What is the role of our ocean in climate change?
The ocean plays an invaluable role in solving the climate crisis, capturing between five to twelve billion tons of carbon dioxide (CO2) from the atmosphere annually. It is fueled in part by phytoplankton at the ocean surface which consume CO2 through photosynthesis. When phytoplankton die or are eaten by larger organisms, the carbon-rich fragments and faecal matter sink to the seafloor and are sequestered into the sediments. Locking carbon into ocean sediments is a key function of the oceanic carbon cycle and necessary in reducing atmospheric carbon.
But what has carbon got to do with rising sea temperature anyway? This video provides the perfect explanation.
How exactly is climate change affecting our oceans?
The ocean and its inhabitants are under attack due to overexploitation and increased levels of destructive fishing methods, but now they have a new worry. The climate crisis poses a plethora of threats including, but not limited to, rising water temperatures, ocean acidification, reduced availability of oxygen, intense marine heat waves, coral bleaching events, and the continent-wide shifting of marine life.
It was previously argued that sharks would likely be uneffected by rising CO2 levels, as modern sharks evolved in the Devonian era, when atmospheric CO2 levels were many times greater than the current day. As more and more studies take place, we are unearthing that this may not be the case.
As most Chondrichthyan's (sharks, rays, skates, and chimaera) are ectotherms, water temperature directly regulates biological and physiological processes that are crucial for survival. A rise in water temperature makes sharks more vulnerable due to their already globally threatened status, poorly characterised basic life history, and an inability to evolve fast enough to survive drastic changes due to slow generation times and low reproductive output. Scientists have already reported that in 28 species of shark, skate and chimaera as water temperature rises, incubation time decreases (more on why this appears to be bad news for sharks below).
This may seem overwhelming, but together we can stop this.
We can change these grim sounding prospects but first we need to understand them, so let’s dive in!
How do sharks affect atmospheric carbon?
Coastal predators such as sharks protect and enhance blue carbon (carbon stored in coastal and marine ecosystems) by limiting grazing on seagrass meadows and kelp forests. These ecosystems are precious not only to the plethora of species they support but also in the role they play in the absorption of carbon due to photosynthesis. For example, tiger sharks frequent shallow seagrass meadows. They prevent overgrazing in concentrated areas from turtles and other prey species, resulting in increased and more evenly distributed seagrass. The increase in meadow mass directly helps rising carbon levels, as seagrass captures carbon from the atmosphere 35 times faster than tropical rainforests.
Large fish such as shark, tuna, mackerel and swordfish are comprised of ~10 to 15% carbon. When they die naturally, their bodies act as direct carbon sinks through 'carcass deadfall'. The carbon held in their bodies is sequestered into the sediment. Put simply, if a carcass is removed from the sea its corresponding carbon is released into the atmosphere but if left to die at sea that carbon is buried. Luckily, these animals are also useful to the climate crisis when they stay alive!
Whaling and Sharks?
The global biomass of whales is less than 25% of pre-whaling times. A study by Pershing et al. shows the implications of removing large marine species on the oceans ability to store carbon. If efforts were increased to restore populations, whales could remove 8,700,000 tonnes of carbon from the environment. That's the equivalent of 110,000 hectares of forest, an area the same size as the Rocky Mountain National Park. If blue whale populations alone were restored they could sequester 3,600,000 tonnes of carbon. The same amount achieved by 43,000 hectares of forest, an area the same size as Los Angeles. But this isn’t the end of the story...
Vicki James, policy manager at Whale and Dolphin Conservation (WDC) told the BBC...
"We need to think of whaling as being a tragedy that has removed a huge organic carbon pump from the ocean that would have been having a much larger multiplying effect on phytoplankton productivity and the ocean's ability to absorb carbon."
The organic pump mentioned above refers to the whales natural ability to provide iron fertilisation to phytoplankton through its faeces. This iron helps phytoplankton blooms thrive and in doing so they have enormous influence, capturing ~40% of all CO2 produced.
As a species high in the trophic chain, the destruction of whale populations has affected both population dynamics and carbon sequestering. As whale populations declined, the orcas that predated them turned to smaller marine mammals like sea otters. As sea otter populations declined, their prey, sea urchins, rapidly increased leading to mass destruction of the kelp forests. These kelp forests are vital in the absorption of carbon from our atmosphere. Sound familiar to anyone?
We cannot let history repeat itself. We cannot allow the same pattern of oceanic disturbance and lost carbon sequestering opportunities that have happened to whales, happen to sharks.
The removal of top predators such as sharks through fishing and finning, not only results in the destruction of precious carbon sinking herbivorous habitats as explained above, but also in higher biomass of prey animals. This shouldn’t come as a surprise to anyone, fewer sharks equals more prey. However, due to greater levels of respiration produced by the increase in lower trophic level fish, a higher net carbon system is produced.
The increase in prey species has also been reported to release the carbon trapped in mangrove and salt-marsh sediments through bioturbation (the disturbance of sedimentary deposits). That is, fish that would normally be eaten by sharks agitate the sediment and release the trapped carbon. The presence of sharks directly limits this release into the atmosphere.
The bottom line? We have to stop mismanaging food webs, they are complex and become unpredictable when we haphazardly remove keystone species. Even the social news and entertainment platform Unilad has got it, with their article titled ’Sharks must be left alone to stop climate change'!
How do rising sea temperatures affect sharks?
A study by Gervais et al. published in October 2020 shows that when assessing how climate change will affect a particular species and ecosystems, it is not only the individual species that need to be considered but intraspecific variations, differences that occur within species, too. Their study showed that even among the same species (the Port Jackson in this case) changing ocean temperatures affected metabolic rates and swimming activity differently dependant on the rearing temperatures of the eggs. In adult populations reared under warmer temperatures, it was found that as water temperature rose their maximum swimming activity reduced. They also showed that eggs reared under cooler temperatures were more sensitive to temperature changes.
"With a rise in water temperature of just 3℃, the energy required to survive is more than twice that of current day temperatures for the Port Jackson sharks in Adelaide."
This energy cannot be spent on other areas such as reproduction and growth, vital for thriving populations. Gervais also noted that populations of sharks from areas that have been subjected to generations of water temperature fluctuations, may unsurprisingly cope better than those that have experienced constant temperatures for many generations.
So what does this mean? The devil is in the detail, we cannot generalise a support scheme for sharks dependent on species alone, the environment the eggs were reared in needs to be considered too. Increasing evidence on the negative impacts of eggs being reared at higher temperatures was released by Sheezer et al. this January. The team showed that epaulette sharks, one of the most resilient shark species, is seemingly unaffected by increased ocean acidity and reduced oxygen levels. Sadly, the same cannot be said for rising water temperatures. At 31°C, the temperature predicted in its natural habitat by the end of the century, epaulette sharks hatched 25 days earlier, at day 100 as opposed to day 125, and weighed less despite eating through the yolk at a faster rate. They also had reduced metabolic performance.
➼ Predicted changes to oceanic currents along the south Australian coast are forecast to increase the availability of favourable water temperatures for bull sharks by three months. Meaning they could appear along the south coast from December to April, as opposed to just January and February.
➼ Fishing fleets have yet more to answer for. In 2016 207 million tonnes of CO2 were released into our atmosphere by fishing vessels, with 48 million tonnes of that from small-scale fisheries. For reference, in the 1950’s 47 million tonnes of CO2 were released. This does not include the potential carbon sequestering opportunities lost by the removal of life from the ocean.
➼ Ocean acidification refers to the increased levels of CO2 absorbed into the ocean and subsequent increase in oceanic pH. This increase acts to acidify the blood and tissues of water-breathing marine organisms. Of the benthic sharks studied, there were pronounced effects on body condition, growth, aerobic potential, hunting and prey detection. However, embryonic survival and development times were mostly unaffected. Is that a silver-lining? It’s the best we could do.
She Changes Climate is a campaign that aims to resolve gender imbalance of the UK Climate Conference of the Parties (COP) COP26 leadership team. Commencing this November, the top level team responsible for the framing and narrative of the agenda is predominantly male, with just 2.3 women for every 12 positions. Over 400 leading women of the environment signed a letter to the UK government asking them to implement a 50:50 ratio immediately.
Why is this important? An inclusive and more accurate representation of the population has been proven to improve the quality of decision-making as it reflects and incorporates a wider range of perspectives and expertise. Half the planet is not represented at a critical juncture in our history. Women and men do not experience climate change in the same way, but despite women bearing the brunt of the incoming climatic changes they remain more resourceful and resilient, something that is well-understood by the UNFCCC. They document just five reasons why climate action needs women here.
What does this have to do with sharks? We need a holistic, global and accurate representation of the threats that the climate crisis presents to be able to implement solutions. Female ocean scientists and specialists need to be heard just as clearly if we are to save our ocean and all its residents, including sharks.
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