According to a new study, the rapid decline of Arctic sea ice has sharply reduced levels of nitrate, a nutrient essential for the growth of plankton that form the foundation of the region's ecosystem. Researchers warn that the shift could affect everything from fish and seabirds to marine mammals, while also weakening the Arctic Ocean's ability to absorb carbon from the atmosphere.

The study found that large areas of shallow Arctic waters, once shielded by sea ice, are now being exposed to much more sunlight. That increased exposure appears to accelerate a natural process that removes nitrate from seawater.

Because plankton rely on nitrate to grow, lower nutrient levels could reduce the amount of life the Arctic ecosystem can sustain.

Declining Nitrate Levels in Arctic Waters

Scientists have observed major changes in Arctic wildlife populations in recent years, but the underlying causes were not fully understood. Researchers from the University of Edinburgh sought answers by examining more than 20 years of ocean sampling data collected from Fram Strait, a key passage where Arctic waters flow into the Atlantic Ocean.

Their analysis revealed a clear turning point beginning around 2009. From that period onward, nitrate levels in water leaving the Arctic steadily declined. Researchers say the timing closely matches a dramatic acceleration in Arctic sea ice loss.

The team concluded that shrinking sea ice intensified a process known as benthic denitrification. During this process, nitrate is converted into nitrogen gas within shallow seafloor regions. These continental shelf areas cover nearly half of the Arctic Ocean.

Scientists say the result is a major change in the Arctic's nutrient balance.

Smaller Plankton and a Weaker Food Chain

Researchers warn that nitrate-poor conditions may favor smaller plankton species in the future. That matters because smaller plankton generally support less productive food webs, leaving less energy and food available for larger marine animals higher up the chain.

The consequences could extend beyond the Arctic itself. Plankton also help remove carbon dioxide from the atmosphere through photosynthesis, meaning reduced plankton growth may weaken the ocean's role in storing carbon.

Because the shift is tied to ongoing sea ice decline, researchers believe the Arctic Ocean is unlikely to return to its previous state.

The team says more research is needed to understand how these changes could affect marine ecosystems elsewhere, including the North Atlantic and important commercial fishing regions.

The findings were published in the journal Communications Earth & Environment. The study was supported by the Natural Environment Research Council (NERC)'s Changing Arctic Ocean project.

Researchers from the Norwegian Polar Institute, Scottish Association for Marine Science, Technical University of Denmark, and Alfred-Wegener-Institut in Germany also contributed to the work.

Marta Santos-García, a PhD student in the University of Edinburgh's School of GeoSciences who co-led the study, said: "For years, sea-ice loss in the Arctic Ocean was expected to increase phytoplankton growth because more sunlight could reach surface waters. 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."

Professor Raja Ganeshram of the University of Edinburgh's School of GeoSciences, who led the study over the last two decades, said: "The changes we report suggest that the Arctic Ocean ecosystem passed a tipping point around 2009. 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."