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Long-term changes in the aquatic biota of Lake Abashiri, a meromictic lake in northeastern Hokkaido, Japan

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Lake Abashiri is an example of a meromictic lake under the stability of upper oligohaline water and lower polyhaline water, but a brackish lake on the whole. Nevertheless, boundary depth of the hypolimnetic layer has gradually decreased with time. At the same time, this has led to an increase in salinity and eutrophication in the epilimnion and to an acceleration of deoxygenation of the hypolimnion, attributed to the upwelling of bluish water. In order to investigate succession of the aquatic biota linked to these hydrological changes, results of occasional long-term studies on the distribution of plankton and benthos were summarized, mostly from Japanese publications of lake surveys. In each result, the boundary of the polyhaline hypolimnetic layer has gradually risen from 13 to 7 m depth between 1932 and 1955, and has been determined, in recent years, to be 5.5 m. Salinity of the epilimnion was lower than 0.1‰ in 1930s, and 0.5–0.6‰ in the 1940s before 1949. Thereafter, salinity increased to 1.8–2.3‰ until recently, with levels now often over 3‰. Following these hydrological changes, the succession of planktonic fauna and flora, particularly of the dominant species, has exhibited a gradual change from freshwater to brackish or salinity tolerant species during the 72 year period from 1926 to 1997, including a period of variable species composition in 1975–1980 and a period of degradation after that. In contrast, according to available data, the benthic community has remained unchanged in terms of the composition of dominant species in all habitats during the same 72 year period. The dominance of some plankton and benthos species has been maintained over several decades, although the community structures have displayed a few phases of abundance and regression during these 72 years.
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Keywords: Lake Abashiri; hydrology; long-term changes; meromictic lake; plankton and benthos

Document Type: Miscellaneous

Publication date: 2001-06-01

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