Marine Vulnerability to OA and OW

Introduction
Ocean acidification (OA) and ocean warming (OW) are two interconnected stressors reshaping marine ecosystems. OA reduces theAvailability of carbonate ions necessary for calcifying organisms to build shells and skeletons, while OW alters metabolic rates, distribution, phenology, and the structure of marine communities. Together, these stressors can amplify each other’s effects, threatening biodiversity, ecosystem services, and the livelihoods tied to healthy oceans. This article surveys a broad range of marine taxa to identify which species and groups are most vulnerable to OA and OW, the mechanisms driving vulnerability, and the uncertainties that shape our understanding. By synthesizing current scientific findings, the discussion highlights both well-established patterns and areas where more research is needed to inform conservation and policy.

Table of Contents

Vulnerability of Calcifiers
Susceptibility of Fisheries-Dependent Species
Vulnerability in Coral Reef Communities
Planktonic Organisms and Primary Production
Mobile Pelagic Species and Migration
Benthos and Sediment-Dwelling Fauna
Ecosystem Engineers and Habitat Formers
Mollusks under Dual Stress
Echinoderms in Acidified Waters
Crustaceans and Shell Consumers
Behavioral and Physiological Sensitivities
Regional Hotspots and Climate Gradients
Socioeconomic Implications and Adaptive Responses
Knowledge Gaps and Research Needs

Vulnerability of Calcifiers
Calcifying organisms, such as corals, mollusks (oysters, clams, mussels), and some echinoderms, are among the most vulnerable to OA due to the direct chemical interference with calcium carbonate formation. The saturation state of aragonite and calcite declines as CO2 dissolves into seawater, making shell and skeleton production energetically more costly or even unfeasible in some conditions. OA can also erode existing shells through increased dissolution, reduce growth rates, and impair skeletal strength. In many regions, juvenile stages are particularly sensitive, potentially altering recruitment patterns and long-term population viability. In addition to direct calcification challenges, OA may interact with thermal stress to exacerbate mortality, disease susceptibility, and reproductive failure. Ocean warming compounds these risks by altering larval dispersal, settlement cues, and habitat suitability, potentially accelerating mismatches between life stages and available habitats.

Susceptibility of Fisheries-Dependent Species
A broad array of species targeted by fisheries—including mollusks, fish with calcified structures, and crustaceans—face heightened risk under OA and OW. For bivalves and gastropods, reduced shell integrity can lower survival during predation and environmental fluctuations, impacting harvest yields. Pelagic and demersal fish may experience altered growth rates, metabolism, and mismatched spawning times with prey availability. In some species, warming temperatures promote range shifts to cooler waters, leading to economic and cultural impacts for coastal communities reliant on traditional fishing grounds. A key concern is the potential for OA and OW to interact with overfishing, habitat degradation, and pollution, compounding resilience limits and elevating the risk of stock declines.

Vulnerability in Coral Reef Communities
Coral reef ecosystems epitomize vulnerability to OA and OW due to their reliance on calcium carbonate skeletons and their sensitivity to temperature anomalies. Ocean warming drives coral bleaching events by inducing stress that causes the expulsion of symbiotic algae (zooxanthellae), reducing energy budgets and increasing mortality during heatwaves. OA weakens coral skeletons and growth, reducing structural complexity that supports diverse fish and invertebrate assemblages. The combined stressors threaten reef accretion, recovery after disturbances, and the provision of critical services such as coastal protection, fisheries, and tourism. The cascading effects propagate through trophic interactions, altering predator–prey dynamics, competition, and habitat availability for dependent species.

Planktonic Organisms and Primary Production
Phytoplankton and zooplankton underpin marine food webs and biogeochemical cycles. OA can alter photosynthesis and calcification in some phytoplankton groups, with potential shifts in species composition and productivity. Calcifying plankton, like coccolithophores, ciliates with calcareous structures, and certain foraminifera, may experience reduced calcification and changes in community structure. These changes can cascade to higher trophic levels, affecting herbivores and the predators that rely on plankton-supported pathways. Conversely, some non-calcifying phytoplankton may thrive under OA and OW, potentially altering carbon cycling and ecosystem productivity. The effects are context-dependent, varying with nutrient regimes, light, and temperature, making predictions complex.

Mobile Pelagic Species and Migration
Species with high mobility, including tunas, billfishes, and pelagic sharks, may respond to OW by shifting distribution to track preferred thermal niches. While mobility offers a buffer against local OA effects, OW can still influence prey distribution, migration timing, and energetic costs of movement. Some pelagic species could experience mismatches with prey availability if primary production shifts in different regions or seasons. Additionally, OW can affect the development and performance of larvae and juveniles in species with complex life cycles, influencing recruitment success and population trajectories.

Benthos and Sediment-Dwelling Fauna
Bottom-dwelling organisms such as polychaetes, bivalves, brittlestars, and certain crustaceans experience OA directly at the sediment-water interface. Sediment chemistry and oxygen conditions modulate OA impacts; some species may tolerate lower pH better than others, while others exhibit reduced growth, altered reproduction, or increased mortality. Temperature increases can intensify metabolic demands and stress responses. Sediment-dwelling communities also influence biogeochemical processes, including nutrient cycling and carbon sequestration, meaning their decline can alter ecosystem functioning and habitat structure for other organisms.

Ecosystem Engineers and Habitat Formers
Organisms that create or modify habitats—such as corals, kelp, seagrasses, and some bivalves—are critical for maintaining biodiversity and ecosystem services. OA and OW threaten the integrity and persistence of these habitats by weakening structural components, altering growth rates, and shifting species interactions within communities that depend on the engineers. The loss or degradation of habitat formers reduces refugia, nursery areas, and feeding grounds for a multitude of species, amplifying vulnerability across the ecosystem.

Mollusks under Dual Stress
Mollusks such as oysters, clams, scallops, and mussels face direct OA-related challenges to shell formation, which can reduce survival, growth, and filtration capabilities. When combined with OW, metabolic costs rise, larval development can be stunted, and disease dynamics may shift. This combination is particularly concerning for aquaculture operations and natural populations that rely on shell integrity for protection and structural stability in reefs and beds.

Echinoderms in Acidified Waters
Echinoderms—including sea urchins, starfish, and brittle stars—rely on calcareous endoskeletal components that can be compromised by OA. OA can weaken skeletal structures and affect larval development, settlement, and juvenile survival. Some echinoderms display resilience in certain contexts, but overall there is concern for declines in key keystone species that influence community structure and predator–prey dynamics, especially in areas with pronounced acidification.

Crustaceans and Shell Consumers
Crustaceans such as crabs, lobsters, and shrimps experience OA-related challenges to exoskeletal calcification and molting processes. While some crustaceans may exhibit tolerance to OA in certain life stages, others show reduced growth, delayed molting, and higher vulnerability to predation due to thinner or weaker shells. OW can alter habitat use and prey availability, affecting energy budgets and reproductive success. The interaction of OA with common stressors like hypoxia and pollution further shapes vulnerability patterns.

Behavioral and Physiological Sensitivities
Beyond structural challenges, OA and OW influence behavior, sensory perception, and physiology in various species. Changes in chemosensory cues can affect foraging, orientation, and predator avoidance. Metabolic rate shifts, acid–base regulation challenges, and stress responses can influence growth, reproduction, and survival. These sublethal effects can have population-level consequences, especially when they alter critical life-history traits or disrupt environmental cues used for habitat selection and reproduction.

Regional Hotspots and Climate Gradients
Vulnerability is not uniform globally. Regions with naturally lower carbonate saturation, high freshwater input, or intense CO2 fluxes—such as polar regions and upwelling zones—tend to exhibit stronger OA impacts. Coral reefs in shallow, well-lit waters may experience rapid OA-driven calcification declines, while polar and subpolar ecosystems face simultaneous temperature and sea-ice changes. Upwelling regions can deliver high CO2 and low pH water, exacerbating stress on local communities. The interaction with local stressors (pollution, overfishing, habitat destruction) determines the net vulnerability and adaptive capacity of species and ecosystems.

Socioeconomic Implications and Adaptive Responses
The vulnerability of marine species to OA and OW has direct and indirect consequences for human communities. Fisheries yields, aquaculture productivity, tourism, and coastal protection depend on resilient ecosystems. Adaptive responses include assisted breeding and selective breeding programs for aquaculture species, restoration of degraded habitats, reduction of local stressors, and the development of climate-smart fisheries management. Integrated approaches that combine mitigation of CO2 emissions with adaptation and conservation planning offer the best chance to lessen negative outcomes. Public awareness, policy frameworks, and international collaboration are essential to align scientific insights with practical governance.