Extra oxygen doesn't protect freshwater creatures from warming waters

Greg Howard
9th November, 2025

Extra oxygen doesn't protect freshwater creatures from warming waters

Temperature at which loss of motor function occurred (CTmax) under normoxia and hyperoxia for 10 fish species, revealing minimal impact of increased oxygen levels on warming tolerance in most cases.

Image adapted from: Raby et al. / CC BY (Source)

Key Findings

  • This global study examined how increased oxygen levels affect the heat tolerance of 14 aquatic species, finding limited protective effects
  • For most tested species (10 of 14), higher oxygen levels did not improve their ability to withstand warmer temperatures
  • A small increase in heat tolerance (0.2-0.3°C) was observed in only four species – two tropical reef fishes and two marine crustaceans – but the effect was minimal
Ocean warming is a major consequence of climate change, posing a significant threat to aquatic life. As temperatures rise, marine and freshwater animals face physiological stress, potentially leading to widespread mortality events, often referred to as marine heatwaves[2]. Understanding how these animals cope with increasing temperatures is crucial for predicting the future of aquatic ecosystems. A key question is whether factors beyond temperature itself – such as oxygen levels – can influence an animal’s ability to withstand heat stress. Researchers at Trent University recently investigated the role of oxygen supersaturation in protecting aquatic animals from heat stress[1]. Oxygen supersaturation occurs when water contains more dissolved oxygen than is normally present, a phenomenon common in shallow waters with abundant photosynthetic activity, like those found in coral reefs or coastal areas. Photosynthesis, the process by which plants and algae convert sunlight into energy, releases oxygen as a byproduct. The study aimed to determine if this excess oxygen could extend the thermal tolerance of various aquatic species. Thermal tolerance refers to the range of temperatures an organism can survive in. The study involved 14 different aquatic species – 10 fish and 4 crustaceans – from both marine and freshwater environments. Researchers measured what is known as the ‘critical thermal maximum’ (CTmax) for each species. CTmax is the temperature at which an animal loses all motor control and is considered to be at its limit of thermal tolerance. These measurements were taken under two different oxygen conditions: normal oxygen levels (normoxia) and high oxygen levels (hyperoxia, specifically 150% saturation). A total of 1,451 animals were used across 24 experiments and 147 CTmax trials, using two different warming rates to simulate different heatwave scenarios. The results were somewhat surprising. In the majority of species – 10 out of 14 – oxygen supersaturation had no noticeable effect on their CTmax. This means that increasing oxygen levels did not help these animals withstand higher temperatures. In the remaining four species – two tropical reef fishes and two marine crustaceans – the effects of oxygen were mixed and generally small, increasing thermal tolerance by only 0.2 to 0.3°C. These findings contrast with some earlier research suggesting that increased oxygen availability could buffer against the negative effects of warming[3]. That study demonstrated that oxygen supersaturation, resulting from photosynthesis, could extend the survival of animals at higher temperatures by fulfilling increased metabolic demands. However, the Trent University study suggests this protective effect is not widespread. The physiological basis for thermal tolerance is complex. Research has shown that heat stress impacts fish at a molecular level, affecting reaction rates, protein structure, and cell membrane function[4]. These molecular changes can lead to mitochondrial dysfunction (impairing energy production), oxygen limitation, and ultimately, organ failure. While oxygen availability is clearly important for overall metabolism, this study indicates that simply increasing oxygen levels isn’t enough to overcome the fundamental physiological challenges posed by extreme heat for most species. The implications of this research are significant for predicting the impacts of climate change on aquatic ecosystems. Current models used to assess climate risk often don’t account for oxygen supersaturation. The findings suggest that incorporating this factor into these models is unlikely to substantially improve their accuracy for most species. This doesn’t mean oxygen isn’t important; it simply suggests that its role in mitigating heat stress is limited and doesn’t offer a widespread protective mechanism. The study highlights the need for a more nuanced understanding of how different species respond to combined stressors like warming and oxygen fluctuations.

EnvironmentEcologyMarine Biology

References

Main Study

1) Oxygen supersaturation has negligible effects on warming tolerance across diverse aquatic ectotherms

Published 4th November, 2025

https://doi.org/10.1371/journal.pbio.3003413

Related Studies

2) Biological Impacts of Marine Heatwaves.

https://doi.org/10.1146/annurev-marine-032122-121437

3) Oxygen supersaturation protects coastal marine fauna from ocean warming.

https://doi.org/10.1126/sciadv.aax1814

4) Physiological Mechanisms of Acute Upper Thermal Tolerance in Fish.

https://doi.org/10.1152/physiol.00027.2022


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