松本城の濠のプランクトン群集の現状とその浄化対策

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Castle or even the ruin of the castle, which is usually situated at the center of the city, is the symbol of the citizen. Thus the moats of the castle consist aquatic environment in downtown supposed to be an oasis. However, this symbolic aquatic environment is often very eutrophicated. It is no exception for the case of Matsumoto castle. The first limnological investigatin to find out some counterplans against the pollution of moats was carried out from 1973 to 1975. Following the observations for three years, we proposed to reduce the sources of pollution as (1) to stop the inflow of sewage, (2) to supply fresh undergroundwater, (3) to minimize the supply of foods for fish. Thereafter the water quality recovered to some extent and the bloom of Microcystis almost disappeared from the moats. However, it has been indicated these years again that the dense bloom of diatoms spoils the view of the castle. Thus, the second serial limnological investigaion was performed from 1987 to 1990 in order to purify the water for the better lanndscape of the castle. Investigation was mainly performed to reveal the efect of some trials of purification and the changes of water quality and plankton community. Profiles of the moats The castle tower is surrounded by five moats including a small one of name F. The surface area of moat AB, which was formerly separated to moat A and B, is 4,407m², moat C is 3,021m², moat D is 5,333m², and moat E is 13,699m², and moat F is 1,158m². All of these moats are very shallow with the mean depth of ca. 0.6～0.8m. The COD concentration of the moats, being 10.9 to 25.4 ppm, was far beyond the category A of the environmental standard for lakes. The transparency of the water decreased both in spring and autumn according to the increasing in the density of palnkton. Observations and Experiments Limnological observations mainly on planktonic community were carried out from March 1987 to October 1990 nearly every month. Experiments for purification were performed in moat D, AB and C. The purification experiment by using submerged stones was first done in moat D in April 1987 then in moat C in the next year. Plankton community Phytoplankton 1. The number of the species appeared in the moats ranged from 26 to 41, 31-46, 23-37, and 19-27 for 1987, '88, '89, and '90, respectively. The maximum number of the appeared species was observed in 1988 and the minimum was in 1990. Diatoms were the dominant phytoplankton which appeared 25 species. Green algae of less then 10 species followed. The number of the species decreasd with year in every moat and a radical decreasing was observed in moat D. 2. Diatoms dominated in winter and others (Green algae, blue-green algae, and flagellates) were abundant in summer. The monthly appeared species number was within 5 to 15 and it fell below 10 in 1990. 3. The dominant species in cell number was Cyclotella kiitzingiana which amounted to 103～104 cells/ml in 1987. The following phytoplankton were Synedra acus and Scenedesmus quadricauda. Aphanocapsa pulchra and Anabaena spiroides temporary bloomed in June in moat AB. The same was found in 1988, but the temporal bloom was brought by Pandorina morum in moat AB and C in July. Cyclotella kiitzingiana bloomed again for a long period in 1989 and 1990 except for moat D. Spirulina jenneri bloomed for a short period in summer 1990 in moat AB. 4. Seasonal changes in cell number of phytoplankton varied large particularity in moat AB St.2 from 300 to 114,800 cells/ml while it was very small in moat D from 2,200 cells/ml in 1987. The maximun, density was above 1,000 cells/ml, while the minimum was less than 100 cells/ml in general. The largest cell density was observed in different months for different moats; in July for moat AB, in November for moat C, and in May for others in 1989. 5. The water bloom caused by Cyclotella kiitzingiana was sometimes observed in moat C and E, though it was not so dense. 6. The cell volume of each phytoplankton varies so large that the dominant phytoplankton in terms of biomass is different from that of cell number. Although the number was insignificant, the large phytoplankton as Synedra ulna, Pandorina morum, Englena g. occasionally became the dominant species in terms of cell volume, while Cyclotella kiitzingiana predominated both in number and biomass. 7. The biomass of phytoplankton was generally high during summer except for moat D. The largest average biomass throughout the observation period was found in moat AB of 2,584 mg/m³ in 1987, while the smallest was in moat D of 1.1 mg/m³ in 1990. The order of the bionass was as follows, AB>C>E>F>D in general. 8. The density of phytoplankton around the point of inflow of fresh groundwater was well diluted, thus there observed heterogenity in the horizontal distribution of phytoplankton. Zooplankton 1. The number of the species appeared in the moats ranged from 6 to 14, 9-17, 7-16, and 5-11 for 1987, 1988, 1989, and 1990, respectively. The yearly change of the number of appeared species of zooplankton was the same as for phytoplankton. 2. The predominated zooplankton in individual number was Brachionus calyciflorus throughout the observed years and in almost all the moats. Charchesium polyopinum temporary dominated in moats AB and C in April 1987. In 1988 the following species appeared dominantly; Colpoda cuculus for moat AB in July, Keratella cochlearis for moat C in May, Stylonichia sp. for moat D in August, and Trichocerca iernis for E in August. In 1989, Brachionus calyciflorus predominated in every moat except for moat D. Brachionus angularis became dominant exceptionally in moat C in 1990. 3. The maximum number of zooplankton in every moat ranged from several tens to a few thousands was observed in summer so far examined. 4. There found little difference in the dominant species between individual number and biomass of dry weight. Brachionus calyciflorus, being 10 - 100 mg/m³, was the dominant zooplankton throughout the observation period and in almost all of the moats. 5. The maximum biomass of zooplankton varied seasonally from 5.4 to 68.4 mg/m³ for 1987, 12.8 - 393.6 for 1988, 12.4 - 452.8 for 1989, and it became large in 1990 being from 13.6 to 1254.9 mg/m³. In most cases Brachionus calyciflorus was the dominant species, while e.g. Mesocyclops leucharti dominated in September in moat AB, Alona affinis in moat D in April, Polyarthra trigla in September in moat E, and Mesocyclops leucharti in September in moat F in 1987. 6. The holizontal distribution of zooplankton in these small moats are to be small by their high mobility. 7. No clear relationship in the biomass between zooplankton and phytoplankton was found probably because of the fluctuation of environmental conditions of moats due to their small capacities. The effect of the supply of fresh groundwater appeared clearly in moat D. When the water supply raised up to 2,800 tons/day of ca. three times of the former treatment, the phytoplankton biomass decreased one-fourth within one month. However in case of moat AB the effect did not appear so clearly for the whole moat, probably due to the structure of the moat as the gradient of the bottom level and the very shallow water depth where the recycling of nutrients from the sediment is to be quite significant. For the moat C the water supply was only 200 tons/day, so the effect of fresh water supply did not appear yet. It was confirmed from the experiments that if the turnover time of the water was minimized below 3 or 4 days, the transparency of the water increased over 30 cm in a few or several weeks and the following treatment could purify the sediment to some extent. Thus, the enlargement of the capasity of water supply is to be solved. It was revealed that the submerged stones acted as a biofilm to scavenge small algae if there is significant water flow. It is thus a supplemental method to purify the water quality, although the possibility to be utilized as a new habitat for the small organisms as larvae of firefly remains. In concluding, we suggest the following four points for the further purfication and recovery of the landscape of the castle: 1)dredging or at least the improvement of the quality of sediments, 2)enlargement of water supply, 3)reducing the turnover time of the water by rearrangement of water current, 4) publicity through the explanation the importance of the landscape and ecology of moats in particular to the pupils and students of elementary and junior high schools.
信州大学理学部の論文
1991-08-29

松本城の濠のプランクトン群集の現状とその浄化対策

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