Highly saline water (salinity-17 psu) in the lower layer of Lake Nakaumi often ascends along the lake bottom and flows into the Ohashi River. In this period, when the upsurge stage of internal oscillation with large amplitudes of 2-3 m appears along the western coast of the lake, a large amount of highly saline and anoxic water goes upstream through the Ohashi River. We carried out a 24- hour observation in the summer of 1996 to detect the internal oscillation and its effects on the vertical distribution of water properties, and found a dynamical relation between the internal oscillation and the anoxic water movement in Lake Nakaumi. We also performed sailing observations along the Ohashi River to examine the inflow process of anoxic water toward Lake Shinji and the dissolved oxygen consumption of the lower anoxic water. We found that the dissolved oxygen concentration showed a value below 6.0 mg 1^-1 in the whole region of the Ohashi River during the observation period, and that a rapid fall in dissolved oxygen concentration did not occur while the anoxic water passed through the river from Lake Nakaumi. The lower water of Lake Nakaumi seems to recover the dissolved oxygen concentration due to vertical mixing with the upper fresh river water at the shallow eastern mouth of the Ohashi River. It then moves upstream through the river with almost no consumption of dissolved oxygen.

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The temporal variation in emergence patterns of Chironomus plumosus (L.) was studied during 1982-1995 (except in 1987 and 1988) in the central part of eutrophic Lake Suwa. The number of emergence periods in any one year occurred two (March-May and August-September) or three times (March-May, June-July and August-September). The appearance of the emergence period of June-July appears to be dependent on the growth of larval populations during May-June from adults that had emerged in March-April. In this population, larvae that had not developed to a mature size emerged in the next emergence period.
The relationships between the emergence periods and mud temperature, dissolved oxygen concentration, chlorophyll-a concentration in 6 m water depth or sedimentation rate at 5 m depth were not clear, whereas the emergence periods were closely related to larval densities during May-June. Thus, the years which exhibited a high larval density of more than 2, 000 ind m^-2 had no emergence period of June-July in almost all cases. These results suggest that competition due to the increased density of the C. plumosus larval population slowed their growth and, consequently, the emergence period of June-July did not appear.

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Vertical attenuation of underwater ultraviolet radiation [UVR; UVA (320-400 nm) and UVB (280-315 nm) together with photo-synthetically available radiation (PAR, 400-700 nm)] was measured at 22 sites from 5 freshwater bodies of various trophic states. The factors controlling the attenuation of UVR were examined. At all the sites, underwater irradiances decreased almost exponentially with depth and were more attenuated in dimension order from UVB, UVA, to PAR. Substantial variation in attenuation was observed among water bodies. Attenuation depths (Z_1%) for UVB ranged from 0.3 m to 1 m in the eutrophic sites, while exceeding 2 m in oligo-mesotrophic ones. Z_1% of UVB and that of UVA were 0.5 and 0.9 times the Secchi disk depth, respectively. Attenuation coefficients (m^-1) of UVA and UVB showed significantly positive linear correlations with both chlorophyll a and particulate organic carbon concentrations. Dissolved organic carbon (DOC) was weakly correlated with attenuation coefficients of UVA and UVB. This disagrees with previous reports that stated DOC is the major factor controlling underwater UVR. Phytoplankton is considered to be the more important factor controlling the attenuation of underwater UVR in water bodies with high phytoplankton content and low DOC.

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In parallel with the monitoring of the distribution profiles of dissolved organic carbon (DOC) in Lake Biwa, Japan, the adsorption characteristics of DOC onto an adsorbent of hydrous iron oxide (HIO) was investigated by varying the pH of the lake water samples from 11.5 to 3.6. The first sign of the adsorption of DOC appeared around pH 8.5 ; the adsorption percentage increased sharply as the pH decreased and reached a constant value around 60% at pH 4-6, followed by a rapid decrease to 0% below pH 4.0. By utilizing the flat region (pH 4-6) of the adsorption percent, the total DOC (tot-DOC) was separated into two fractions ; the adsorption active (act-DOC) fraction and the inert (int-DOC) fraction. The latter showed a steady concentration around 0.4 mg Cl^-1 in the lake throughout a one-year period, whereas the former showed variations between 0.7 and 1.2 mg Cl^-1 depending on the season. Act-DOC was found to be reactive with permanganate ion, and the amounts of permanganate consumed for the reaction can be described as follows : the chemical oxygen demand (COD) (mg O₂l^-1)=0.97×act-DOC (mg Cl^-1)-0.50, where COD is the concentration of an equivalent amount of oxygen demanded for the oxidation. The chemical characteristics and the origin of act-DOC are discussed in light of the values of the slope and intercept of the linear relation. The int-DOC fraction had no such clear relation to COD and appeared to remain as a refractory fraction of organic carbons occurring in the lake.

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Dissolved organic carbon (DOC) in Lake Biwa and its four inflowing rivers (forestal, industrial, agricultural and urban with catchment areas) was fractionated using three resin adsorbents into six classes : hydrophobic acids, bases and neutrals ; hydrophilic acids, bases and neutrals. Hydrophobic acids (aquatic humic substances, AHSs) and hydrophilic acids (HiAs) were found to dominate in both lake and rivers. In the lake water, HiAs prevailed over AHSs (AHSs : 25-27%, 0.37-0.41 mg Cl^-1; HiAs : 40-48%, 0.58-0.78 mg Cl^-1) ; in the river waters, the opposite was observed (AHSs : 37-73%, 0.32-0.71 mg Cl^-1 ; HiAs : 23-35%, 0.11-0.45 mg Cl^-1). Especially in the forestal area, the contribution of AHSs was greatest among the rivers. The UV absorbance over DOC ratio (UV: DOC) for the lake samples increased in the order of HiAs, lake-water DOC, and AHSs. The UV : DOC ratios of both the lake-water DOC and AHSs isolated from Lake Biwa were similar to those reported as algal-derived, suggesting that a significant portion of the DOC in L. Biwa may be aquagenic and algal-derived. The UV : DOC ratio of the river-water DOC was 60% greater than that of the lake-water DOC. The Pedogenic nature of the river-water DOC was implied. However, the UV : DOC ratio of the AHSs isolated from the rivers was low as compared with those reported. It is of interest that the UV : DOC ratio of the urban-river AHSs was slightly lower than that of the lake.

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Annual and seasonal changes in the community structure of metazoan plankton in Lake Kawaguchi were studied from April 1993 to December 1995. Sixteen taxa of rotifer, 4 taxa of cladoceran and 5 taxa of copepods were identified in this study. Among zooplankton species, Kellicottia longispina, Synchaeta stylata, Bosmina longirostris, and Daphnia galeata were found abundantly. Copepod species were grouped into Cyclopoida. A comparison of the present with previous studies indicates that some cladoceran species, Bosmina fatalis and Bosminopsis deitersi, have vanished altogether. It may be concluded that the structure of the zooplankton community in this lake has changed remarkably in recent years.

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Two large-celled bacterial species, Chromatium sp. and Macomonas sp., bloom at an upper boundary of the H₂S layer of Lake Kaiike all year long. Most of the blooming cells of Chromatium sp. appeared to submit to a low level of photosynthetically active radiation of 0.2-3μ mole m^-2 sec^-1 due to a limited supply of H₂S, at which level the bacterial cells were just able to float in lake water. Even after a 1.5 year-dark incubation, the bacterium began to grow when exposed to light.

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The purpose of this work is to investigate the relationships between the stream water chemistry and the geological situations, and to examine the possibility of depicting a water quality map comparable to the geological map for the streams flowing out from mountainous regions in Nagano Prefecture, central Japan. For the first report of the study, this paper describes diurnal and seasonal variations of the major ions in the stream waters and a comparison among streams from granitic and adjacent sedimentary rock areas forming the Hida Mountains. The measurements were performed by ion chromatography for the waters, collected at 6 stations every two weeks and/or once a month during almost three years (Feb. 1994 - Oct. 1996). Seasonal variations of the major ion concentrations showed temporal decreases in spring-summer and fall, which were, however, practically small, in view of the differences among the streams investigated. The concentrations of magnesium, calcium and sulfate ions were obviously low in the waters from the granitic and high in the waters from the sedimentary rock area. Another parameter obtained by ion analyses, the difference between total base cations and total strong acid anions, i.e., [Σ (BC)- Σ(SA)], was discussed in relation to the buffering capacity of the stream waters against the current acid precipitation.

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