Melting sea ice reshuffles Arctic nitrate, risks food web

Arctic nitrate – New research suggests Arctic warming has crossed a tipping point: phytoplankton growth has slowed since 2009 because blooming on the Pacific side is consuming nitrate that other regions—especially the Atlantic side—need to keep thriving. The shift could alter

On satellite images, the Arctic can still look like a promise: in stretches of open water near Svalbard, phytoplankton blooms paint the sea green. The pigment chlorophyll has been increasing across parts of the Arctic, and algal blooms there have broken records.

But that green hue masks a harder story now unfolding across the food chain. In the same Arctic system. phytoplankton growth in many areas has slowed since 2009—and on the Atlantic side it has even begun decreasing. The reason. new research suggests. is that extra growth in one part of the Arctic is starving another part of a crucial nutrient.

Raja Ganeshram at the University of Edinburgh. UK. and colleagues propose that flourishing phytoplankton on the Pacific side are depriving neighbouring areas of nitrate. a chemical vital for growth. Arctic warming, Ganeshram says, “is not just a reduction in sea ice and temperature; it is more than that. It’s having an effect on the ecosystem. ” adding that it can reshape “food resources in ways that we don’t fully understand. both within the Arctic as well as in the north Atlantic. at our doorstep.”.

Nitrate is one of the three main nutrients needed for plant life. In the Arctic. Pacific water coursing through the Bering Strait brings nitrate into the Chukchi Sea. part of the Arctic Ocean. Currents then carry that nitrate around the region and eventually into the Atlantic—largely through the Fram Strait between Greenland and Svalbard.

To see what has changed. Ganeshram’s team analysed measurements of nutrient levels in the Fram Strait collected by regular icebreaker expeditions from 1998 to 2023. They found a sharp fall in nitrate levels from 2009 onwards. The drop, the researchers say, coincided with a “regime shift” towards lower sea ice extent.

Once sea ice retreats, more light reaches the water. In the Chukchi Sea, that means more phytoplankton can grow, bloom, then die and sink. There, aerobic bacteria and archaea in the sediments decompose the sinking material, consuming oxygen. If oxygen becomes depleted, anaerobic microbes take over and break down dead phytoplankton while consuming nitrate.

By the time those waters reach other parts of the Arctic—like the Fram Strait—they are depleted of nitrate.

The shift is showing up in the biology. In the Fram Strait, diatoms, a kind of algae that thrive when nitrate is available, have ceased to be dominant. Instead. most phytoplankton there are microplankton. which are more efficient at sourcing their nitrogen from ammonium produced by bacteria and zooplankton in the water column.

That change has consequences for the way energy moves through the ecosystem. Smaller phytoplankton must be eaten by smaller zooplankton before that energy reaches larger zooplankton and then fish. Because some energy is lost at each step. a longer food chain could ultimately mean less food for fish. seals. polar bears—and even human communities like the Inuit.

The altered flow of nutrients into the north Atlantic could also change the composition of phytoplankton there, with possible effects on commercial fishing.

Jean-Éric Tremblay at Laval University in Quebec City. Canada. who was not involved in the research. puts it plainly: “What this is showing is that the Arctic Ocean is not going to become the oasis of the future. ” he says. “If you increase [phytoplankton] production, you enhance denitrification, you remove nitrate, and further down the line you reduce productivity.”.

The researchers behind the study argue the ecosystem has crossed a tipping point. Team member Marta Santos-García. also at the University of Edinburgh. says. “That sea ice is not going to come back. even if interannually there might be some fluctuations.” She adds: “So you can basically imply then that this loss of nitrate will likely not be recovered.”.

In other words, the Arctic’s story is shifting from sunlight-driven growth to nutrient scarcity—and the tipping point may already be in motion.

Arctic Ocean phytoplankton chlorophyll chlorophyll satellite imagery nitrate nitrogen cycle sea ice retreat Bering Strait Chukchi Sea Fram Strait diatoms microplankton denitrification food web seals polar bears Inuit commercial fishing icebreaker expeditions University of Edinburgh