A study by Hokkaido University, the University of Tokyo, and Nagasaki University has introduced a new way of thinking about the mechanisms of nutrient transfer from deep-sea water in the northwest subarctic Pacific, and has confirmed the importance of iron from the Okhotsk Sea in supporting the growth of phytoplankton.
The western subarctic Pacific Ocean, including the Bering Sea – with the largest salmon catches in the world – is so productive because it has lots of phytoplankton, the base of the ocean food chain. Phytoplankton are free-floating, photosynthetic microorganisms that grow in lakes, streams, and oceans. They include algae, cyanobacteria, protists, and diatoms. The phytoplankton in turn require nutrients such as nitrate, phosphate, and silicate that are lifted from the deep ocean, as well as iron.
The research article (Jun Nishioka et al., Subpolar marginal seas fuel the North Pacific through the intermediate water at the termination of the global ocean circulation, PNAS, 27 May, 2020) helps explain why the western subarctic Pacific Ocean, which accounts for only 6 percent of the world’s ocean surface area, produces an estimated 26 percent of the world’s marine resources.
“Marginal seas” in this case means the Bering Sea and the Sea of Okhotsk, which lie at the margins of the subpolar North Pacific, separated from the main body of the Pacific by the Aleutian and Kuril Island chains, respectively.
“Intermediate water” refers to the middle layer of water. Ocean water tends to become stratified into surface, intermediate, and deep layers because temperature, salinity, and mineral content affect the density of the water. Cold, mineral-rich water sinks and tends to stay at the bottom, while warm water devoid of minerals tends to rise. The lack of nutrients, and iron, near the surface limits phytoplankton growth. Intermediate water lies between the surface and deep layers and mixes to a certain degree with the surface.
The “global ocean circulation,” also called the “ocean conveyor belt,” is a constantly-moving system of deep-ocean circulation driven by temperature and salinity. Cold, salty nutrient-rich water sinks in the North Atlantic and moves south as warm surface water takes its place, then it runs east along the Antarctic until it branches off to the Indian Ocean and the North Pacific where it ends with an upwelling that brings the nutrients back to the surface.
It is the mechanism of this upwelling that the research team researched by sampling the water at different levels at various locations in the Okhotsk and the Bering Seas aboard a Russian and Japanese vessel, respectively.
Associate Professor Jun Nishioka of the Institute of Low Temperature Science at Hokkaido University, a primary author of the study, said the findings provide a much more complete picture of the regional marine food chain.
“Before our finding, everybody believed the nutrients which accumulate in the deep water come up to the surface directly. But that is not true,” Nishioka said. “The intermediate water accumulates the highest nutrients, because the intermediate water can move into the straight – Kuril Island Strait or Aleutian Strait – and hit some small seamounts, and make a lot of turbulence in the intermediate layer. And the intermediate water comes up and spreads to a wide area. And many phytoplankton grow, and nutrients accumulated in the phytoplankton decompose into the intermediate layer again. So, it circulates [between] the intermediate layer and the surface layer. But some of the intermediate water escapes from the subarctic and cannot return, so that amount should be supplied from the deep water.”
The team was also able to clarify the route of iron from its origin in the Sea of Okhotsk. This sea has high iron production because of its heavy production of sea ice. When ice freezes, the salt and minerals are forced out and sink, which drives movement of the intermediate water. This turbulence mixes iron from sediments at the ocean bottom into the intermediate water. The water moves along Sakhalin Island and exits through straits in the Kuril Islands, where the same type of mixing in the intermediate water as discussed for the Aleutians occurs.
A previous paper by Nishioka and others addresses this phenomenon. While some iron is contributed to the Pacific from airborne dust, that paper found that the main contributor was sedimentary iron from the Sea of Okhotsk.
Nishioka said that while iron is contained in the continental shelf everywhere, the Sea of Okhotsk also has sea ice production and input from the Amur River in the same area. “That is a miracle condition – different from other areas,” he said.
Reduced sea ice formation due to global warming may threaten these conditions, Nishioka said.
“Our institute already has the evidence that sea ice production is decreasing, and intermediate water transport is weakening now. And now we are trying to simulate if the iron transport is also decreasing, or it will affect the phytoplankton primary production,” he said. “That is ongoing research.”
Nishioka said initial findings show climate change is already altering this intricate food web.
“We cannot detect yet if the iron is reduced, but the phytoplankton species is changing in the Oyashio [Current] area, and the phytoplankton production is also decreasing,” he said. “We have evidence about that. So we have to connect the sea ice reducing and the biological changes.”
Photo courtesy of Jun Nishioka
