Sea ice loss in the Arctic has triggered a critical tipping point that's destroying the food chain

The Arctic Ocean has crossed a tipping point that is wreaking havoc on the region's food chain, with potentially dire consequences for commercial fishing and the ocean's capacity to soak up carbon, a new study reports.

Scientists found that vast areas of melting sea ice in the Arctic are leading to a significant reduction in nitrate, a key nutrient that forms the base of the marine food web and thus underpins important regional fisheries. As the ice disappears, more light hits the water's surface, promoting the growth of microscopic, plant-like organisms called phytoplankton. When phytoplankton die, their cells sink to the seafloor and are decomposed by nitrate- and oxygen-consuming bacteria.

The new study, published May 28 in the journal Communications Earth & Environment, found that the bacteria are consuming more nitrate than the Arctic ecosystem can withstand.

This effect, known as "denitrification," is irreversible under current climate conditions because we have passed a threshold where so much sunlight reaches the ocean that it's supercharging phytoplankton's productivity, said Marta Santos-García, a doctoral student of Arctic marine biogeochemistry at the University of Edinburgh in Scotland and the first author of the study.

"Even if sea ice were to increase temporarily, the Arctic nutrient system responds over much longer timescales," Santos-García told Live Science in an email. "Short-term increases in sea ice would be unlikely to rapidly reverse the decline in nitrate inventories, which may take much longer to recover."

Dropping nitrate levels may eventually come back to bite phytoplankton, because these tiny organisms need nitrate to carry out photosynthesis. As a result, the transition to a low-nitrate regime could accelerate climate change, as nitrate plays an essential role in the ocean's biological pump, which takes carbon dioxide from the atmosphere via photosynthesis and locks it away at depth when phytoplankton and the animals that eat it die.

"With nutrients such as nitrate in limited supply this mechanism cannot work effectively," Santos-García said.

To understand ecosystem changes in the Arctic, the researchers analyzed two decades of data from the Fram Strait, a passage between Greenland and Svalbard, Norway, that is the main gateway through which Arctic waters flow into the Atlantic Ocean. They found a sharp decline in nitrate levels in this region after 2009, which coincided with a dramatic reduction in Arctic sea ice and a gradual shift in phytoplankton communities toward smaller species that can cope with low nutrient levels.

"Shifts towards smaller phytoplankton have already been observed in parts of the Arctic, although these changes have not previously been linked to nitrate losses," Santos-García said. "This matters because smaller phytoplankton are generally less efficient at transferring energy up the food web. More of the energy is recycled within microbial communities rather than being passed on to larger zooplankton, fish, seabirds, and marine mammals."

Phytoplankton sit at the very bottom of the marine food chain, so the impacts of nitrate depletion will ripple through the Arctic ecosystem, impacting species at the highest levels. This could also affect fisheries in regions that depend on Arctic nutrient exports, such as the North Atlantic. But pinpointing what will happen in ecosystems downstream of the Arctic Ocean requires more research, Santos-García said.

For years, researchers thought the long-term impact of sea ice loss in the Arctic would be an increase in phytoplankton, because more organisms can bathe in sunlight and multiply when the sea ice extent is small. However, the increase in phytoplankton since 2009 has depleted nitrate levels enough to limit future phytoplankton growth.

Whereas phytoplankton proliferation used to be limited by how much sunlight reached surface waters, it is now controlled by nitrate levels. Therefore, nitrate must be considered as a key driver of future changes in the Arctic, Santos-García said.

"As nitrate is the nutrient that limits Arctic productivity, understanding these changes is therefore important not only for Arctic communities and ecosystems, but also for improving projections of future climate change," she said.