ARCTIC OCEAN — A new study published May 28 in Communications Earth & Environment reports that bacterial nitrate consumption in the Arctic Ocean exceeds sustainable limits, contributing to reduced regional nitrate levels. This decline is driven by melting Arctic sea ice and increased phytoplankton growth.

Nitrate serves as a foundational nutrient in the Arctic marine food web. The study indicates that decreased sea ice coverage increases sunlight exposure at the ocean surface, which stimulates phytoplankton growth in Arctic waters. Decomposing phytoplankton cells then sink to the seafloor where bacteria metabolize them, consuming nitrate in a process called denitrification.

Researchers analyzed ecological data collected over twenty years from the Fram Strait. The Fram Strait is a marine passage between Greenland and Svalbard, Norway, that channels Arctic waters into the Atlantic Ocean. Nitrate concentrations in the Fram Strait declined after 2009, corresponding with decreased Arctic sea ice. This period also saw a compositional shift toward smaller phytoplankton species. Increased phytoplankton biomass since 2009 has depleted nitrate reserves to a level that restricts future growth.

Marta Santos-García, a doctoral researcher in Arctic marine biogeochemistry at the University of Edinburgh and lead author of the study, said, "Shifts towards smaller phytoplankton have already been observed in parts of the Arctic, although these changes have not previously been linked to nitrate losses." Nitrate depletion is projected to affect higher trophic levels within the Arctic ecosystem. Reduced nitrate availability may also decrease the efficiency of the oceanic biological pump, which sequesters atmospheric carbon dioxide.

Altered nutrient exports from the Arctic could impact fisheries in the North Atlantic region. "Even if sea ice were to increase temporarily, the Arctic nutrient system responds over much longer timescales." Santos-García said.