Many of us know how climate change is causing an increase in ocean acidification, warming sea water temperatures and coral bleaching, but did you also know it causes an increase in the number and severity of ‘hypoxic’ or low oxygen events? Understanding how this decrease in oxygen (and its reciprocal increase in carbon dioxide) impacts species dependent on oxygen is important if we are to effectively predict and manage the impacts of future climate change on marine life.

University of Exeter PhD student Dan Montgomery tells us about his new paper, working to understand the tolerance of European Sea Bass to hypoxic events.

Words by Dan Montgomery, PhD Student, University of Exeter

Key message: During periods of low oxygen in the oceans fish are also faced with high CO2 levels. Previous research investigating responses to hypoxia by fish hasn’t considered this change in CO2. We found that including realistic changes in CO2 during hypoxia tolerance tests increase hypoxia tolerance of European seabass by 20 %. This has important implications for assessing impacts of hypoxia on fish species and predicting potential effects of climate change.

Oxygen is key to most animals found on earth and a lack of oxygen has large consequences, potentially including death. For animals that live on land or in the air low amounts of oxygen (otherwise known as hypoxia) are relatively rare, however for animals that live in water (like fish) hypoxia is much more common 1. In order to determine the impacts of low oxygen on these animals we need to know how tolerant they are to these low oxygen conditions. Scientists have been conducting research to discover the tolerance of fish species to hypoxia for over 50 years but crucially these experiments are carried out in laboratories and aquariums where oxygen is reduced in water by bubbling them with nitrogen (or a mix of nitrogen and air). Whilst this reduces the oxygen levels in the water it does not account for changes in another key gas, carbon dioxide!

European Sea bass in Exeter University’s aquarium

Low oxygen levels in the world’s oceans are usually caused by respiration of bacteria. As a by-product of this respiration carbon dioxide is produced. This means that whenever oxygen levels are reduced carbon dioxide levels increase. The reciprocal relationship between carbon dioxide and oxygen is well known and has been recorded many times in oceanographic surveys 2,3. Our research, using European sea bass, aimed to understand if this increase in CO2 during a hypoxic event changed the hypoxia tolerance of fish when compared to normal experimental techniques which induce hypoxia without changing CO2.

Working in the lab to measure blood chemistry of sea bass

We found that sea bass which experienced environmentally realistic increases in CO2 during a hypoxia challenge were 20 % more tolerant to hypoxia than fish exposed to a hypoxia challenge with no CO2 change. We believe this increase in tolerance is related to changes in the chemistry of the sea bass’s blood which increase the affinity of haemoglobin for oxygen in their red blood cells. This means that as O2 levels drop in water the bass can maintain transport of oxygen in their blood for longer! This result may mean that previous research investigating hypoxia has miscalculated the true tolerance of fish in the wild.

Juvenile seabass in the holding tanks in the Aquatic Resources Centre at the University of Exeter

Improving our understanding of how hypoxia impacts fish species is crucial as climate change is causing an increase in both the prevalence and severity of hypoxic events. If calculations of hypoxia tolerance are incorrect this could affect our ability to predict impacts of climate change on fish. Our aim is to now investigate whether this response is common in marine fish or if individual species have differing responses.

The study, published by Scientific Reports, is freely available at here.

You can follow Dan on Twitter @DanWMont


  1. Breitburg, D. et al. Declining oxygen in the global ocean and coastal waters. Science (80-. ). 359, (2018).
  2. Melzner, F. et al. Future ocean acidification will be amplified by hypoxia in coastal habitats. Mar. Biol. 160, 1875–1888 (2013).
  3. Sunda, W. G. & Cai, W.-J. Eutrophication Induced CO2-Acidification of Subsurface Coastal Waters: Interactive Effects of Temperature, Salinity, and Atmospheric PCO2. Environ. Sci. Technol. 46, 10651–10659 (2012).

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