Like terrestrial animals, most marine animals require oxygen for aerobic respiration. Oxygen availability influences where species can live, how much energy they can obtain, and how successfully they grow and reproduce.
Oxygen enters the ocean through exchanges with the atmosphere and through photosynthesis by marine plants, algae, and microorganisms. Circulation and mixing transport oxygen into deeper waters.
Climate change disrupts these processes. Warmer water holds less dissolved oxygen at saturation, while stronger temperature-driven layering can reduce the exchange of oxygen between surface and deeper waters. In some locations, changes in currents and ventilation further influence oxygen supply.
At the same time, warmer conditions can increase biological respiration and decomposition, consuming oxygen more rapidly in certain environments.
The combined effect is ocean deoxygenation: a decline in dissolved oxygen concentrations across parts of the ocean.
4.2 The growth of oxygen-poor habitats
Oxygen minimum zones occur naturally in some ocean regions where biological oxygen consumption is high relative to the supply of oxygen from circulation and mixing. Climate change and other human pressures can expand or intensify low-oxygen conditions.
In coastal waters, excessive nutrient inputs from agriculture, sewage, and other sources can stimulate algal growth. When this organic material dies and decomposes, microorganisms consume oxygen. Under suitable conditions, this process produces hypoxia, meaning oxygen concentrations become too low for many organisms to function normally.
Severe hypoxia can cause fish and invertebrates to leave an area, reduce feeding and reproduction, or die. Some species are more tolerant of low oxygen than others, meaning that communities may become dominated by a narrower range of organisms.
These changes can alter predator-prey relationships and disrupt habitats that support fisheries. In extreme cases, oxygen depletion creates so-called dead zones, where most oxygen-dependent marine life cannot survive normally.
The term does not mean that every organism disappears. Microorganisms and certain specialized animals may remain. However, the loss of many fish and invertebrates can substantially reduce ecosystem diversity and economic value.
4.3 Consequences for large marine animals
Large, active fish often require substantial amounts of oxygen to support their metabolism. When oxygen-poor waters expand, suitable habitat may shrink or shift toward the surface and other areas with better oxygen conditions.
This can compress the vertical space available to marine organisms. Fish may be forced into narrower temperature and oxygen ranges, increasing competition and potentially making them easier for fishers to locate.
The ecological effects can extend to sharks, tuna, and other predators, although the exact response depends on species physiology, migration patterns, and local ocean conditions.
Ocean deoxygenation also affects nutrient cycling and microbial processes. Some low-oxygen environments favor chemical pathways that alter the availability of nitrogen and other nutrients, with implications for productivity and the wider marine carbon cycle.
Breitburg and colleagues highlighted the global significance of ocean oxygen loss and the combined role of climate change and nutrient pollution in their 2018 review in Science.
Addressing deoxygenation requires both climate mitigation and local pollution control. Reducing nutrient runoff, improving wastewater treatment, restoring wetlands, and monitoring oxygen levels can help limit coastal hypoxia, while global emissions reductions are needed to address warming-related oxygen loss.
5. The Deadly Trio: Interactions Between Warming, Acidification, and Deoxygenation
Ocean warming, acidification, and deoxygenation are often described as interconnected climate-related stressors. Their importance lies not only in their individual effects but also in how they interact.
A marine organism may be able to tolerate a moderate increase in temperature when food is abundant and oxygen is plentiful. The same organism may struggle when warmer water increases its metabolic demand while oxygen availability declines and the chemistry needed to build shells becomes less favorable.
These combined pressures can influence growth, reproduction, immune function, and survival. They may also affect organisms differently at different life stages. Eggs and larvae, for example, can be particularly sensitive to environmental conditions.
The interaction between stressors is not always simply additive. In some cases, one pressure intensifies another; in others, the effects may partially offset each other or vary according to local conditions. Scientists therefore study multiple stressors together rather than assuming that each can be understood independently.
The broader ecological consequences can be substantial. A reduction in shell-forming plankton may affect food availability, while lower oxygen levels change where fish can feed. At the same time, coral bleaching can remove the complex habitat that supports numerous reef species.
When these effects occur alongside overfishing, pollution, and habitat destruction, ecosystems may have fewer opportunities to recover.
The IPCC has emphasized that climate-related drivers are already changing marine communities and that risks to biodiversity and ecosystem functioning increase as warming intensifies. IPCC, 2022.
This evidence supports an integrated management approach. Protecting a species from fishing pressure is important, but conservation plans must also account for changing temperatures, water chemistry, oxygen availability, and habitat connectivity.
6. Consequences for Marine Biodiversity Beyond Coral Reefs
6.1 Mangroves and seagrass meadows
Coral reefs are among the best-known climate-sensitive marine ecosystems, but mangroves and seagrass meadows also play essential ecological roles.
Mangroves occupy tropical and subtropical coastlines, where their root systems provide shelter for juvenile fish, stabilize sediment, and reduce wave energy. Seagrass meadows support fish, invertebrates, and sea turtles while storing carbon in their biomass and underlying sediments.
Rising sea levels, stronger coastal development pressures, changes in sediment supply, and extreme weather can threaten these habitats. Some mangrove systems can migrate inland as sea levels rise, but seawalls, roads, and urban development may block this movement.
Seagrass meadows are vulnerable to warming, marine heatwaves, turbidity, and nutrient pollution. When vegetation disappears, sediments can become more easily disturbed, reducing water clarity and making recolonization more difficult.
The loss of these habitats affects biodiversity while reducing the benefits they provide to coastal communities. Their conservation should therefore be treated as a priority within climate adaptation and marine protection strategies.
6.2 Polar ecosystems and sea-ice loss
The Arctic and Antarctic oceans contain distinctive ecosystems shaped by cold temperatures, seasonal light, and sea ice.
Sea ice provides habitat for algae, supports food-web processes, and influences the behavior and distribution of animals such as seals, penguins, and polar bears. In polar marine ecosystems, changes in sea ice can alter the timing and productivity of primary producers and the availability of prey.
As sea ice declines in the Arctic, some species lose essential habitat, while others expand into newly accessible areas. These shifts can change competition, predator-prey interactions, and the distribution of commercially valuable fish.
Polar waters also face ocean acidification. Cold water can absorb relatively large quantities of carbon dioxide, and some polar regions are particularly vulnerable to changes in carbonate chemistry.
Changes affecting plankton and other small organisms can propagate upward through food webs, influencing fish, seabirds, and marine mammals. However, responses vary by region, and not every population responds in the same way.
Protecting polar biodiversity requires international cooperation, long-term observation, responsible fisheries management, and efforts to reduce greenhouse gas emissions.
6.3 Deep-sea biodiversity
The deep ocean is home to a remarkable diversity of organisms, including corals, sponges, crustaceans, worms, and animals adapted to extreme pressure and limited food availability.
Many deep-sea species grow slowly and reproduce infrequently. These life-history characteristics can make recovery from disturbance particularly slow.
Although the deep ocean is less exposed to rapid daily temperature fluctuations than surface waters, it is not isolated from climate change. Warming, oxygen loss, changing circulation, and alterations in the supply of sinking organic material can influence deep-sea habitats and food webs.
The consequences remain uncertain for many species because large areas of the deep ocean have not been adequately studied. This lack of information creates a major challenge for conservation, especially as human activities expand into deeper waters.
Precautionary management is therefore important. Deep-sea mining, destructive fishing practices, and other potentially damaging activities should be assessed carefully, with strong environmental safeguards and sufficient scientific information before sensitive habitats are disturbed.