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Title Limnological Studies of Lake Yogo-Ko (II) Title Limnological Studies of Lake Yogo-ko (II) Author(s) HORIE, Shoji Bulletin of the Disaster Prevention Research Institute (1967), Citation 17(2): 31-46 Issue Date 1967-10 URL http://hdl.handle.net/2433/124736 Right Type Departmental Bulletin Paper Textversion publisher Kyoto University Bull. Disas. Prev. Res. Inst. Kyoto Univ., Vol. 17, Part 1, No. 122 October. 1967 Limnological Studies of Lake Yogo-ko (II) By Shoji Hoktc (Manuscriptreceived Aug. 15, 1967) Abstract The secondpart of the writer'slimnological studies of LakeYogo-ko, deals with the Neolimnology,based on his monthlyobservations during 1961-4962. The writeralso givesa historicalreview of limnologicaldata for the purposeof discerningthe real situationof lakeeutrophication or waterpollution, or both,in LakeYogo-ko. Historical Review of LimnologiealStudies of Lake Yogo-ko In recent years, changes of natural water quality have been becomingre- markablein connectionwith the increaedactivity of people. In addition, changes of climate affectthe physics of the lake causing changes in its chemistry and the biota in it. One proof is given by the change in the transparencyof Lake Yogo-koas the writer has shown in Fig. 1". Thus, for purposes of discerning the real situation of lake eutrophicationor water pollution, limnologicaldata of the past are particularly important. On the basis of it, we can try to discuss the transition in lake typologyand then make some recommendations concerningthat problem. Of course, sporadic data of past observationsis less significantfor a detailed discussion"and therefore only a general trend can be discerned. The chemicalstudy of this lake's water was brieflymentioned by Yoshimure. He made analyses of surface water, taken on September27, 1930,as follows: pH Si02 Cl Ca KMn04 cons. 6.9 3.3mg/1 6.0 mg/1 4.7mg/1 10.5mg/1 0.175mg/1 Yoshimuras,"supplemented the followingchemical data of water samples, taken at the same date. Si02 Fe203 Mn CaO 0 m 2.3 mg/1 1.0mg/1 0.04mg/1 (1.2) 9 3.8 1.6 0.8 12 10.1 20.0 1.8 A year later, he publishedthe remainingdata on water which was also sampled on September27, 1930. NI13 N205 Alb. NI13 Sum. P205(soluble) 80mg/cbm. 15mg/cbm. 80mg/cbm. 175mg/cbm. 60?mg/cbm. On th basis of such data on nitrogenouscompounds and phosphate,he regarded Lake Yogo-koas a mesotrophiclake5•e). In addition,Yoshimura" denoted 1.0 mg/1 Ca0 and the chemicalcomposition of water sample on Sept. 27, 1930. 32 S . RORIE / a , • I• to° , ' 4 - __ _1423L erli% \it. • '•.. HAI 3- 04/ I 0 JAN. FEB. MAR. APR. MAY JUNE JULY AUG . SEPT.OCT NOV. DEC. Fig. 1. Figure showingthe changein the figureof transparency for Lake Yogo-ko(Compiled by Horiefrom various sources. including his observationaldata of 1961-1962).(After Horie unpublished") Free CO2(mg) Fixed CO2(mg) Ca (mg) m 8.4 7 4 . 6 9 5 . 2 12 4 . 2 12 10.8 24 3 . 9 According to the later data puplished by the same author'', the nitrogen content of surface water of Lake Yogo-koon December 15, 1930 was 0.07 mg/1. As for the physics of this lake, Yoshimura°' cited the data obtained by the Shiga Fishery Experimental Station on August 15, 1928. Transparency was 5.5 m and VA/h was 83.5. Water temperature of the upper hypolimnion 16.6°C. Thermocline 5-11 m Thermocline gradient 1.8t/m M de ax= (falayer 7-8m -7 gradient 3.5°C Thermal type Ib As he listed'°', the winter water temperature obtained by the same Station on February 3, 1928 showed the same figure of 3.8°C throughout the whole water mass and transparency was 4.0 m. Besides, the other summer data, August 8, 1930, given by Yoshimura"' is as follows ; Principal thermocline position 5-12 m e, gradient 1.7°C/m Inflexion layer position 8-10 m gradient 3.3t/m Thermal type C II However, continuousobservation of the limnology of Lake Yogo-kowas carried out by Miyadi and Hazama"). Between late October and the end of March, Limnological Studies of Lake Yogo-ko (11) 33 the water temperature was almost or completely uniform throughout the lake. The pH of lake water also showed the same figure at each level of depth bet- ween late October and the end of February, though it indicated an acidic tendency in February. At the end of March, figures between depths of 0 m and 10 m indicated 6.8-7.0, though below 11 m they remained at 6.6. Between late October and the end of March the water kept a sufficient amount of dis- solved oxygen, which makes it possible to deduce that there was continuous circula- tion throughout the winter. The lake stratified conspicuously during the summer. At the end of May, the thermocline developed evidently and the decrease of dissolved oxygen in the hypolimnion began. Such a tendency became more obvious as the season advanced. Eventually dissolved oxygen below the depth of 11.5 m was completely depleted and there was little between 8 m and 11.5 m. Thus the thermocline was found at a depth of about 8 m in mid-summer, and pH was the lowest between about 8 m and 10 m. Transparency was maximum in August (8 m) and two minima occurred in both October and March (3.0 m and 3.1 m respectively). In other months it varied between 4.2 m and 5.5 m. These physico-chemical features of the lake offered basic data for their study in relation to the bottom fauna which indicate the eutrophication of lakes ; Miyadi"' classified Lake Yogo-ko as a Corethra (with Chironomus plumosus) lake. Such data obtained in 1930 is important for the writer's present study, since it shows the limnological characteristics of Lake Yogo-ko in its natural state, not after its artificial deformation. Tanaka's".15'16' recent papers on the chemistry of the lake water showed that the amount of total Fe, Fe", Mn, P, and Si increased remarkably (Fe" 6.85 mg/1, Mn 2.25 mg/1, P dissous 0.66 mg/1, Si dissous 6.1 mg/1), as the amount of dissolved oxygen in the profundal part of the lake decreased in mid-August. In the writer's opinion, it may be presumed that the redox potential in the mud- water interface drops in mid-summer and the black color of the bottom mud which was discovered by Miyadi during the summer stagnation period is a proof of the state of reduction. Conspicuous absorption of the oxygen of the bottom mud which was proved by Miyadi's study coincides with the existence of anaerobic stratum in this lake. The other important study was carried out by Koyama who made analyses of water and mud which were obtained in this lake on August 21 and 23, 1951, though this data is still unpublished (Table 1, 2, 3)*. At the same time, Yamagutim found the transparency to be 3.5 rn ; it evidently decreased during the last twenty years, when we compare this with the figures given by Miyadi in 1930. Yamaguti concluded that a figure of 3.5 m at that time was reasonable because the compensation depth, which was esti- mated from the lower limit of vertical distribution of higher aquatic plants, almost coincides with twice the depth of transparency"). Negoro's' also noticed that Lake Yogo-ko showed water blooms consisting of Lyngbya limnetica Lem- mermann and Anabaena macros pora Klebahn in late August. He mentioned that about 60 % of the diatoms found in Lake Yogo-ko were common to those of Lake Biwa-ko, ten species among them being found only in Lakes Biwa-ko and Yogo-ko. It is of interest that Ste phanodiscus carconensis, Melosira ilahca, and * The writer is much indebted to Prof. T. Koyama who gave permission for his data to be used in this paper. TABLE 1. Chemical Composition of lake water of Lake Yogo-ko (Aug. 21, co 1951, at St. 1), cited from the unpublished data of T. Koyama (mg/I)..‘,. unfiltered Depth m Ca Mg CI SO,Fe1Mn Al Si02 P NHeN NO2-N NO3-N KMnO, cons. Fe"Fetotalsoluble 0 2.94 1.28 6.66 1.09 <0.01 0.215 0.042 <0.02 0.223 3.86 <0.01 <0.02 <0.001 <0.02 12.2 2.5 - - - 1 - <0.01 0.244 0.047 <0.02 0.353 4.78 <0.01 <0.02 <0.001 <0.02 10.0 5 2.90 1.33 7.00 : 1.09 ' <0.01 0.265 0.070 0.12 0.243 5.27 <0.01 <0.02 <0.0015 <0.02 11.2 7.5 - - - - <0.01 0.320 0.094 0.17 0.302 5.78 <0.01 <0.02 <0.001 <0.02 10.3 103.40 1.44 6.58 1.871 2.66 3.58 1 3.28 2.05 0,288 7.07 0.392 <0.02 <0.001 <0.02 11.0 TABLE 2. Chemical composition of lake sediments of Lake Yogo-ko(Aug. 21, 1951 at St. 2), cited from the unpublisheddata of T. Koyama (mg/I). unfiltered De ( .4 pth cm Ca Mg CI SO, 1 Fe Fe Mn Al SiO2 Fe' total soluble C) . A 0 3.49 1.50 - 0.43 6.85 - 6.90 2.25 0.438 13.0 Exi 5 3.07 1.97 6.90 1.20 6.55 - 7.70 2.18 0.302 10.0 10 - - - - 6.12 - 7.05 2.17 0.287 10.0 25 3.60 1.63 6.79 0.98 6.02 - 6.95 1, 2.02 0.263 10.0 50 - - - 6.02 - 6.18 2.12 0.284 9.2 TABLE 3.
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