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How Low Can You O?

Scientists have predicted that we will see many changes in our oceans as a result of climate change: rising sea levels, higher water temperatures, changing currents, more acidic oceans... But what you may not have heard about is how climate change could affect the levels of oxygen in our oceans. So what is "ocean deoxygenation"? How does it happen? And what effect will this have on sharks?


Blue sharks are currently the most broadly-distributed off all shark species (image Credit: Greg Skomal, National Marine Sanctuaries, NOAA / WikimediaCommons)

Breathe With Me

We often say fish like sharks can breathe underwater, but what we actually mean is that they are able to use oxygen (also known as O2) dissolved in the water for "respiration". You may have flashbacks from high-school biology, where you learnt that the equation for respiration is: Glucose + Oxygen → Carbon Dioxide + Water + Energy


In just the same way that we humans need to breathe to fuel respiration, in sharks respiration is vital to release energy from the foods they eat. This in turn, is used to power their bodies. Seawater naturally contains dissolved oxygen, which sharks are able to extract through their gills.



O, My!

As sharks are active predators, their oxygen requirements are relatively high. Therefore, sharks cannot venture into regions where dissolved oxygen levels are very low (known as “hypoxic” areas). The majority of sharks cannot withstand hypoxia in the region of 3.0 – 3.5 ml of oxygen per litre. Therefore, the amount of oxygen in the water, is one factor which drives the distribution of sharks in the oceans (Vedor et al, 2021).


Sharks' distributions are affected by the amount of oxygen in the water because they cannot survive in hypoxic water (Image Credit: Diego Delso / WikimediaCommons)

There are regions in the oceans which are naturally lower in oxygen. This is caused by global sea water circulation, water temperatures and climate. Hypoxic areas with dissolved oxygen as low as 0.45 – 1.00 ml oxygen per litre are considered to be ‘dead zones’, meaning there is too little oxygen for most life. Some of these dead zones are fixed in relatively permanent locations. For example, in the eastern tropical Atlantic ocean, there exists a “permanent oxygen minimum zone”, with as little as 2.0 ml of oxygen per litre of water. In this area, sharks are unable to descend into the hypoxic layer of water below the oxygen-rich surface layer. This means their distribution is limited on the vertical plane (Vedor et al, 2021).


Whilst hypoxic regions are perfectly natural, the problem is that they are changing… When water is warmer, dissolved oxygen naturally escapes from the water as a gas. Therefore, cooler waters have higher levels of dissolved oxygen, where warmer water is comparatively lower in O2. This means that, as our global climate continues to change and sea temperatures gradually rise, hypoxic dead zones are expanding or even appearing in areas where they had not been seen before (Vedor et al, 2021).


Low oxygen areas occur naturally in the ocean, but they are growing as our oceans warm (Image Credit: R. Diaz, The GO2NE network & The World Ocean Atlas 2009 / WikimediaCommons)

Deep Blue Sea

So what does this mean for ocean life, like sharks? Tagging studies around the eastern Atlantic Ocean dead zone have shown that blue sharks (Prionace glauca) will actively avoid swimming into the hypoxic region. If they do enter, their diving behaviour is drastically different to normal - they do not dive to such great depths and they don't dive as often whilst in the low-oxygen area (Vedor et al, 2021).

Scientists don't know if the sharks avoid these regions because they simply can't breathe there or the area is not attractive to them because there's no food (as their prey species cannot survive in the hypoxic zones). It's also possible that blue sharks may suffer "hypoxia-related visual impairment", where the low oxygen makes them unable to see to hunt, so they avoid the dead zone (Vedor et al, 2021).


But whatever the reason, experts are very concerned that ocean deoxygenation and the expansion of dead zones will lead to "habitat compression" for the blue shark (and maybe many other species of sharks and other animals). This means that their distribution will contract (and most likely their populations decline) because the amount of available habitat has shrunk (Vedor et al, 2021).


Blue sharks behave differently when they enter an hypoxic region of the ocean (Image Credit: Diego Delso / WikimediaCommons)

Feeling Blue

Blue sharks are the most heavily-harvested of all shark species. As a result, their populations are declining in the wild and they are already classified as Near Threatened by the IUCN and (Rigby et al, 2019).

We cannot add oxygen to our oceans in order to make them more habitable for marine life, like blue sharks. Besides, that would be treating the symptoms, rather than tackling the cause of the disease! To stop ocean deoxygenation we must mitigate the effects of climate change! The only way we can halt the march of ocean acidification, rising sea surface temperatures and ocean deoxygenation is to stop our contribution to greenhouse gas emissions before it is too late. That means reducing our reliance on unsustainable energy sources now.



If you would like to reduce your carbon footprint, there are many things you can do to help right from the comfort of your home:

  • Recycle everything you can, including metals, glass and plastics,

  • Avoid buying products in unnecessary packaging,

  • Walk or cycle wherever you can, rather than relying on your car (it’s good for your health too!),

  • Avoid single-use items and reuse anything you can,

  • Switch to biodegradable products, like bamboo, for your toiletries and clothing,

  • Buy used items (Vintage is green!),

  • Improve the insulation in your home, so you use less energy on heat (this lowers the cost of your bills too!),

  • Switch to an electric car, solar panels and other renewable energy sources.

If we all make one small change, it adds up to a big improvement! Together we can do this!


Scientists are concerned that the blue sharks' distribution will constrict as the oxygen levels in the oceans drop (Image Credit: Ben Phillip / Pexels)

References

Rigby CL, Barreto R, Carlson J, Fernando D, Fordham S, Francis MP, Herman K, Jabado RW, Liu KM, Marshall A, Pacoureau N., Romanov E, Sherley RB & Winker H (2019). Prionace glauca. The IUCN Red List of Threatened Species 2019: e.T39381A2915850. Access online.


Vedor M, Queiroz N, Mucientes G, Couto A, da Costa I, dos Santos A, Vandeperre F, Fontes J, Afonso P, Rosa R, Humphries NE & Sims DW (2021). Climate-driven deoxygenation elevates fishing vulnerability for the ocean’s widest ranging shark. eLife, 10, e62508. Access online.



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