The northern North Pacific is known to be high in nutrients and low in chlorophyll (HNLC); dissolved macro nutrients (nitrate, phosphate and silicate) in the surface water cannot fully be utilized by phytoplankton because of the low availability of iron. Iron is usually supplied to estuarial and coastal regions from the land surface and it is difficult for iron to be transported to the remote central area of the northern North Pacific.
The neighboring Sea of Okhotsk is also characterized by sufficient nutrients supplied by the winter convective mixing of surface and deep waters. The Sea of Okhotsk is, however, not an HNLC region. This is because sufficient dissolved iron is transported from the Amur River. The Amur River, including major tributaries like the Shilka, Argun, Zeya, Bureya, Songhua Jiang (Sungari) and Ussuri Rivers, is 4444 km long and has a drainage area of 2,129,700 km2. The major part of the drainage area is covered by boreal forest, mixed forest and swamps. The lower part of the drainage area is cultivated land and major cities such as Blagoveshchensk, Harbin, Khabarovsk and Komsomolsk-na-Amure. The relatively less developed Amur River basin enables the river to transport various kinds of terrestrial materials to the Sea of Okhotsk. Of particular importance is dissolved iron, which is considered to originate in an anoxic environment such as swamps.