Scientists say climate change is causing large-scale and permanent changes in the Arctic Ocean, fundamentally disrupting the marine food chain.
The rapid decline in Arctic sea ice is sharply reducing levels of nitrate, a nutrient essential for the growth of plankton that forms the basis of the region’s ecosystems, according to a new study. Researchers warn that this change 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 the Arctic’s shallow waters that were once protected by sea ice are now exposed to more sunlight. This increased exposure appears to accelerate the natural processes that remove nitrate from seawater.
Because plankton relies on nitrate to grow, lower nutrient levels can reduce the amount of life that arctic ecosystems can support.
Declining nitrate levels in the Arctic Ocean
Scientists have observed major changes in Arctic wildlife populations in recent years, but the underlying causes were not fully understood. Researchers at the University of Edinburgh sought answers by examining more than 20 years of ocean sampling data collected from the Fram Strait, a key passageway for the Arctic Ocean to flow into the Atlantic Ocean.
Their analysis revealed a clear turning point starting around 2009. Since that period, nitrate levels in water exiting the Arctic have steadily decreased. Researchers say this timing roughly coincides with a dramatic acceleration in Arctic sea ice loss.
The researchers concluded that shrinking sea ice has intensified a process known as benthic denitrification. During this process, nitrate is converted to nitrogen gas within the shallow seafloor region. These continental shelf areas cover almost half of the Arctic Ocean.
Scientists say this will result in major changes in the nutritional balance of the Arctic.
Plankton becomes smaller and the food chain becomes weaker.
Researchers warn that low nitrate conditions may favor smaller plankton species in the future. This is important because small plankton typically support less productive food chains, leaving less energy and food available to larger marine animals up the chain.
The impact could extend beyond the Arctic itself. Plankton also help remove carbon dioxide from the atmosphere through photosynthesis. In other words, reduced plankton growth could weaken the ocean’s role in storing carbon.
Because this change is associated with ongoing sea ice loss, researchers believe it is unlikely that the Arctic Ocean will return to its previous state.
The researchers say more research is needed to understand how these changes will affect marine ecosystems in other regions, including the North Atlantic and important commercial fishing areas.
The research results were published in a magazine Communication Earth and Environment. This research was supported by the Natural Environment Research Council’s (NERC) ‘Changing Arctic Ocean’ project.
Researchers from the Norwegian Polar Institute, the Scottish Marine Science Society, the Technical University of Denmark and the Alfred Wegener Institute in Germany also contributed to the study.
Marta Santos García, a PhD student at the University of Edinburgh’s School of Geosciences who co-led the study, said: ‘For years, the decline in Arctic sea ice was expected to increase phytoplankton growth as more sunlight reaches surface waters. “Our findings suggest that this relationship has changed. The Arctic Ocean appears to be moving from a system primarily limited by light to one increasingly limited by nitrate availability, with far-reaching implications.” The role of the Arctic in marine ecosystems, food chains, and Earth’s climate. ”
Professor Raja Ganeshram, from the University of Edinburgh’s School of Geosciences, who led the study over the past 20 years, said: “The changes we report suggest that the Arctic ecosystem crossed a tipping point around 2009. How this change cascades through the food chain has important implications for us, including commercial fishing in the North Atlantic, and needs to be closely monitored.”
