Ocean Acidification

Highlights

• 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.

Background

Cold-water corals (CWCs) are key ecosystem engineers that create complex three-dimensional habitats much like tropical reefs, but in deep, cold seas. However, like other reef-building systems, they are increasingly threatened by climate change and ocean acidification. CWC communities in the Mediterranean Sea may be especially vulnerable because these waters absorb more atmospheric CO₂ than the global ocean, making it a mesocosm that mirrors broader global trends affecting marine life. Since calcification is energetically costly and likely becomes even more demanding as pH and carbonate ion availability decline, understanding how the decrease in aragonite saturation state (Ω_arag) affects biomineralization is essential for predicting the future of these corals.

Results

Here, we investigated skeletal structural and compositional changes of the scleractinian CWC Desmophyllum dianthus along an Ω_arag gradient in the Mediterranean Sea using specimens collected between 400 and 1200 m depth. Our findings indicate that skeletal porosity increases at the macro-scale with decreasing Ω_arag, while micro- and nano-scale structural and compositional features remained unaffected.

Conclusions

The persistence of micro- and nano-scale skeletal features across an 800 m depth gradient suggests that D. dianthus maintains tight biological control over mineralization at these scales, even as Ω_arag declines. This control does not extend to the macro-scale, where increasing porosity alters the skeleton’s overall architecture under lower Ω_arag. D. dianthus thus appears to preserve the fundamental “building blocks” of its skeleton while changing its larger-scale structure, a decoupling that may make macro-scale porosity an early marker of acidification stress in CWCs.

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 hpf^0.18) compared with in the control treatment (length = 24.24 hpf^0.54). 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.

Ocean acidification, driven by rising atmospheric CO₂, threatens the ability of corals to build their skeletons by reducing their capacity to maintain an elevated pH at the calcification site (pH_cf), a process essential for calcium carbonate precipitation. Boron isotopes have commonly been used to show that the response of pH_cf to ocean acidification is highly species-specific. However, the physiological mechanisms underlying this variability remain poorly understood. Recently, lithium (Li) isotopes have been used to trace the activity of ionic transport involved in cellular pH regulation and calcification (e.g. H⁺, Na⁺ and Ca²⁺), and may therefore help resolve these mechanisms. Here, we investigate multiple coral species from Tutum Bay (Papua New Guinea), a natural CO₂ seep system creating pH gradients (mean pH_T = 7.66 at seeps vs. 8.01 at control sites) analogous to future ocean acidification scenarios. Our results show a relationship between seawater pH, calcifying fluid chemistry, and lithium isotopic composition. Corals exposed to low seawater pH exhibit significantly altered δ⁷Li values relative to colonies from the control site, with some species becoming enriched in ⁷Li (up to 2‰) as pH_cf declines. This isotopic shift is consistent with reduced efficiency of Na⁺/H⁺ exchangers (NHEs), active transporters that preferentially incorporate the lighter ⁶Li isotope under optimal conditions but may become less effective under elevated proton concentrations. By linking Li isotopes to calcifying-fluid chemistry, these results provide geochemical evidence that ocean acidification may disrupt ionic regulation in corals and that Li isotopes can help to resolve biogeochemical controls of carbonate-systems.  

Highlights

• Aerosols and their major sources are seasonally variable at Visakhapatnam.
• Total suspended matter was higher during winter than during summer.
• Biomass burning is a dominant source of aerosols during winter.
• Fossil fuel and coal combustion are the major sources during summer.

Abstract

The continuous rise in anthropogenic aerosol emissions degrades ambient air quality, and their deposition onto the surface ocean alters chemical and biological characteristics. Identifying the sources of aerosols is crucial for taking appropriate measures to minimize their impacts. Stable isotope ratios of carbon (δ¹³C) and nitrogen (δ¹⁵N) are promising tools for identifying sources of carbonaceous aerosols. The objective of this study is to identify the dominant sources of carbonaceous aerosols over an urban region using stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope ratios, and to evaluate their potential influence on surface ocean acidification in the coastal Bay of Bengal. Aerosol samples were collected between March 2016 and February 2017 at a fortnightly interval, over an urban region, to examine the sources of carbonaceous aerosols and to evaluate the possible impacts on surface ocean acidification. Significantly high concentrations of total suspended particulates (TSP) during winter (112 ± 26 μg m⁻³) compared to summer (58.8 ± 8 μg m⁻³), associated with an insignificant seasonality in δ¹³C_TC (−26.9‰ to −22.9‰), indicating ageing of organic aerosol through oxidation. In contrast, higher δ¹⁵N_TN during winter (2.2‰ to 12.1‰; 5.4 ± 2.9‰) than summer (−12.9‰ to −1.8‰; −4.7 ± 3.3‰) indicate different sources. Based on source characteristics of δ¹³C_TC, δ¹⁵N_TN and the isotope mixing model, biomass burning and coal combustion are the major sources of carbonaceous aerosols during winter, whereas coal and fossil fuel burning contributed during summer. Since biomass burning contains higher concentrations of acidic aerosols, such as sulfates, and its deposition over the surface ocean results in higher level of pH levels compared to coal ashes. A higher decline in pH of the coastal waters during winter than summer was reported in the coastal Bay of Bengal. This study confirms that the deposition of higher sulphate and nitrates due to biomass burning in the Indo-Gangetic Plain (IGP) region is responsible for a greater decline in pH of the surface ocean during winter than summer. Taking appropriate measures to reduce biomass burning in the IGP region would decrease ocean acidification and allow the atmospheric CO₂ sink into the coastal Bay of Bengal to achieve net zero carbon emissions in the future.

Marine ecosystems are undergoing rapid transformation under climate change, yet the responses of many marine invertebrates remain vastly understudied. In particular, for many benthic gastropods there is a striking imbalance between their traditional appreciation by shell collectors—and, consequently, their consistent representation in Natural History Collections—and the limited attention they receive in ecological and conservation studies. Focusing on the northeastern Atlantic and the Mediterranean, the cowries Luria lurida, Naria spurca, Zonaria pyrum and the frog-shell Talisman scrobilator are emblematic examples of this knowledge gap, despite being frequently mentioned as species of conservation concern. Using long-term occurrence records spanning more than a century, we modelled past and present distributions of these species and explored their potential responses to future climate scenarios through a multi-temporal Species Distribution Modelling framework. Our results show that intermediate climatic conditions—both in time (2050–2060 vs. 2090–2100) and scenario intensity (moderate SSP2-4.5 versus high-emission SSP5-8.5)—may represent a critical transition phase, leading to habitat contractions without compensatory gains in newly emerging suitable areas. The Mediterranean Sea is expected to increasingly function as a cul-de-sac, with the dominant circulation patterns strongly limiting outward movements towards cooler regions for species relying on planktic larvae for dispersal. Furthermore, incorporating larval sensitivity to reduced pH suggests that large areas of the Atlantic Ocean may actually result unsuitable for larval persistence, substantially reducing the habitat effectively available for completion of the full life cycle; this highlights the need to account for connectivity, life-history constraints and juvenile-stage sensitivity when assessing climate-driven range shifts in shelled organisms with planktic larvae.

The northern Gulf of Mexico (nGOM) is a river‑dominated marginal sea with strong physical‑biogeochemical variability. We reconstruct sea surface partial pressure of CO₂ (pCO₂) at 4‑km, 8-day resolution from 2003 to 2024 using a satellite‑based, season‑specific random forest model (independent validation R² = 0.82, RMSE = 27.6 μatm). The climatological pCO₂ distribution exhibits a sharp coastal‑to‑offshore gradient: river‑influenced coastal waters (SSS < 33) have persistently low pCO₂ with high spatial variability, while offshore waters (SSS > 33) have higher pCO₂ with weaker heterogeneity and lower seasonal amplitude. The nGOM acts as a net CO₂ sink for atmospheric, largely concentrated in the river‑influenced plume region due to riverine nutrient‑stimulated biological uptake. Seasonal pCO₂ variation is dominantly controlled by temperature but counteracted by spring‑summer biological drawdown (reducing pCO₂) and autumn‑winter vertical mixing with CO₂‑rich deeper water (raising pCO₂). Interannual pCO₂ variability is dominantly affected by year-to-year changes in river discharge and nutrient loading, with higher discharge leading to lower pCO₂ via enhanced biological uptake. On a decadal timescale, sea surface pCO₂ increased at a rate of 0.50 ± 0.20 μatm yr^-1, much slower than atmospheric pCO₂ (2.13 ± 0.04 μatm yr^-1), leading to a strengthening oceanic CO₂ sink with the sea-to-air flux becoming more negative at −0.41 ± 0.06 mmol C m^-2 d^-1 yr^-1. Furthermore, a decreasing frequency of easterly winds has reduced the westward transport of the Mississippi River plume, causing a higher pCO₂ increasing rate on the western Texas‑Louisiana shelf.