The disappearance of Arctic sea ice not only changes the face of one of the fastest-warming regions on Earth. Scientists say it also leads to the depletion of nitrates in the ocean.
Nitrates help support tiny organisms at the base of the marine food chain. These changes could impact fish, seabirds, and marine mammals.
A new study published in the journal Communications Earth & Environment found that nitrate levels in Arctic waters flowing through the Fram Strait have been declining since around 2009. This downward trend coincides with accelerating sea ice melt.
Scientists from the University of Edinburgh and partner institutions analyzed oceanographic data from over 20 years of research in the strait, a vital channel between the Arctic Ocean and the Atlantic Ocean. Their findings point to what the researchers call a significant turning point for the Arctic system.
As sea ice retreats, shallow shelf areas that were previously in shadow are now receiving more sunlight. Researchers say this process likely enhances benthic denitrification—the natural process by which nitrates are converted into nitrogen gas on the seafloor.
Since the Arctic's continental shelf covers nearly half the ocean, the consequences could be widespread.
«"For many years, it was thought that declining sea ice in the Arctic Ocean would lead to increased phytoplankton growth because more sunlight would reach surface waters," said Marta Santos-García, one of the study's leaders, in a ScienceDaily press release. "Our results show that this relationship has changed.".
The press release also noted that nitrate is essential for plankton growth, and plankton forms the foundation of the Arctic marine ecosystem. Without nitrate, the ocean as a whole may support less life. The species that do thrive may be smaller plankton, supporting less productive food webs.
A weakening of the food chain could increase pressure on fish populations, which in turn could have consequences for commercial fisheries in the North Atlantic.
Plankton absorb carbon dioxide from the atmosphere through photosynthesis, so slowing plankton growth could reduce the Arctic Ocean's ability to store carbon. The same loss of sea ice, driven by rising temperatures and disrupting ecosystems, could also weaken the natural climate buffer.
Researchers say these changes may not be temporary. Because the shift is linked to ongoing sea ice loss, they believe the Arctic Ocean is unlikely to easily return to its previous state.
«"The Arctic Ocean appears to have shifted from a system primarily limited by light to one increasingly limited by nitrate availability, with far-reaching implications for marine ecosystems, food webs, and the Arctic's role in Earth's climate," Santos-García said.
Professor Raja Ganeshram added: "Our data suggests that changes in the Arctic Ocean ecosystem reached a tipping point around 2009.".
The international team of researchers who conducted the study stated that scientists need to closely monitor how declining nitrate levels are spreading through the food chain. Further research will allow them to better understand how this affects ecosystems beyond the Arctic, including key commercial fishing regions.
Subscribe to TCD's free newsletters to receive helpful tips, actionable advice, and a chance to win $5,000 toward your home improvement. To see more stories like this, change your Google preferences here.