諏訪湖集水域生態系研究

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The purpose of our project is to clarify the mechanism of eutrophication in lake, and to gain a better understanding to the relation between a natural ecosystem and human activies. Lake Suwa is a shallow eutrophic lake located in a fault basin in the central part of Japan. This lake was categorized as a mesotrophic lake at the beginning of the 20th century, but gradually the trophic level became higher as the human activites in the watershed area became more active. Since 1960's the high growth of Japanese economy has exerted a great influence upon Lake Suwa and brought such a change in the water condition that it is now classified as a hypertrophic lake. As a result of eutrophication, the heavy water-bloom has been caused by Microcystis, a kind of the blue-green algae, in every summer. In order to cope with this situation, the inhabitants around the lake started to inquire into the possible measures to clean the lake in 1966, and wosked out a plan of waste water disposal through a sewage system, and finally in October 1979, part of the sewage plant started to operate. In 1982, the sewerage construction of lake Suwa area covered 733 ha, which was corresponded to about 16% on the plan, and collected the domestic water of about 44,000 inhabitants, was corresponded to 23% of the plan. At present the sewage plant have treated the water of 23,000m³ in a day. Judging from the inflow amount of waste water into the sewage plant and the water quality of the inflowing, the reduction rates of total nitrogen and total phosphorus into the lake were about 26% and 30%, respectively. The following studies were carried out by the three main research subteams. The first team dealing with the lake water concentrates the detail study on the present state of lake Suwa. The routine field study have been carried out at 10 days' interval in each year (1977,1980～1986) and monthly in the second and third fiscal year (1978 and 1979). The contents of the routine field study are follows; the primary production and the community respiration, the standing stock of the biomass (phytoplankton, zooplankton, bacteria and benthos), the nutrients, and the other environmental parameters (Kurasawa, 1987). Furthermore some experimental works (the evaluation of biological available nutrient by the new bioassay technique of the modified BOD method, and the release rate of nitrogen and phoshorus from the bottom sediment, and so on) have been also done on lake Suwa. The second team dealing with the streams in the catchment area concentrated the studies of the lake and concentrated the studies of the nutrient loads to the lake and of the role of the aquatic insects and the community of sessile microorganisms (bacteria, algae and so on) on the stream bed. And the vegitation of riverside was listed and analyzed the function as nutrient trap. The third team dealing with the terrestrial ecosystem concentrated the studies on the natural loads of the nutrients from grasslands and forests, and the artifical loads of the nutrients from grasslands and forests, and the artificial loads from the agricultural fields and the urban areas. The catchment area was divided into 10 differest drainage areas and the estimation of the leaf standing stock on each area has been carried on. And on the three different forests (Abies, Quercus and Larix), the works of the decomposition rate, the soil respiration, the standing stock and the activity of the soil respiration, the standing stock and the activity of the soil animals and so on, have been carried on. In addition to above activities, another many works have been carried on in the catchment area such as the estimation of the wash-out nutrient load from the diffent vegitation, the climatological observation, the simulation analysis of the lake ecosystem and so on. The results taken by our regular observation at the lake center from 1977 to the present time are as follows; (1) the minimum and the maximum value of transparency were 30cm and 154cm in 1977, respectively. In 1981 and 1982 after starting to operate the sewage plant, the transparencies were 51-195cm in 1981 and 35-170cm in 1982. The maximum amount of chlorophyll-a in summer was 1,150mg/m² in 1977, but they were 490 mg/m² in 1981 and 671mg/m² in 1982. According to our previous report, the results of our simulation studies showed that the transparency will be improved from 30-40cm in summer of 1977 upto 70-80cm, when the nutrient loading into the lake are reduced by about 50% of total nitrogen and about 70% of total phosphorus. At the same time it showed that the maximum amount of chlorophyll-a can be reduced by half. However, the reduction rate of both nutrients did not yet attain the rate of goal in 1982. It is necessary to watch the progress of the change of lake. (2) In summer the dominant species of algae was unchanged, but the blooming of Anabaena spiroides occurred a short period in the beginning of summer season since 1980. (3) In 1977 and 1978 at the peak of the standing crop of Microcystis is the summer the amount of dissolved inorganic nitrogen has become extremely small. On the contrary, the amount of reactive phosphorus (phosphate phosphorus) tends to remain unchanged. Since 1980, however, the tendency of the diminish of the dissolved inorganic nitrogen in summer did not clear, and the low level of the dissolved inorgaic nitrogen and reactive phosphorus tended to remain. (4)Before starting to operate the sewage plant the annual gross production were 756, 699 and 777gCm⁻² yr.⁻¹ for three years, and the energy transfer efficiencies were 0.61-0.73%. While, after that the annual gross production were decreased by 576gC in 1981 and 629gC in 1982, the efficiencies were 0.48% and 0.55%, respectively. Annual sedimentation rate was calculated by using in situ method and they were 86.9gCm⁻²yr⁻¹, 16.0gNm⁻²yr⁻¹ and about 2gPm⁻²yr⁻¹. At the present stage, there are many points which are uncertain. It is necessary to continue observation of lake and to discuss with climatic condition such as solar radiation and precipitation.
信州大学理学部の論文
1987-12-05

著者

倉沢秀夫
東邦大学理学部生物学教室

諏訪湖集水域生態系研究

スポンサーリンク

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