Deep oceans face starvation by end of century

The deep ocean floor, earth’s largest habitat, will be starved of food by the end of this century, scientists have warned.

New research published on open-access journal Elementa today shows that food supply to some areas of the Earth’s deep oceans will decline by up to half by 2100.

Dr Andrew Sweetman, based at the Lyell Centre for Earth and Marine Science and Technology at Heriot-Watt University in Edinburgh, and colleagues from 20 of the world’s leading oceanographic research centres have used earth system models and projected climate change scenarios, developed for the Intergovernmental Panel on Climate Change (IPCC), to quantify impending changes to deep oceans.

The team looked at a number of sea and ocean beds, from the Arctic to Antarctic Oceans, focusing on bathyal (200-3000m) and abyssal (3000-6000m) depths. As well as measuring how the deep oceans’ food sources will decline, the team examined the impact that increased seabed temperatures, declining oxygen levels and increasingly acidic seawater will have, under the sea and across the planet.

Sweetman, associate professor at Heriot-Watt’s Lyell Centre for Earth and Marine Science and Technology, said: “The rate of change underway in our oceans is faster than at any point we know of in geological history.

“Deep seafloor ecosystems provide services that are vitally important to the entire ocean and biosphere; we should all be concerned at what’s happening on our ocean floors.

“The organic matter cycling that occurs in the deep sea helps to buffer the ocean against pH changes and the effects of ocean acidification.”

The changes that are projected in the deep ocean, which accounts for more than 95 % of the volume of the Earth’s oceans, are likely to significantly alter the health and sustainable functioning of the planet over the next couple of centuries.

Most of the deep sea currently experiences a severe lack of food, but according to Dr Sweetman and his research team, it is about to face a famine.

Sweetman continued: “Abyssal ocean environments, which are over 3000m deep, are some of the most food-deprived regions on the planet.

“These habitats currently rely on less carbon per m2 each year than is present in a single sugar cube.

“We’ve shown that large areas of the abyss will have this tiny amount of food halved by 2100.

“For a habitat that covers half the earth, the impacts of this will be enormous.”

The researchers also describe an imminent, significant temperature increase that will happen at the deepest parts of the ocean.

Dr Andrew Thurber, co-author of the study and a professor at Oregon State University (USA) said: “Deep-sea ecosystems are not just going to experience a reduction in food, but will likely also experience an increase in ocean temperature of 1°C within 85 years.”

“This is very worrying because increasing temperature will increase the metabolism of animals and microbes that live in the sediment, meaning they will require more food at a time when much less is available.”

The scientists also examined how certain human activity will continue to affect the deep ocean. Dr. Sweetman explained: “The deep sea is fast becoming a target area for increased exploitation of key resources and the dumping of pollutants. Pressure from fishing has led to many deep-sea fish species being severely exploited through trawling and longlining, with some species having been fished to commercial extinction.

“There is also extensive interest in mineral mining at hydrothermal vent systems along mid ocean ridges, at seamounts and polymetallic nodule areas at abyssal depths, such as the Clarion Clipperton Zone of the Pacific Ocean.”

Dr Lisa Levin, co-author and professor at Scripps Institution of Oceanography UC San Diego in La Jolla (USA), said: “Because many deep-sea environments are naturally very stable in terms of environmental conditions, even slight changes in temperature, oxygen, food supply, and pH are likely to significantly lower the resilience of deep-sea communities to the impact of human activity.

“These many challenges call for intensified observations of and spatial planning for the deep ocean, coordinated at an international level.”

Dr Sweetman warned that the stresses and strains put on the depths of the ocean cannot be ignored. He said: “Politicians the world over must recognise the vulnerability of life on the ocean floor to climate-related stressors, and the direct influence that the surface climate can exert on the world’s largest ecosystem.”

Heriot-Watt University’s chief scientist, Professor John Underhill, said: “Andrew and his colleagues’ research has produced a timely reminder of the impacts of the world’s changing climate on ecosystems living on the ocean floor.

“Their work showcases the quality, depth and breadth of research being undertaken into understanding the deep oceans at Heriot-Watt in the Lyell Centre for Earth and Marine Science & Technology”

Summary of key findings

·         Over the next 84 years the highest temperature changes are likely to occur at the abyssal seafloor in the North Atlantic, Southern and Arctic Oceans (0.5-1oC).

·         Bathyal depths are also likely to experience increasing temperatures of approx. 4 °C in the Pacific, Atlantic, and Arctic Oceans.

·         Bathyal seafloor habitats in the North Pacific, North Atlantic, Arctic and Southern Oceans could experience a reduction in bottom-water oxygenation by 0.03–0.05 mL L–1 by the year 2100, which represents a reduction in water column O2 levels by 0.5–3.7 %.

·         Ecosystems within and on the fringes of oxygen minimum zones could be particularly affected by the O2 and warming changes predicted for bathyal environments.

·         Bathyal seafloor habitats (N. Atlantic and the Weddell Sea, Antarctica) in other areas of the world´s oceans will also experience significant reductions in pH (e.g., a decrease of 0.29 to 0.37 pH units) by the year 2100, as a result of the entrainment of CO2-rich seawater to the seafloor at sites of bottom-water formation.

·         The areas likely to be impacted by significant declines in food supply lie in the North and South Pacific, North and South Atlantic, and North and South Indian Oceans. The abyssal and bathyal regions of the Indian Ocean are predicted to experience declines in food supply by as much as 40 % and 55 %, respectively by 2100.

Dr Sweetman led a team of researchers from 20 oceanographic research institutes from seven countries in the research project. To read the paper in full, click here: https://www.elementascience.org/articles/10.1525/elementa.203/

Contact: Sarah McDaid, Director, McDaid Public Relations, 07866789688 | sarah(at)mcdaidpr.co.uk | www.mcdaidpr.co.uk

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