Sea of Okhotsk

Rivers pour fresh water into the sea. It floats on top of the salty water.

Cold winds blow over it. The thin top layer freezes into ice.

Three things help the Sea of Okhotsk freeze. First, the Amur River and other fresh water make a less salty top layer about 50 meters deep, floating on saltier, heavier water. Only this top layer has to cool down before it freezes.

Second, land surrounds the sea, so little water mixes in from the open ocean. Third, in winter, high pressure over Siberia and low pressure over the sea send cold Siberian air across the water. Ice begins near the mouth of the Amur in early November and reaches the coast of Hokkaido by mid-January.

The ministry gives three conditions. The double-layered structure: the Amur supplies about 37% of the sea's fresh water, creating a surface layer of about 32.5‰ salinity to roughly 50 m, above denser intermediate cold water (about 33.5‰) formed by brine sinking from growing ice; convection is confined to the top 50 m, so the surface cools quickly to freezing. The sea's limited exchange with the open ocean preserves this layering. And a winter Siberian high and an Okhotsk low drive cold air over the sea.

Ice forms along the Siberian coast and the Amur mouth from early November, spreads south with northwesterly winds and the East Sakhalin Current, reaches Hokkaido in mid-January and covers about 80% of the sea by mid to late March; it is gone by the end of June.

Tiny plants called ice algae live in and under the ice.

When spring comes, they grow very fast. Many animals eat them.

Sea ice does not stop life. As the ice grows, cold, salty water sinks and stirs up nutrients from deeper down. Light still gets through the ice, so tiny plant-like organisms called ice algae, mostly diatoms, grow inside and under it.

When the ice starts to melt in early spring, the ice algae multiply quickly in a spring bloom, and other phytoplankton bloom along the melting ice edge. These blooms start the food web for fish, birds and mammals.

Ice formation drives water circulation and brine rejection, lifting nutrients from the lower layer to the surface; under the ice there is enough light for photosynthesis and a stable temperature near minus 1.8°C, so the water just beneath the ice suits phytoplankton. Ice algae, mainly diatoms, grow within and on the underside of the ice and bloom rapidly as it melts in early spring; other phytoplankton then bloom at the retreating ice edge, and primary production starts earlier than in seas that never freeze.

UNESCO credits this supply of nutrient-rich intermediate water, from sea-ice formation in the Sea of Okhotsk, with the successive phytoplankton blooms that underpin Shiretoko's marine ecosystem.

Salmon swim up the rivers from the sea. Bears and owls catch them to eat.

Sea lions and whales swim in the sea.

At Shiretoko, on the sea's southern edge in Japan, the food web reaches from the ocean into the forest. Salmon swim up small rivers to lay their eggs, and brown bears and the rare Blakiston's fish owl eat them. UNESCO says Shiretoko has one of the highest densities of brown bears recorded anywhere.

The area is also important for Steller's sea lions, some kinds of whales and dolphins, threatened seabirds and migrating birds.

UNESCO inscribed Shiretoko in 2005 under criteria (ix) and (x). Food webs starting from the plankton blooms involve fish, birds and mammals across ocean, rivers and forest; salmonids swimming upstream to spawn feed terrestrial species such as the brown bear and the endangered Blakiston's fish owl. Pacific salmonids there include white-spotted charr, masu salmon, chum salmon and pink salmon, and its watersheds are the southernmost sea-run habitat of the Dolly Varden.

The site has one of the highest recorded brown bear densities in the world and is globally important for Steller's sea lion, some cetaceans, threatened seabirds and migratory birds.

Never walk out onto sea ice. Stay on safe ground.

Watch sea lions and seals from far away.

NOAA, the United States ocean agency, advises staying at least 50 yards (about 46 meters) from seals and sea lions, never feeding them, and keeping well away from pups, because a mother is likely nearby. If an animal stares, fidgets or flees into the water, you are too close.

PlayOwl's advice for icy coasts: never walk on sea ice or ice-coated rocks, and watch drifting ice only from safe ground with a grown-up.

NOAA Fisheries recommends a distance of at least 50 yards (about 46 m) from seals and sea lions, never feeding them, keeping pets away, limiting viewing to 30 minutes or less, and watching for signs of disturbance such as staring, fidgeting or fleeing into the water; close encounters can cause stress and even make mothers abandon pups. These are US guidelines, but their reasons apply on any coast.

PlayOwl's own advice is to stay off sea ice and ice-glazed rocks, which can be slippery or break away, and to watch from safe ground.