Photo by Yannis Papanastasopoulos
Ocean oxygen levels are declining at an unprecedented rate, with new research revealing that ancient episodes of deoxygenation caused widespread collapse of deep-sea fish populations, offering a stark warning for marine ecosystems facing accelerating oxygen loss today.
A groundbreaking study published in Communications Earth & Environment demonstrates how mesopelagic fish communities disappeared during past deoxygenation events, providing crucial insights into how marine life responds to oxygen-depleted waters.
Ancient lessons from the Mediterranean
Researchers led by the Institute of Environmental Science and Technology at the Universitat Autònoma de Barcelona analysed fossil remains of lanternfish—one of the most abundant fish families in the deep ocean—preserved in Eastern Mediterranean sediments dating back more than 10,000 years.
The study revealed that lanternfish were largely absent during periods of extreme oxygen depletion and only reappeared in large numbers once oxygen levels increased again, about 6,000 years ago.
Sven Pallacks, the study’s main author, warned: “The case of lanternfish clearly illustrates what may happen on a larger scale if ocean deoxygenation continues. If a group with such massive biomass disappears, other marine species are also likely to be at risk.”
Critical role of deep-sea fish
Lanternfish, despite their small size, represent an estimated biomass of 600 million tons, possibly making them the most abundant vertebrates on Earth by weight. They play a crucial role in climate regulation and ocean food webs by connecting surface waters with the deep ocean through their daily vertical migrations.
During daylight hours, lanternfish inhabit the dark mesopelagic zone between 200-1,000 meters deep to hide from predators, while at night they swim to the ocean surface to feed on zooplankton. This behavior makes them particularly vulnerable to oxygen depletion in mid-water depths.
Accelerating global oxygen loss
The historical evidence comes at a critical time as modern oceans face rapid deoxygenation. According to the International Union for Conservation of Nature, oceans have lost around 2% of dissolved oxygen since the 1950s and are expected to lose about 3-4% by 2100 under current emission scenarios.
The oxygen loss has two primary drivers: ocean warming, which reduces water’s capacity to hold oxygen and creates stronger stratification that prevents mixing of oxygenated surface waters with deeper layers; and eutrophication from agricultural runoff and pollution, which promotes excessive algae growth that consumes oxygen as it decomposes.
Regional impacts in UK waters
The oxygen decline is not just a global phenomenon but is being observed in UK coastal waters as well. The Marine Climate Change Impacts Partnership reports that sustained observations in the North Sea reveal the recent onset of oxygen deficiency in late summer, partly due to ocean warming. Significantly, both the intensity and extent of oxygen deficiency have increased over time.
Short-term measurements indicate the Celtic Sea also shows the onset of oxygen deficiency in late summer, suggesting the problem extends across multiple UK marine regions. Models project that annual mean oxygen concentration will decline most strongly in North Sea regions and the Celtic Sea, with decreases of 5.6 to 5.9% by 2100 under high emission scenarios.
The scale of the problem has grown dramatically. In 2011, there were around 700 reported sites worldwide affected by low oxygen conditions – up from only 45 before the 1960s. The volume of completely oxygen-depleted waters has quadrupled since the 1960s.
Recent research suggests this trend is accelerating, with evidence pointing to widespread loss of ocean oxygen becoming evident across large parts of the ocean between 2030 and 2040.
Ecosystem-wide implications
The study’s findings suggest that mesopelagic ecosystems are particularly vulnerable to oxygen loss, with their collapse potentially destabilising ecological balances, impairing the ocean’s role in global carbon cycling, and threatening marine biodiversity and food security.
The research showed that during ancient deoxygenation periods, while surface-dwelling fish like European anchovy increased in abundance due to enhanced surface productivity, deep-water species virtually disappeared. This pattern highlights how oxygen loss reshapes entire marine food webs.
Recognition as planetary boundary
Scientists are increasingly calling for ocean deoxygenation to be recognised as a critical planetary boundary. An international study published in Nature Ecology and Evolution argues that the progressive loss of oxygen threatens not only ecosystems, but also the livelihoods of large sectors of society and the entire planet.
The researchers emphasise that oxygen loss affects waters from local ponds to the global ocean, requiring coordinated monitoring, research, and political action at all levels.
Conservation and management challenges
The IUCN identifies several consequences of ocean oxygen decline, including decreased biodiversity, shifts in species distributions, displacement or reduction in fishery resources and expanding algal blooms. These changes threaten the ocean’s food provisioning ecosystem services.
In UK waters specifically, researchers warn that continued warming and reduced oxygen availability will affect the metabolism, health, and reproduction of many marine organisms, which could have major impacts on ecosystems and commercial fisheries. The risk of oxygen deficiency in summer is expected to increase due to lower oxygen levels experienced during winter and spring.
Temperature increases explain approximately 50% of oxygen loss in the upper 1,000 meters of the ocean, where species richness and abundance are highest. Even slight reductions in dissolved oxygen can induce stress in marine organisms by depriving them of adequate oxygen supply at the tissue level.
Future research and monitoring needs
The Mediterranean study represents one of the few investigations using fossil evidence to understand long-term fish responses to deoxygenation. The research team emphasises the need for expanded paleontological studies to capture broader regional patterns and account for spatial variability in fish communities.
Future research priorities include stable isotope and trace element analyses of fossil records to better understand environmental changes and species responses across periods of extreme climatic variability. Such studies could provide valuable insights into marine life resilience and adaptability under future climate scenarios.
Implications for marine protection
The findings have significant implications for marine conservation strategies. Understanding how fish communities responded to past oxygen changes can inform the design of marine protected areas and fisheries management under future deoxygenation scenarios.
The study suggests that protective measures may need to account for species’ varying sensitivity to oxygen levels and their ability to adapt to changing ocean conditions. Species confined to deep waters, like lanternfish, may face greater risks than those able to exploit surface habitats.
Global monitoring imperative
As ocean oxygen continues to decline, the research underscores the urgent need for comprehensive global monitoring systems to track changes in oxygen levels and their impacts on marine ecosystems. Early detection of oxygen loss could enable adaptive management strategies to protect vulnerable species and maintain ecosystem services.
The study’s lead author concluded that the research demonstrates the “”potentially dramatic importance of very low oxygen levels for the mesopelagic fish community, overriding the effects of ocean warming and productivity changes” – a finding with profound implications for marine ecosystems facing accelerating deoxygenation in the coming decades.