A naidid oligochaete, Ophidonais serpentine (MÜLLER, 1773) is redescribed based on specimens from Lake Yunoko, central Japan. This is the first record of this species from Pacific Asia. Among 1988 specimens, eleven species of oligochaetes were recorded from Lake Yunoko. Ophidonais serpentine was abundant in the littoral zone, but disappeared in the deeper profundal. Tubifex sp. and Limnodrilus spp. (L. hoffineisteri and L. claparedianus) composed 86-100% of the oligochaete assemblage except at the central eastern inlet. The oligochaete densities were higher in the northern basin, accounting for several tens of thousands of individuals m^-2 in the upper profundal.

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Changes in constituents of the bacterial assemblage in the pelagic zone of Lake Biwa were surveyed monthly by using several different media for MPN (most probable number) counts. LW medium (lake water medium) for MPN counts was prepared with the same lake water samples as for the inoculum samples. LWd medium (different lake water medium) for MPN counts was prepared with previously sampled lake water. All media for MPN counts were sterilized through filtration.
In most cases, LW medium for MPN counts provided the higher cell numbers at each sampling time, as compared with cell numbers of LWd medium. However, LWd medium provided almost the same cell numbers as LW medium, whenever the previously collected lake water for LWd medium contained the same dominant phytoplankton. LWd media, in which other species of phytoplankton dominated, provided extremely low MPN values.
These facts suggest that successions of phytoplankton cause seasonal changes of bacterial assemblage structures through EOC.

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The temperature, conductivity and turbidity of lake water are very important indices of a lake environment. Conductivity reflects the total amount of concentration of major ions (Na⁺, K⁺, Mg²⁺, Ca²⁺, Cl^-, SO₄^2-, HCO₃^-). Turbidity correlates the concentration of SS (Suspended Substance). A TCT-profiler was designed for monitoring of a lake environment. It is able to quickly measure temperature, electric-conductivity and turbidity at every 0.5m interval in a water column. A high speed boat equipped with the TCT-profiler makes it possible to collect data over a wide area of a lake. Measured data are automatically memorized and stored within the sensor unit, as the profiler, suspended from an ordinary rope, moves up and down beneath the lake surface.
Observations were carried out at several locations in Lake Biwa, and useful data were obtained. Turbid water due to heavy rains was transported from the Ane River into the center of lake by the lake current (cyclonic gyre). The transportation speed of turbid water was estimated over a two day period at about 5cm⋅sec^-1, which corresponded to the gyre speed.

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Ferromanganese crusts were sampled from the surface of a stone collected at a depth of 20 m in the northern part of Lake Biwa, Japan. These samples were analysed for 37 elements by neutron activation, X-ray fluorescence, and ICP-AE. The crusts were found to be enriched with Ba, P, B, As, and sometimes with Co, Ni, Cu and Sb. The elements were classified into 4 groups based on the varieties of host minerals (Fe-oxides, Mn-oxides or allochthonous materials) in which they were incorporated : elements mainly associated with 1) Mn-oxides : Ba, Ni, Cs, Sr and Co ; 2) Fe-oxides : P, B and As; 3) allochthonous materials : Na, K, Rb, Al, Ti, Sc, Hf and Th ; and 4) Mn-oxides plus allochthonous materials : rare earth elements and major heavy metals. The elemental compositions in the Lake Biwa concretions, including the crusts and Mn-deposits studied previously by these authors, were compared with those in other freshwater and oceanic concretions. As a result, the concentrations of rare earth elements and major heavy metals were found to be much lower, whereas those of B, P and As were higher in the Lake Biwa than in the oceanic concretions. These differences could be well explained in terms of the effects of sea salt, growth rates of the concretions, and pH of the formation environment.

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The chironomid fauna was studied in the emergent plant zone of Lake Kasumigaura in May, August and November of 1986. A total of 2, 181 adults were reared and emerged from plant samples and sediments in the emergent plant zone. We identified 1957 specimens belonging to 31 species of 18 genera. Ten species were abundant, of which five were recovered from plant samples, two from sediments and others from both. Species composition did not differ much from May to August, which might have corresponded with the change of in situ larval density. The abundance of the chironomids was highest in May and low in August for plant samples, while it was highest in August for sediments. On plant samples, it correlated with the chlorphyll a amount on the reed stems, which was higher in May than in August. In November, only two species predominated. Species composition differed more markedly among substratum types than among localities in the lake. Some chironomid species seemed to select specific sediments. Chironomid fauna in littoral hydrophyte zones was more diverse than in pelagic zones in Lake Kasumigaura.

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Source of suspended sediment was identified from grain size distribution in two headwater basins in West Central Hokkaido, Japan. The fine grains in river bed sediment were supplied to the river water as suspended sediment in the early snowmelt season, whereas those in hillslope sediment were washed out into the rivers in the summer rainy season. The fine grains probably accumulated in the bed sediment during winter, and were stirred up into river water as the river discharge increased. Since the fine grains in the river bed sediment were gradually exhausted, further stirring up could occur only when the river discharge exceeded that season's previous maximum. The fine grains in the hillslope sediment were most likely to be stored during the dry period from May to July, only to be washed away during the summer rainy season. Thus the suspended sediment supply from the hillslope is sparse in the snowmelt season. This type of seasonal variation in suspended sediment supply is probably common to headwater basins in the cold and humid climate of West Central Hokkaido, Japan.

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A study was conducted on seasonal fluctuations in water quality, environmental factors and bacterial density in the river water and epilithon on the riverbed at Koremasa, marking the mid course of the Tamagawa River in Tokyo. Water quality and environmental factors at Koremasa were largely influenced by the fluctuation of discharge at the study site. Consequently, the amount of P-compounds was correlated with the discharge summed for 10 days before the sampling day. The amount on the epilithon, however, fluctuated with discharge only on the sampling day. Thus, the turnover of P-compounds was thought to be slower than that of other elements in the river water. Principal component analysis of water quality and environmental factors showed that the first principal component was composed of the TDP, PO₄-P, NH₄-N, DOC, water temperature and ignition loss of the epilithon. The bacterial density in the river water and epilithon was determined by the acridine orange direct count (AODC), and plate count methods on media of three different compositions. In spite of marked fluctuations observed in bacterial numbers determined using these agar media, very slight fluctuations in the AODC were evident throughout the year. The bacterial densities calculated using each plate as a percentage of AODC were larger in the epilithon community than in flowing river water. The bacterial community in the epilithon on the riverbed must therefore be actively growing in the river ecosystem.

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