Climate Change and the Forgotten Deep Ocean

'In the deep ocean, warming, acidification and deoxygenation, as well as changing food supply are already occurring and we have barely begun to study this...'

It just so happens that the latest issue of Science Magazine features a special section focusing on the effects of climate change on the oceans. Never one to miss an opportunity, here I review Levin and Le Bris’ article ‘The deep ocean under climate change’ published yesterday.

The deep ocean is defined as the ocean below a depth of 200m and makes up most (90%) of the habitable space for life on Earth. Largely unseen and unmonitored, the deep oceans play a fundamental role in quelling the effects of climate change by absorbing vast amounts of heat and carbon dioxide as well as recycling nutrients for surface ecosystems. The problem is that in acting as a ‘critical buffer to climate change’, vulnerable ecosystems are exposed to the combined pressures of warming, ocean acidification, deoxygenation and altered food inputs.

In their paper, Levin and Le Bris argue that more must be done to mitigate the effects of climate change on the deep ocean as changes that could arise from these stresses may ‘threaten biodiversity and compromise key ocean services that maintain a healthy planet and human livelihoods’.

There are few long-term data series available for the deep ocean on climate-relevant time scales though repeat hydrographic surveys have provided estimates of decadal warming in deep basins of up to a 0.1°C increase per decade in the global ocean (Purkey et al. 2010). However, warming of deep-sea basins is not homogeneous and much higher rates of warming have been documented in the Arctic and Southern Oceans.

It is known that most deep-sea species live in very stable thermal regimes, growing and reproducing slowly and experiencing great longevity- for example, fish in these settings can live for hundreds of years while some forms of colonial coral can live for thousands. As a result these species are thought to be extremely sensitive to change where warming of even 1°C or less may ‘exert stress or cause shifts in distributions and alter species interactions’. A prime example of where this has already occurred is in the Palmer Deep near the Antarctic Peninsula, where warming above a 1.4°C threshold has prompted the invasion of lithodid crabs: voracious predators that are responsible for the dramatic plunge in benthic invertebrate numbers. And what is more worrying, according to the authors, is that many species of the deep ocean are yet to be described and the impacts of climate change on such species unknown. Armstrong et al. (2012) argue that ‘potential loss of deep-sea biodiversity may suppress adaptation capacity and limit the living library of species, genes and biomolecules available to future generations’.

Fig 1. A) Lithodid crabs invading Palmer Deep, Antarctica enabled by warming; B) expansion of cold seep fauna due to methane release induced by warming; C) Mediterranean cold-water coral reefs affected by warming and acidification; D) Low-oxygen tolerant Humboldt squid have expanded their distribution along the East Pacific margin

Further, it is warned that warming of the seafloor also has the potential to release methane from gas hydrates buried along continental margins. 

In a similar manner to that of warming, data relating to the effects of acidification on the deep ocean are rare and direct observation of the biological consequences of acidification on organisms is lacking.

Increased CO₂ is predicted to decrease the habitat range suitable for calcifying marine species because the depth at which water becomes undersaturated for aragonite or calcite will move upward. The effects on deep-sea coral reefs is of particular concern because they support extremely diverse communities as well as providing fundamental habitats to commercial fishes.    

Warming of the ocean also results in a decrease in its ability to hold oxygen and waters become more stratified because warm water being less dense than cold water creates strong density gradients which reduce vertical mixing. The combined effects of reduced oxygen solubility in warmer water and increased stratification create widespread deoxygenation, with effects greatest at 200-700m. This is leading to the expansion of the world’s naturally occurring low oxygen zones (OMZs), causing shifts in habitat usage. For example, intolerant billfishes are faced with a reduction in suitable habitat whereas for hypoxia-tolerant species such as the Humboldt squid, habitat ranges are expanding. 

The paper also explains that increased stratification also reduces nutrient supply to surface waters from the deeper ocean where organic matter is recycled and vast abyssal habitats beneath oligotrophic (nutrient-poor) waters may be further deprived of organic matter supply. They argue this could lead to decreased benthic biomass and altered respiration and bioturbation rates and predictions worryingly suggest a similar trend could affect 80% of biodiversity hotspots.

Interactions between many different climate stressors in deep sea ecosystems are complex and many areas expected to experience the greatest climatic changes are also subject to anthropogenic pressures from mining, fishing and oil and gas exploration.

Fig.2 The current and proposed human exploitation activities in the deep ocean with CO2-induced change in the temperature, pH and oxygenation of the deep ocean (Levin and Le Bris 2015)

But what can be done?

As it stands nobody is responsible for protecting the mitigation potential or resilience of ecosystems in the vast deep oceans and 64% of the world’s ocean is beyond national jurisdiction meaning it isn’t covered by the United Nations Framework Convention on Climate Change (UNFCCC). As Levin and Le Bris state, ‘the biodiversity and climate change vulnerability’ of the deep ocean exist in a ‘policy vacuum’. 

  

As with so many other climate change problems, the first and foremost effort must be put into reducing CO₂ emissions although even if this were to occur, the deep ocean will continue to experience the effects of these emissions for years to come. Other ways to mitigate the effects of climate change on the deep sea are limited- possibilities include spatial planning to limit human activities that may help to create safe havens for endangered species and attempts to reduce physical and chemical disturbances from bottom trawling, oil and gas extraction and seabed mining.

Levin says there’s an urgent need for a deep-ocean observation network to improve climate modelling and evaluate feedbacks between the ocean that can act to speed up or slow down the rate of warming. The ocean’s capacity to absorb heat and carbon dioxide is not indefinite, she warns.