2004 WORLD HERITAGE LIST NOMINATION ● JAPAN3a. 3 Sea ice
The nominated site is located on the Shiretoko Peninsula which juts out
into the southern part of the Sea of Okhotsk. Due to topographical andgeographical conditions, the Sea of Okhotsk is the most southern(lowest latitude) ocean in the world as a seasonal sea ice area. The
coastal waters of the Shiretoko Peninsula support a marine ecosystemthat is based on the abundant plankton supplied by sea ice.
3a. 3. 1 Formation of sea ice
Sea ice forms in the coastal waters of the Shiretoko Peninsula but notalong the Sea of Japan or the Pacific Ocean at the same latitude. This isdue to three unique conditions affecting the Sea of Okhotsk.
The first condition is the double-layered water structure of the Sea ofOkhotsk. There is a surface layer and a lower layer with a large difference insalinity. This double-layered structure with different salinity is the primaryreason that the Sea of Okhotsk is the southern limit for sea ice.

Description

The Amur, which is the longest river in northeastern Eurasia, drains alarge volume of freshwater into the Sea of Okhotsk (approx. 37% of thefreshwater supply). The freshwater creates a low salinity surface layer(salinity approx. 32.5‰ ) at about 50 meters in depth. This surface layer
is unique to the Sea of Okhotsk (Aota and Ishikawa, 1991). Therefore,
the salinity of the seawater increases significantly at about 50 to 60
meters below the surface. In addition, salt is discharged from sea ice as itforms and, as a result, the salinity of sea ice becomes far lower thanseawater. Consequently, the melting of sea ice in spring also contributes
to maintain the low salinity of the surface layer (Figure 3-3).

(sea-surface)
00
3.2

5
3.3

10 (°C)
3.4 ( % )watertemperaturelight water50
depth (m)heavy water100

salinity

150

200
densty

250
1025

1026

1027 (g/l)

Figure 3-3 The double-layer saline structure of salinity in the sea of Okhotsk(Aota, 1993)30

As sea ice forms and develops in winter, the cold brine produced by sea ice
descends and creates an intermediate cold water below the surface layer,
which is high in salinity and heavy (salinity approx. 33.5‰). In summer, thesurface layer is lighter due to the warmer water temperature from solar
radiation, and it is difficult to mix the surface layer and the intermediate
cold water. Therefore, the temperature of the intermediate cold water ismaintained around zero degree even in summer. The presence of this coldlayer of seawater is another unique characteristic of the Sea of Okhotsk.
As the ocean surface cools, the surface layer increases in density anddescends while the less dense, warmer seawater ascends. This
successive circulation of seawater continues and as the whole seawaterreaches the freezing temperature (approx. −1.8°C) it begins to freeze.
Therefore, the deeper the seawater circulates, the longer it takes for theseawater to reach the freezing temperature, and it requires longer before
the seawater begins to freeze.

Description

As described above, at depth of more than 50 meters, there is theintermediate cold water that is denser and higher in salinity than thesurface layer in the Sea of Okhotsk. Therefore, even when the cooled
surface layer of water increases in density, it does not exceed the densityof the intermediate cold water, as its salinity is lower. This creates abarrier and the surface layer only circulates within a depth of 50 meters.
Therefore, the surface water is cooled rapidly to the freezing
temperature and sea ice is formed (Sakuma, 1994; Aota, 2003).
The second condition is that the Sea of Okhotsk is surrounded by lands
and there is little exchange of seawater with the open sea. The Sea ofOkhotsk is surrounded by the Eurasian Continent, KamchatkaPeninsula, the Kuril Islands, Hokkaido and Sakhalin. Exchange ofseawater is mainly through several channels between the Kuril Islands
with the Pacific Ocean and the inflow of the Soya warm current (abranch current of the Tsushima warm current) through the Soya Strait.
As a result, the Sea of Okhotsk is relatively closed off with little
exchange of seawater with the open sea (Aota and Ishikawa, 1991). The
low level of exchange contributes in maintaining the double-layered
structure of different salinity.

Sea ice of the Sea of Okhotsk photo by Ministry of the Environment

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2004 WORLD HERITAGE LIST NOMINATION ● JAPANThe third condition is the distribution of atmospheric pressure in thearea. In winter, a high pressure system develops in the west over Siberia
while a low pressure system develops in the east over the Sea ofOkhotsk. This weather pattern causes cold air from Siberia to blow into
the area and it is very effective in chilling the seawater. Theseconditions result in the rare phenomenon of the waters around theShiretoko Peninsula freezing even though they are located at therelatively low latitude of 44° N.
As described above, the nominated site is a prime example representing
the unique mechanism of sea ice formation in the Sea of Okhotsk.
3a. 3. 2 Development of sea ice
6262
Nine percent (32×10 km ) of the global sea surface (361×10 km ) is covered
with ice in winter. Of this ice-covered area, 56 percent is located in the southernhemisphere and 44 percent is located in the northern hemisphere. Some ofthese areas are covered in ice all year around (perennial sea ice area) whileothers are covered only for a certain period of the year (seasonal sea ice area).
6262
The seasonal sea ice area accounts for 70 percent (22.5×10 km ; 15.5×10 km
62in the southern hemisphere, 7×10 km in the northern hemisphere) of the total
sea ice area (Takahashi and Shirasawa, 2002).

Description

The coastal water of the Shiretoko Peninsula is the lowest latitude ofseasonal sea ice area in the world. In fact, it is at the border between thecold zone (arctic zone) and the warm zone (temperate zone) andtemperatures would normally be quite inadequate for the formation ofsea ice. The unique feature of the Shiretoko Peninsula can be recognizedby comparing its latitude (44° N) with the famous wine region ofBordeaux (44° 50' N) and the canal-filled city of Venice, Italy (45° 30' N).
The Shiretoko Peninsula is situated south of these two locations.
In the northwestern part of Sea of Okhotsk, sea ice begins to form along
the eastern coast of Siberia, and at the mouth of the Amur River, fromearly November. Due to the effects of the northwestern seasonal winds
and sea currents (e.g. East Sakhalin Current), the sea ice area expands
southwards along the western part of the Sea of Okhotsk and reaches
the coast of Hokkaido in mid-January. By this time, the coastal waters ofnorthern Hokkaido have also dropped to freezing point and the sea ice
formed along the coastline combined with the sea ice from the northcreates an expansive ice field. The area covered by sea ice reaches its
maximum size in mid to late March, covering approximately 80 percentof the Sea of Okhotsk (Aota, 1993).
The sea ice begins to melt from the end of March, with the ice edgeslowly receding northward along the same course of its expansion, but inreverse. All of the ice disappears by the end of June and there is nopermanent ice left until the next winter (Figure 3-4).

3a. 4 The marine ecosystem under the sea ice around Shiretoko
The coastal water of the Shiretoko Peninsula is located in the lowest
latitude among sea ice area in the world. The physical changes brought
upon by sea ice and the resulting dynamic food chain supports a richmarine ecosystem.
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March 1986

January 1986

April 1986

Feburary 1986

May 1986

Description

December 1985

Figure 3-4 Seasonal distribution of sea ice in the Sea of Okhotsk (1985-1986) (Kurasawa, 2001)
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Description

2004 WORLD HERITAGE LIST NOMINATION ● JAPANSea ice photo by KURASAWA Eiichi

The base of the marine food chain is phytoplankton which consists
mainly of diatoms. Blooms of phytoplankton develop on the nutrients
supplied by sea ice. The phytoplankton become a source of food for krill
and zooplankton such as small shrimp, and then, they become food forsmall fish, benthos including sea urchins and sea squirts on the oceanfloor, crustacean and shellfish. These animals in turn become foodsources for large migratory fish, marine mammals such as seals and sea
lions and birds such as Steller’s sea eagles and white-tailed eagles. In
addition, salmon and trout that swim upstream to spawn become food forthe brown bear, red fox and Blakiston’s fish-owl.
The formation of sea ice plays an integral role in the proliferation ofphytoplankton which is the primary producer in the marine ecosystem.
Similar to terrestrial plants, phytoplankton need nutritive salts such assilicate, phosphate and nitrate in addition to sunlight for photosynthesis.
Usually, the concentration of these nutritive salts is lower in the surface
layer and higher in the deeper layers of the sea. In lower latitudes, thestrong sunlight promotes active photosynthesis in the surface layer ofthe sea. Therefore, the nutritive salts are depleted due to theconsumption by phytoplankton. Furthermore, the surface layer iswarmed by sunlight which reduces its density. As a result, there is little
circulation with the lower layers. Consequently, the nutritive salts stored
in the lower layer are not circulated up to the surface layer and the lackof nutrients restricts the proliferation of phytoplankton.
However, from autumn onwards in sea ice areas, there is an active
circulation of water due to the cooling of the surface layer. In addition,
the brine descends with the development of sea ice, which further
promotes circulation of the nutritive salts of the lower layer to the34Description

Ice algae photo by SUZUKI Yoshifusa

surface. In other words, the circulation of deep seawater causes thenutrients to be used at the surface layer in the ice sea area. In addition, it
is not totally dark under the sea ice and there is sufficient light forphotosynthesis. Therefore, the sea ice is not a negative factor for thegrowth of phytoplankton. The layer of water just under the sea ice is aperfect habitat for phytoplankton as there are high amounts of nutritive
salts, sufficient levels of sunlight, a stable temperature around minus 1.8
degrees Celsius and an adequate level of air capsules containing oxygenand carbon dioxide (Takahashi and Shirasawa. 2002; Aota, 1993).
Phytoplankton (called “ice algae”) consist mainly of diatoms which growwithin and on the bottom surface of the sea ice. The ice algae rapidly
multiply in early spring when the ice starts to melt (spring bloom)(Matsumoto et al., 2002; Shirasawa et al., 2002). Primary productionlevels are low when the sea is covered with ice. However, the food supply
becomes abundant with the bloom of ice algae. In addition, as the ice
melts along the ice edge, a type of phytoplankton different from ice
algae begins to multiply rapidly with the increase of sunlight and theorganic matter supplied by the melting ice. These primary production
activities occur earlier in seasonal sea ice areas compared with sea
which is never covered by ice (Takahashi, 2002). In fact, there arereports that the standing stock of phytoplankton in the coastal water of
Hokkaido in the Sea of Okhotsk are more than ten times higher thanthat of the Pacific Ocean side (which is not covered with sea ice)
(Furuhashi, 1980). Accordingly, the sea ice nurtures phytoplankton
which in turn feed the zooplankton, supporting a food chain that extends
to higher level consumers such as large fish, marine mammals andterrestrial wildlife.
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2004 WORLD HERITAGE LIST NOMINATION ● JAPANLarga seal Phoca largha photo by TAZAWA Michihiro

Primary production activity by organisms such as phytoplankton occur
earlier in the waters around Shiretoko Peninsula than in any other placein the Sea of Okhotsk, the melting and breakdown of ice also occurs
earlier here than in the rest of the Sea of Okhotsk. This period of highlevel primary production coincides with the shoals of fish fry and theweaning of seal pups that need a good quality and abundant food source.
As a result, the plankton plays an important role in the life cycle ofmarine wildlife. For example, fry of walleye pollack and chum salmon as
well as pups of the larga seal all feed upon the zooplankton thatproliferate with the melting of sea ice. In addition, salmon fry that swim
downstream into the ocean have been observed to follow the abundantlevels of phytoplankton and zooplankton along the ice edge to higher
latitude areas as they feed and grow on this food source (Takahashi,2002). The coastal waters of the Shiretoko Peninsula are an important
starting point of the food chain for the integrated ecosystem thatconsists of the ecosystems of sea, river and land.

Description

As described above, the coastal waters of the Shiretoko Peninsula
support an abundant food source for high-level producers and promote adiverse biota by producing a high level of food compared to thesurrounding ocean which is not covered with ice. Furthermore, this richfood source is supplied at the right time. The nominated site is anoutstanding example representing this mechanism.

3a. 5 Plants
The majority of the vegetation covering the Shiretoko Peninsula,
including the nominated site, is in virgin condition. While the altitudechanges is only some 1,600 meters from the coast to the mountain peaks,
alpine plants such as Japanese stone pine Pinus pumila and other alpine
plant communities are developed at relatively low altitudes, and diverse
vegetation is distributed vertically at the site. In addition, the flora
contains both northern and southern species and therefore, it is rich inspecies diversity.
3a. 5. 1 Vegetation communities
With regard to horizontal distribution of the forest vegetation in the low
altitude areas of the Shiretoko Peninsula, it is a Pan-Mixed Forest Zone
according to Tatewaki (1958) and consists of a mosaic of three types offorests:
1. The cool temperate deciduous broad-leaved forest with species
such as Japanese oak Quercus mongolica var. grosseserrata,
painted maple Acer mono and Japanese linden Tilia japonica,
2. The subarctic evergreen coniferous forest with species such as
Sakhalin fir Abies sachalinensis, Yezo spruce Picea jezoensis andSakhalin spruce Picea glehnii, and3. The mixed forest which is a combination of the above cooltemperate deciduous broad-leaved forest and subarctic evergreen
coniferous forest.
The fore st b elongs to the “Manchu-Japane se mixed fore st ”
biogeographic province of Udvardy which covers nearly all of Hokkaido
excluding Kuromatsunai lowland zone and areas south of this zone, as
well as southern parts of the Far East including southern Kuril Islands,
36