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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, more acidic oceans... But what you may not have heard about is how climate change could affect how the levels of oxygen in our oceans. Scientists have discovered that, under future conditions, oxygen levels in seawater could be markedly lower, known as “ocean deoxygenation”. This could have serious consequences for marine animals, like sharks...


The blue shark (Image Credit: George Karbus, Source: www.georgekarbusphotography.com)

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.



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”). The majority of sharks cannot withstand hypoxic areas 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).


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).


Locations of 'dead zones' (with very low oxygen) in our oceans (Image Source: www.robertscribbler.com)

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).


The blue shark (Image Source: www.wikipedia.org)

So what does this mean for ocean life, like sharks?


In a recent study, scientists sought to understand how the “movement ecology” of blue sharks (Prionace glauca) is changing around the eastern Atlantic Ocean dead zone. They fitted 55 blue sharks with satellite tags. These fancy electronic devices are able to measure environmental conditions, whilst also tracking the animal’s location, to then submit this data back to scientists via satellite. They allowed the researchers to understand how blue sharks were using the space in the eastern Atlantic, depending on the levels of oxygen (Vedor et al, 2021).


Two different satellite tags show that blue sharks do enter hypoxic waters [in red] in the Atlantic (Vedor et al, 2021)

Tag data showed that blue sharks do enter the oxygen minimum zone in the eastern Atlantic Ocean, but whilst they are there, their diving behaviour is drastically different to normal. When the sharks were experiencing lower oxygen conditions, they did not dive to such great depths as they did when outside the dead zone. Whatsmore, the number of dives they performed was significantly lower when in the hypoxic area and they spent much less time diving to deep depths (Vedor et al, 2021).


In low oxygen water [left] blue sharks do not dive as deep [A] of for as long [C] compared to normal [right B & D]

These findings are consistent with a theory known as the “habitat compression hypothesis”. Therefore, the scientists concluded that the range of the blue shark was significantly contracted within the eastern Atlantic due to hypoxic conditions. They theorised this could be caused by three factors:

  1. Hypoxia-related visual impairment - meaning the sharks are not able to see to hunt, so they avoid the hypoxic water,

  2. Prey species avoiding hypoxic waters - so the sharks have no reason to go into these areas and/or

  3. The physiological limitations of the blue shark in hypoxic conditions (known as “hypoxia tolerance”) - they simply cannot breathe in these oxygen starved waters.

It is possible a combination of all of these factors plays a role too (Vedor et al, 2021).


What is troubling about these findings is that this shows us environmental conditions are critical for blue sharks. This is alarming because it is predicted that several oxygen minimum zones will expand and new dead zones will appear under future climate conditions. This could mean that the distribution of blue sharks will be significantly contracted… With a reduced habitat volume, blue shark populations could begin to decline (Vedor et al, 2021).


Blue sharks are already classified as 'near threatened' by the IUCN and their populations are known to be declining in the wild (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!



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.


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.


By Sophie A. Maycock for SharkSpeak.

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