence of behavioural acclimation or progressive deterioration was detected over time. Contrary to the initial hypothesis, larger individuals appeared more susceptible to environmental stressors than smaller individuals.
Climate change is altering marine ecosystems through concurrent changes in temperature, carbonate chemistry, and dissolved oxygen. These changes are particularly important in coastal upwelling systems, where organisms already experience naturally variable environmental conditions. This dissertation integrates synthesis, experimentation, and modeling to evaluate how ocean change influences marine invertebrates and the fisheries they support. First, I conducted a meta analysis examining the effects of ocean acidification and deoxygenation on marine invertebrates. Both stressors produced broadly negative effects on fitness-related traits, including survival, growth, development, and reproduction. Although vulnerability varied among taxa, responses were not strongly structured across broad taxonomic groups, suggesting that species-specific traits and environmental history are important determinants of sensitivity. Second, I investigated the effects of multi-stressor upwelling conditions on juvenile Dungeness crab (Metacarcinus magister). Crabs maintained net calcification across a range of moderate conditions but exhibited significant declines under the most severe treatments, indicating threshold responses to environmental stress. Short-term environmental variability had little effect relative to mean conditions, suggesting that exposure severity is a stronger driver of performance than exposure pattern. Finally, I incorporated experimentally observed reductions in calcification into a size-structured yield-per recruit model to evaluate potential fishery consequences. Reduced growth delayed attainment of legal harvest size and decreased projected fishery yield, demonstrating how sublethal physiological responses can scale to population and management-relevant outcomes. Together, these chapters show that ocean change can affect biological systems across levels of organization, from individual performance to fishery productivity. By linking broad patterns of vulnerability to species-specific responses and applied fishery outcomes, this dissertation provides a framework for understanding and managing the impacts of global change in coastal marine ecosystems.
Scleractinian (stony) corals can build highly ordered aragonite (CaCO3) exoskeletons, which are vital for marine ecosystems, coastal stability, and of cultural and economic importance through supporting fisheries and tourism. However, they face multiple challenges through global climate change, unsustainable human activity, and coral-specific diseases. In the latter category, Stony Coral Tissue Loss Disease (SCTLD) has recently emerged as one of the most destructive coral diseases, spreading rapidly and causing widespread tissue mortality across many marine species [1]. We have recently started exploring remnant effects of this disease on Montastraea cavernosa exoskeletons, abundant throughout the Caribbean Sea, Gulf of Mexico, and Atlantic Ocean. Indeed, through use of multi-scale electron diffraction characterization techniques, we find that their skeletogenisis is impacted by this disease from the micro- through atomic scale [2]. Notably, at the nanoscale we observe pockets of more soluble amorphous calcium carbonate (ACC) within centers of calcification (CoCs; i.e. the center of the three-dimensional fans containing arranged elongated aragonite crystals) for healthy corals, which appear absent in STCLD-afflicted corals. At the atomic level, we reveal planar defects in diseased coral, which are much lower in density in healthy corals, presumably inflicted through dysregulation processes after tissue death.
Yet, the most severe threat to global coral reefs and their exoskeletons is climate change. As oceans increase the uptake of anthropogenic CO2 primarily from burning fossil fuels, ocean acidity has increased. The reduction in pH because of this Ocean Acidification (OA) not only reduces the rate of net ecosystem calcification, but also increases net dissolution of skeletons. At current trends, most coral exoskeletons are expected to dissolve starting in 2050 [3]. Currently, it is unclear how exactly (the onset of) dissolution proceeds and affects their aragonitic framework. Given our expertise in characterization of coral skeletons, herein we discuss a developed in-situ platform to investigate OA effects at the nanoscale.
As proof of concept, we sandwiched crushed geological aragonite nanoparticles between own fabricated SiNx-based chips, compatible with a Protochips Atmosphere gas cell holder [4]. Thereafter, we introduced water vapor at 14 Torr at room temperature (∼60% relative humidity) for 10 minutes to create a hydrated environment for the particles (Fig. 1b). In a third step, we introduced gaseous CO2 at a pressure of 1 atm (Fig. 1c). The formed unstable carbonic acid produces HCO3– and H+, which increases acidity (i.e. lowers pH). Indeed, we observe rapid dissolution of aragonite particles after CO2. While we expect roughly a pH ∼4 in this system, this experimental observation matches theoretical expectations that aragonite would dissolve under these conditions (Fig 1c-f). We further observe nucleation and growth of new particles in a dendritic fashion in the vicinity of the dissolved particles (Fig 1c-f). Likely, this is crystallization of calcite, the most stable crystalline polymorph of CaCO3, induced by local dissolution of aragonite [5]. Although pCO2 in the oceans is expected to be much lower (∼400 – 500 µatm) and thus dissolution timescales are expected to shift, this illustrates our platform can capture aragonite dissolution.
To expand our platform methodology, using conventional Ga+ FIB-methods, we prepared a lamella of geological aragonite with a thinned region (∼100 nm), which was then transferred on top of a SiNx-based chip, and attached this in one of the corners between the Si support and the SiNx layer (Fig. 2a). We were able to sandwich the lamella between both chips when observing the cell in the TEM (Fig 2b,c). Next, we will target healthy and STCLD-afflicted coral sections to investigate how nanoscale dissolution proceeds at/near more soluble areas including defects due to the devastating OA process. This understanding may allow for more accurate forecasting of marine ecosystem collapse, enabling targeted mitigation strategies, protecting food supplies, and predicting climate feedback loops [6].
Fig. 1. In-situ OA platform experiment showing TEM snapshots of: a) Initial geological aragonite nanoparticles. Inset: selected area diffraction pattern indicating aragonite spots. b) Introduction of water vapor (14 Torr) after 10 min. The white arrow indicates an apparent hydration layer surrounding the large particle. c) Introduction of CO2 at 1 atm pressure after 10 s. d) 60s e) 90s and f) 150s. White arrows in c-f) mark the outer layer of the large particle dissolving in time, red arrows illustrate growth of new crystals.
Fig. 2. a) SEM image of geological aragonite lamella attached to SiNx chip. b) TEM image of lamella after cell assembly (top and bottom chip). c) In-situ selected area diffraction pattern of the lamella.
Foraminifera inhabit coastal and deep-sea marine environments. These microscopic, single-celled organisms are widely used as proxies for relative age determination and reconstructing past climates. The ratio of calcium isotopes (44Ca/40Ca) in their calcium carbonate shells depends on the amount of carbon dioxide (CO2) and carbonate saturation in seawater at the time of the shells’ formation. On page 527 of this issue, Chen et al. (1) report geochemical evidence that a rise in atmospheric CO2 concentration and ocean acidification at the end of the Aptian Stage [~113 million years ago (Ma)] may have caused the largest extinction of planktic foraminifera of the Cretaceous Period (146 to 66 Ma) (2–4), which is second only to the extinction event at the end of the Cretaceous Period (66 Ma) (5–7). The findings could be important for understanding potential effects of present-day increases in atmospheric CO2 levels on marine organisms.
Paraticinella rohri collected from the South Atlantic is a species of planktic foraminifera that went extinct at the Aptian–Albian boundary. IMAGE: HUBER AND LECKIE (3)
Seafloor-dwelling benthic foraminifera appeared more than 500 million years ago. It took at least 350 million years for them to evolve into planktic species that float in the upper ocean (8–10). The earliest species of planktic foraminifera lacked morphological diversity and were minor constituents of marine sediments. The first major morphological diversification of planktic foraminifera occurred (2, 11) during the Aptian Stage (125 to 113 Ma) of the mid-Cretaceous Period. New species of planktic foraminifera arose, with elongated chambers (Leupoldina) and larger, more heavily calcified shells (Globigerinelloides and Hedbergella), and the first species with a peripheral keel (Pseudoplanomalina cheniourensis) appeared by the late Aptian. Large increases in atmospheric CO2 concentration caused by massive volcanic eruptions triggered substantial depletion of oxygen in the oceans, including Oceanic Anoxic Event 1a and other similar events. Atmospheric CO2 likely drove warming of the atmosphere and water; warmer waters hold less dissolved oxygen compared to cooler water.
About three quarters of planktic foraminifera species disappeared in the Aptian-Albian extinction event. What role might ocean acidification (OA) have played in this die-out? Chen et al. measured calcium isotope ratios in foraminifera, using them as a proxy for biocalcification and carbonate saturation (see the Perspective by Leckie). Their data reveal a dramatic reduction in calcification rates accompanied by decreases in the size, abundance, and diversity of planktic foraminifera. These results are consistent with the hypothesis that ocean acidification drove the extinctions of these organisms at the Aptian-Albian boundary. —Jesse Smith
Abstract
The second-largest extinction event in the evolutionary history of planktic foraminifera occurred at the Aptian–Albian boundary. This extinction may reflect ocean acidification (OA) associated with Oceanic Anoxic Event 1b. As calcium isotope ratios (δ44/40Ca) can track how biocalcification rates respond to OA, we measured δ44/40Ca records for planktic and benthic foraminifera, bulk carbonates, and authigenic calcite across the Aptian–Albian boundary in the South Atlantic. Benthic and bulk δ44/40Ca data display a distinct sequence of negative and positive excursions, similar to δ44/40Ca variations across other OA events. Planktic δ44/40Ca values increase markedly, tracking a reduction in calcification rates coincident with decreases in the size, diversity, and shell thickness of planktic foraminifera. These results suggest that OA drove extinctions of planktic foraminifera at the Aptian–Albian boundary.
Low salinity linked to warming may intensify coastal pH reduction.
Low salinity reduced righting and emergence in the limpet Patelloida pygmaea.
Low salinity reduces Mg/Ca ratios in newly formed shell layers.
Low pH increases shell thickness and Mg/Ca ratios despite dissolution.
CHS2 upregulation suggests compensatory resistance to low pH.
Abstract
Rising anthropogenic carbon dioxide emissions have driven ongoing ocean warming and associated climate changes. In the Yellow Sea, this warming is associated with enhanced monsoonal rainfall, which increases freshwater inputs and lowers coastal salinity. Increased freshwater input can weaken seawater buffering capacity, thereby leading to lower pH conditions in coastal environments. Here, we examined the effects of low pH and low salinity on the intertidal limpet Patelloida pygmaea. Adult limpets were exposed for 31 days to four experimental artificial seawater conditions combining two pH levels (8.0 and 7.5) and two salinity levels (30 and 21 psu). Survival and condition factor were not influenced by pH or salinity. However, low salinity reduced righting and emergence behavior. In addition, the Mg/Ca ratio in the M + 1 layer was lower at 21 psu than at 30 psu. Low pH resulted in a thicker M + 2 layer with higher Mg/Ca ratios despite shell dissolution, potentially helping to maintain shell integrity. While there was no change in heat shock protein (HSP70) expression, these shell modifications were accompanied by an upregulation of chitin synthase (CHS2) genes under low pH. These findings suggest that P. pygmaea is negatively influenced by low pH and low salinity, but also demonstrate compensatory mechanisms that enhance resistance to low pH.
Marine fish survival is threatened by ocean acidification, but the hormonal mechanisms for pH compensation are not well understood, limiting mechanistic understanding of stress responses in marine fish. We examined isotocin signaling in marine medaka (Oryzias melastigma) exposed to year-2100 ocean acidification conditions (Pco2 ∼0.14 kPa, pH 7.6). Our analysis demonstrated that isotocin receptor b (ITRb) was selectively upregulated at 6 h postexposure in adult gills, though it showed only a nonsignificant trend at 5 days postfertilization (dpf) embryos, whereas adenylyl cyclase 5 (ADCY5) showed hypercapnia responsiveness primarily at hatching. Using immunofluorescence and confocal microscopy, we found that both ITRb and ADCY5 proteins localize to the basolateral membrane of Na+-K+-ATPase-positive ionocytes, partially separated from apical H+-secretion machinery. Knockdown experiments showed that ITRb-ADCY5 coupling is crucial for pH compensation, with individual knockdown moderately reducing H+ secretion and combined knockdown causing severe impairment (>70% reduction) and decreasing transcription of acid-secretion genes (nhe3, ca2, and rhcgb) by 44%–60%. Paradoxically, double knockdown triggered a twofold cAMP increase that failed to restore function, whereas wild-type embryos maintained stable cAMP levels across pH conditions, consistent with the hypothesis that ITRb-ADCY5 coupling may organize cAMP production within specific basolateral microdomains, though direct subcellular imaging would be required to validate this compartmentalization model. The developmental asynchrony between ITRb (5 dpf) and ADCY5 (hatching) responses indicates life-stage-specific vulnerabilities. Our findings reveal that basolateral ITRb-ADCY5 coupling represents a critical control point for pH compensation capacity.
NEW & NOTEWORTHY Isotocin signaling, through ITRb-ADCY5 coupling, enables marine fish to compensate for ocean acidification via compartmentalized rather than global cAMP production. Paradoxically, disrupting this pathway doubles cAMP levels yet abolishes H+ secretion, revealing that signal localization determines function. The developmental asynchrony between ITRb and ADCY5 responses suggests that pH-stress vulnerabilities are specific to each life stage and require further investigation.
High carbonate alkalinity can impose composite hydrochemical stress on bivalves by altering inorganic carbon speciation, buffering capacity and ion-exchange conditions. The Hong Kong oyster (Crassostrea hongkongensis) naturally inhabits estuarine areas where low salinity can occur together with carbonate-system disturbance, yet its physiological response to high carbonate alkalinity under a low-salinity background remains unclear. In this study, oysters were exposed at salinity 5 to a Control treatment, A10 (carbonate alkalinity 10 mmol L⁻¹) and A20 (carbonate alkalinity 20 mmol L⁻¹). Salinity was treated as a controlled background condition, whereas the alkalinity treatments represented a composite carbonate-alkalinity perturbation involving alkalinity, pH and carbonate chemistry. Survival, feeding rate, oxygen consumption, ammonia excretion and hemolymph ammonia were measured across Control, A10 and A20. Hemolymph catalase (CAT), glutathione peroxidase (GSH-Px), glucose and ATP were analyzed as A10 and A20 time-course endpoints because a parallel Control was not included for these assays. A 24-h mantle transcriptome was used to screen candidate transcripts associated with innate defense and glutathione metabolism. Oysters under A10 retained partial compensatory capacity, whereas A20 significantly reduced survival. A10 and A20 suppressed early oxygen consumption and ammonia excretion, and altered feeding allocation among microalgae. Within the A10 and A20 biochemical time-course dataset, CAT, GSH-Px, glucose and ATP showed stage-specific variation, suggesting redox and energy-metabolism adjustment under alkalinity stress. Overall, high carbonate alkalinity reduced survival and was associated with metabolic depression, feeding compensation and hemolymph redox-energy remodeling in C. hongkongensis. This study will provide a theoretical basis for water quality regulation in saline-alkaline water aquaculture and the sustainable development of aquaculture.
Climate change models predict future ocean-wide decreases in pH and increases in temperature, posing a serious threat to calcifying marine invertebrates. In Hawaiʻi, local seawater temperature and pH are predicted to change even more rapidly. The Hawaiian oyster Dendostrea sandvichensis, an endemic species, remains understudied despite its ecological roles in reef- building, biofiltration, and as a food source. While previous studies have revealed the alarming impacts of ocean acidification and warming on bivalves, little is known about how the planktonic larvae of D. sandvichensis will respond to projected climate conditions.
To investigate these effects, the larvae of D. Sandvichensis were reared under present-day conditions in Pearl Harbor, HI (pH 8.1, 26.5 °C) and projected future conditions (pH 7.7, 30.0°C) for one week. Shell growth, density, and degradation were then measured using micro-CT, confocal, and scanning electron microscopy. Additionally, two larval husbandry methods were evaluated, with a static-flow system paired with UV water treatment yielding approximately 11- fold higher survival than an open-flow system. Elevated temperature reduced larval survival (~67%) compared to ambient conditions. Reduced pH significantly decreased shell length (~11.8μm), while both stressors reduced shell density in Experiment 3. Shell density decreased by ~12% under elevated temperature, ~18% under reduced pH, and ~43% under the combined stressors, and was accompanied by increased shell degradation and abnormalities.
These findings indicate that elevated temperature and reduced pH impact larval oysters through multiple pathways, including reduced survival and compromised shell integrity. This study provides new insights into the vulnerability of an endemic Hawaiian species and essential data for predicting its resilience under future climate change scenarios.
Absorption of CO2 by global oceans is decreasing pH resulting in ocean acidification (OA). Impacts on shellfish have been documented in ecologically and commercially important species. We examined the influence of diet and OA between two populations of red abalone (Haliotis rufescens) a species of aquaculture importance and declining wild populations. Populations experience different exposure histories: strong upwelling (Van Damme, California [VD]) historically exposed to low-pH conditions and weak-intermittent upwelling (Santa Barbara, California [SB]). Abalone were cultured under control-pH or OA-conditions and fed crustose coralline algae (CCA) or diatoms used in aquaculture. We tested treatment effects of population, settlement diet, and OA-exposure on survival as influenced by larval-energy stores. Survival in both populations was enhanced by CCA when cultured under both treatment conditions; however, by later stages, this effect remained only for SB. SB had reduced post-settlement survival when cultured under OA-conditions, whereas post-settlement survival of VD was not. Diet affected the relationship between larval-energy and post-settlement survival; a positive relationship when fed diatoms and a negative relationship with CCA. The relationship between larval energy and post-settlement survival was stronger in VD. CCA enhanced juvenile growth in SB cultured abalone at both three-months and one-year post-settlement. Settlement diets can reduce the impacts of OA on early-life stages of abalone, but population differences driven by underlying energetics affect the consistency of this outcome. These findings illuminate the impacts from OA, suggesting populations may be at risk, and inform strategies for developing and sustaining shellfish aquaculture in the face of changing ocean conditions.
This study evaluated how the combination of ocean warming, acidification, and deoxygenation (“deadly trio”) affects the early development of the fish Sparus aurata. Embryos and recently hatched larvae were exposed to increased temperature (Δ + 4 °C; 22 °C), elevated CO2 levels (pCO2 ~1000 μatm, Δ − 0.4 units; pH 7.7), and reduced oxygen (Δ − 60% O2 saturation; 3 mg O2 L −1). Deoxygenation emerged as the primary stressor, significantly reducing hatching rates, larval survival, and heart rates. These effects were further intensified when combined with warming and acidification. Acidification alone also reduced larval phototactic behavior by 50%, while exposure to all three stressors eliminated phototactic responses entirely. Overall, the results demonstrate that multiple climate-related stressors together severely harm fish early life stages, emphasizing the need to study combined environmental changes to better predict future impacts of climate change on marine fish populations and ecosystem functioning.
Abstract
The interaction between increased dissolved carbon dioxide, rising temperatures, and oxygen loss—the so-called “deadly trio”—is expected to strongly affect marine biota over the coming years, undermining ocean services and uses. Nonetheless, no study has so far scrutinized the cumulative impact of these three stressors on fish embryos and larvae. To fill this knowledge gap, we conducted a fully multi-factorial experiment to investigate the effects of warming (+4 °C: 22 °C), acidification (Δ − 0.4 pH units: 7.7 pH, pCO2 ~1000 μatm), and deoxygenation (Δ − 60% O2 saturation: 3 mg O2 L−1) on physiological and behavioral responses of the commercially important species Sparus aurata. Deoxygenation was the primary factor reducing hatching rates (64.25%), survival (46.71%), and heart rates (31.99%) of recently hatched larvae, being generally further exacerbated when combined with warming and acidification. No larvae exposed to the interaction of the three treatments reacted to the phototactic behavior test. However, acidification alone caused a 50% reduction in phototactic behavior. Our findings demonstrate that the deadly trio is detrimental to early fish development, impacting several key features at this critical life stage, and the need to assess the impacts of stressors’ interaction on marine taxa to better predict future ecosystem responses to ocean changes.
This study assessed the potential synergistic effects of harmful algal blooms (HABs) and ocean acidification (OA) on the survival and grazing potential on HABs by brine shrimp (Artemia salina). The effects on the fitness of three life stages (i.e. newly hatched, 1-week old, and 2-week old post hatch) of A. salina was evaluated over a 24-hour period. Using well plates, A. salina were individually exposed to toxin producing HABs (i.e. Alexandrium catenella or Margalefidinium polykrikoides), a non-toxin producing HAB (i.e. Gymnodinium aureolum), and fed or unfed non-HAB controls. pH conditions administered were ambient (pH ~ 8) or acidified (pH ~ 7.2). Survival analysis revealed a significant effect of treatment on mortality across life stages. Hazard ratios showed elevated mortality in HAB treatments relative to controls, while OA alone did not. HAB exposure or ontogeny are more important for survival of A. salina than short-term OA exposure. Regarding grazing potential, significant reductions in HAB cell density were more commonly found in treatments with older individuals than younger conspecifics. Cell density of A. catenella was never significantly reduced as compared with other HAB treatments. Overall, exposure to OA conditions did not affect the survival or grazing potential of A. salina, but the species of HAB did.
Oyster aquaculture is expanding in Rhode Island, yet key farming regions in the lower West Passage of Narragansett Bay (WPNB) lack the in-situ, high-temporal resolution monitoring needed to evaluate emerging stressors and support production. At the same time, the industry is undergoing rapid technological development aimed at improving production while reducing labor and overall costs. In recent years, a low maintenance, alternative surface gear was introduced in WPNB; however, its impact on oyster performance relative to traditional cultivation methods has not been quantified. This study established a 1.5-year continuous water quality time series in WPNB and paired these observations with physiological assessments of Crassostrea virginica grown in three cultivation methods (i.e., traditional surface, alternative surface, and bottom gear). This work aimed to better understand the impact of environmental variability and gear type on oyster health, growth, and survival. Multiparameter sondes and discrete bottle samples were used to monitor water quality and calculate carbonate saturation state (Ω) at a 4-acre oyster farm. Nine mesh bags containing early-life stage C. virginica (16 ± 0.4 mm; n = 300 per bag) were deployed in July 2025, with three replicate bags in each gear type. Subsamples (n = 15) from each gear type were collected over a 6-month period for morphometric measurements and scope for growth (SFG) determinations. Minimal differences in carbonate chemistry were observed among sampling sites and Ω remained supersaturated (Ω >1) for the majority of the study period, indicating that ocean acidification was not a major stressor for farmed oysters. Survival was highest (82 ± 4.5%) in surface gear types compared to bottom gear (27 ± 0.58%), while physical growth (shell and tissue) and SFG were largely similar among all gear types. These results suggest that the alternative surface gear may support comparable production and product quality to traditional gear types, while also reducing maintenance and labor demands. Overall, this study provides important physiological and biological context for oyster aquaculture while informing gear selection and farm management strategies.
Unveils Climate-Driven Disease Mechanisms Across Aquatic Systems.
Integrates Multistressor Impacts Including Pollution, Eutrophication, and Salinity Fluctuations.
Explores Shifts in Species Distribution, Reproduction, and Food Web Dynamics.
Highlights Adaptive Traits and Resilience Mechanisms in Aquatic Organisms.
Provides Science-Based Recommendations for Climate-Responsive Management.
Abstract
Climate change is rapidly transforming aquatic ecosystems, posing complex environmental challenges with far-reaching ecological and socio-economic implications. Rising temperatures, sea-level rise, altered precipitation patterns, shifting hydrological regimes, and sea-ice loss are intensifying pressures on coastal, estuarine, freshwater, and polar systems. These stressors contribute to habitat degradation, increased frequency of hypoxic events, and altered species distributions. Crucially, while some dual stressors, such as warming and acidification, can paradoxically increase primary producer biomass, our findings reveal that this resultant biomass often accumulates as detritus rather than being efficiently transferred to higher trophic levels. This observation directly challenges the simplistic assumption that “more growth” is invariably beneficial, highlighting complex indirect effects on food web dynamics and ecosystem function. Ecological perturbations propagate through trophic networks, resulting in biodiversity loss, reduced ecosystem resilience, and declining fisheries productivity, thereby threatening food security and coastal livelihoods. Marine and freshwater organisms are increasingly exposed to multiple, interacting stressors, including warming, acidification, salinity fluctuations (requiring distinct osmoregulatory strategies, e.g., heterosmotic regulation in teleosts vs. isosmotic intracellular regulation in crustaceans), pollution, and overexploitation. These cumulative pressures can exacerbate disease outbreaks, modify host–pathogen dynamics, and facilitate the emergence and spread of aquatic pathogens, with consequences for ecosystem stability and human health. In aquaculture systems, climate-driven stress often acts synergistically with anthropogenic disturbances, amplifying production risks and economic vulnerability. Furthermore, anthropogenic infrastructure like reservoirs can act as unintended hubs facilitating species dispersal following extreme events like floods, altering community structures. At the biogeochemical scale, climate-induced alterations in nutrient cycling, primary productivity, and carbon sequestration are reshaping ecosystem functioning, particularly in high-latitude and freshwater environments where adaptive capacity is comparatively constrained. Changes in food web architecture and energy transfer efficiency further compromise ecosystem services. This review specifically centers on the biological and ecological mechanisms underlying these climate-driven changes, including organism-level stress responses, shifts in species interactions, and alterations in pathogen dynamics. Recognizing the societal implications and public discourse surrounding climate change underscores the urgency of examining its tangible impacts on sensitive environments, such as estuarine ecosystems, which serve as critical interfaces between terrestrial and marine realms and are thus highly susceptible to both climatic shifts and human influence.
Acidification in coastal habitats is increasing in duration and amplitude under the continued influence of ocean acidification and contributing coastal processes. The impacts of low pH conditions on calcifying organisms, especially echinoderms, is well established, with the early developmental stages being especially vulnerable. This is the first study to assess the impact of locally relevant coastal acidification scenarios on the early development of the Cape urchin Parechinus angulosus. Our findings suggest that the early larval stages of this species are unlikely to survive when exposed to low pH conditions, specifically during the onset of skeletogenesis. In our laboratory experiments, larvae that were exposed to the low pH treatment (pH 7.32) showed significantly reduced growth (GLMM, Time × Treatment interaction: β = −0.361 ± 0.019, z = −19.06, p < 0.001) and developmental regression compared with those from the control treatment (pH 7.95). Substantially slower growth rates were observed in the low pH treatment (length = 72.3 hpf0.18) compared with in the control treatment (length = 24.24 hpf0.54). There was also evidence of abnormal and delayed development and potential dissolution of skeletal structures under the low pH condition. However, fertilisation success and larval survival did not differ significantly between the experimental treatments, suggesting that developmental impacts of low pH over short durations, even though substantial, may be sublethal. The developmental impacts are likely to impair the transition of larvae to the adult stages, which may ultimately affect populations of this ecologically important species under future coastal acidification scenarios.
In the face of a rapidly changing climate, assessing organismal responses to future stressors in the context of current, natural exposure to stress could provide key insights to understanding marine ecosystem resilience. I used Balanophyllia elegans, a cold-water, solitary, azooxanthellate coral as a model to better understand how varying oceanographic conditions across its geographic range have shaped its ability to tolerate and potentially adapt to current and future ocean acidification conditions. I collected B. elegans individuals from four sites across 2,500km of their range and subjected them to two pH treatments to investigate site-specific protein expression in response to low pH. Using proteomic analysis, I found that corals from each site responded differentially to low pH, mainly through changes in regulation of metabolism, calcification, and homeostasis-related proteins. Additionally, health condition varied significantly between sites after exposure to low pH, providing further evidence of site-specific responses. These results demonstrate site-specific variation in responses and tolerance to low pH, a pattern that could inform future investigations into environmental-driven adaptive expression. Such site-specific responses highlight the importance of multi-source studies for predicting a species’ ability to navigate future climate changes.
This paper studies the consequences of ocean acidification (OA) on the growth and survival of the larvae of commercially significant shellfish species such as oysters, mussels, and scallops. The authors of the study are particularly concerned with the negative processes of OA with respect to the growth of larvae, shell development, and behavior, which result in decreased survival rates, particularly the consequences of the reduced availability of calcium carbonate on the weakening of shells and larvae, which are preyed upon. The paper also studies the interference with the behavior of larvae, particularly with respect to the adequate detection of sites to settle, which is harmful to recruitment success. Additionally, the study looks at the OA-induced metabolic stress, where the larvae are expected to expend higher energy to maintain homeostasis at the expense of growth and immunity. By focusing on this issue, the paper outlines the OA’s impacts on the shellfish populations and industries. The paper also looks at the available soft measures, such as the implementation of buffering solutions to limit the acidification in hatcheries, the use of genetic selection to incorporate acidification-resistant traits, and coastal management measures to limit local sources of acidification. The paper also suggests some potential new ways to increase the resilience of shellfish stock, including more flexible adaptive aquaculture practices. With commercial shellfish interests emerging, this paper fills some of the more critical gaps in the existing literature and offers insight into the impact of OA on the sustainability of the shellfish industry. It also provides OA mitigation strategies to preserve shellfish stocks in a changing climate.
