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 in the 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.

Andrew Sweetman of the Lyell Centre for Earth and Marine Science and Technology at Heriot-Watt University in Edinburgh, Lisa Levin, a biological oceanographer at Scripps Institution of Oceanography at the University of California San Diego, and colleagues from 20 of the world’s leading oceanographic research centers used earth system models and projected climate change scenarios to quantify impending changes to deep oceans in the review released Feb. 23.

The team looked at a number of sea and ocean beds, from the Arctic to Antarctic oceans, focusing on bathyal (200-3,000 meter or 650-9,800-foot) and abyssal (3,000-6,000 meter or 9,800-19,700-foot) depths, the areas defined in the paper as deep ocean. As well as measuring how the deep oceans’ food sources will change, the team examined the impact that predicted seabed temperature increases, oxygen level declines, and increasingly acidic seawater will have, under the sea and across the planet.

“The rate of change underway in our oceans is faster than at any point we know of in geological history,” said Sweetman. “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 percent 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.

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

Organic compounds produced through primary production – the creation of chemical energy by algae and other phytoplankton through photosynthesis – sink to the deep ocean and make up much of the food supply there. Most of the deep sea currently experiences a severe lack of food, but according to Sweetman and his research team, it is about to receive even less. That’s because the phytoplankton deep sea organisms rely upon are themselves facing a dwindling supply of nutrients in the surface oceans as warming makes waters more stratified.

Sweetman continued: “Abyssal ocean environments, which are over 3,000 meters deep, are some of the most food-deprived regions on the planet. These habitats currently rely on less carbon per square meter 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.

“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,” said Andrew Thurber, co-author of the study and a professor at Oregon State University. “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.

Sweetman said 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 long-lining, 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,” he added. 

The review is based on a workshop led by Sweetman and funded by the Norwegian Research Council.

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 (0.5-1^oC). 
• Bathyal depths are also likely to experience increasing temperatures of approximately 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 milliliters per liter 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 in the North Atlantic and the Weddell Sea, Antarctica in other areas of the world´s oceans will also experience 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 percent and 55 percent, respectively by 2100.

– This news release was adapted from an original issued by Heriot-Watt University

Related Image Gallery: Deep Oceans Face Starvation by End of Century