The Arctic Ocean may have crossed an irreversible chemical tipping point, with climate-driven sea ice loss now disrupting the food chain from plankton to fish, seabirds and marine mammals.
New research led by the University of Edinburgh suggests the loss of Arctic sea ice has triggered a sharp decline in nitrate, a key nutrient needed by plankton at the base of the marine ecosystem.
For years, scientists expected melting sea ice to boost plankton growth because more sunlight could reach surface waters. But the new study suggests the opposite may now be happening.
As ice disappears from vast shallow areas of the Arctic Ocean, sunlight is driving chemical changes that remove nitrate from seawater and convert it into nitrogen gas.
This process, known as benthic denitrification, is now increasing across the continental shelves that sit beneath nearly half of the Arctic Ocean.
That matters because nitrate is essential for plankton growth.
If nitrate levels keep falling, the Arctic may support less plankton overall and smaller plankton species in future, reducing the amount of food available to fish, seabirds and marine mammals.
The impact could also reach beyond wildlife.
Plankton help remove carbon dioxide from the atmosphere through photosynthesis, meaning lower plankton productivity could weaken the Arctic Ocean’s role as a carbon sink.
The researchers analysed more than 20 years of sampling data from Fram Strait, the main gateway where Arctic waters flow into the Atlantic.
They found a clear shift from around 2009, with nitrate levels in waters leaving the Arctic falling steadily at the same time as sea ice loss accelerated.
Marta Santos-García, a PhD student at the University of Edinburgh’s School of GeoSciences, said: “For years, sea-ice loss in the Arctic Ocean was expected to increase phytoplankton growth because more sunlight could reach surface waters.”
She added: “Our findings suggest that this relationship has changed: the Arctic Ocean appears to have shifted from a system mainly limited by light to one increasingly limited by nitrate availability, with far-reaching consequences for marine ecosystems, food chains and the role of the Arctic in the Earth’s climate.”
The study says the change is very unlikely to reverse because it is being driven by ongoing sea ice loss.
Researchers say more work is now needed to understand how the shift could affect marine life beyond the Arctic, including commercial fisheries in the North Atlantic.
Professor Raja Ganeshram, of the University of Edinburgh’s School of GeoSciences, said: “The changes we report suggest that the Arctic Ocean ecosystem passed a tipping point around 2009.”
He added: “How this change cascades through the food chain needs to closely monitored as this has profound implications for us, including on commercial fishing in the North Atlantic Ocean.”
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