A limnological study of the small lakes at the foot of Mt.Shirataka-yama west of the city of Yamagata

Abstract

On the surface of the volcanic mud-flow at the northeastern foot of Mt. Shiratakayama ca. 14 km. west of the city of Yamagata in the Tohoku district, there are at an elevation of about 650 m. a number of shallow depressions, some of which are filled with water forming small lakes. Four of them are described in the present paper. They are smaller than 0.1 km² in superficies, the longest one not exceeding 0.7 km. Although the lake-basins do not exceed 6 m. in maximum depth, most of them becoming deeply depressed in a small area. The morphological characteristics of the original lake-basins seem however to be fairy well preserved, nevertheless some parts of shore have been artificially changed. The morphometric data for them are summarized as follows : phometric data for them are summarized as follows :
Name of lake Ô-numa Aré-numa Koké-numa Magari-numa
Area (km2) 0.09 0.088 0.053 0.03
Maximum length (km) 0.7 0.5 0.4 0.36
Maximum width (km) 0.3 0.2 0.3 0.13
Length of shore-line (km) 1.9 1.3 1.2 0.8
Maximum depth (m) 5.7 5.7 4.4 1.0
Mean depth (m) 2.7 3.3 2.1 0.3
Volume (km3) 0.00024 0.00029 0.00011 0.00001
Shore development 1.79 1.23 1.46 1.33
The primary productivity of these lakes is extremely small in midsummer, only Koke-numa producing a pretty quantity of Microcystis aeruginosa. The plankton is composed chiefly of animals especially of rotifers. The typical eutrophic oxygen curves obtained in these lakes in midsummer suggest that, besides the morphological characteristics of the lake basins, more algal production may be expected in other seasons, such algae as certain diatoms occurring in early spring.
The region where these lakes lie seems to be poor in nutrition, i. e. an oligotrophic region. It therefore suggests that in these lakes the later rise in productivity which was initially very low has been checked relatively for a long period. This is shown by the sediments which are composed largely of silt the brackish grey gyttja composed mainly of organic materials is deposited in the deep depression of a limited area. The deep stagnated water cut off from circulation in the upper layer kept from wind agitation due to the surrounding topography of the lake will bring about the oxygen lost in summer, but it is not necessarily due to the high productivity. The low productivity in the two lakes, Koke-numa and Magari-numa, seem to be accerelated by their waters which are humose to a certain degree owing to humic materials brought in from the surrounding peaty deposits. So far as the oxygen curves are concerned, the lakes under consideration remind us to be eutrophic in certain stage succession. They are however not of true eutrophy but may be said to be “pseudoeutrophic” due to the morphological features of the lake-basins and the topographical peculiarities of the surrounding areas.