Growth of shoots and the in situ turion formation rate of Ceratophyllum demersum wereinvestigated in the shallow eutrophic Lake Sagata (37°49'N, 138'53E), Japan. Almost 90% of the young shoots in spring originated from turions. Active growth of main shoots was observed from June to the end of July, with a growth rate of 0.98 cm day^-1main shoot^-1;that of the total shoot (including lateral shoots) was 5.21 cm day-1. Auto-fragmentation was observed at the end of August, and shortened shoots floated on the water surface. Water temperature at the onset of growth was suggested to be one of the more important factors for growth. Shoots had turions at the apices from October. The in situ rates of turion formation from shoots ranged from about 20% to a high of 55% from main shoots, against about 10% from lateral shoots. C. demersum produced 35.7 turions per 1 g dry weight of plant material.Fragmentation of shoots and turion formation on them are important means of distributing this species to new habitats.

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A new Eucyclops species similar to E. serrulatus and E. speratus is described as Eucyclops roseus.The species seems to be distributed widely in Europe and Asia. The confusing taxonomical status of E. serrulatus and E. speratus in Japanese waters has been partly solved by the recognition of this new species. Eucyclops speratus s. str. does not occur in Japan, while E. roseus, E. serrulatus, and the speratuslike species complex are distributed from the Ryukyu Islands to Hokkaido. This species complex may be divided into several taxa;one is distributed mainly in the northern half of Japan, and the others mainly in the southern half. The habitats of E. roseus and the E. speratus-like species complex in Japan are ponds, lakes, and the middle and lower reaches of rivers and streams, while E.serrulatus is generally restricted to mountain waters and springs.

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The trophic status of Lake Akimoto, which was formed together with sister Lakes Hibara and Onogawa by the volcanic eruption of Mt. Bandai about 100 years ago, is judged to be on the boundary between oligo- and mesotrophic based on some physicochemical parameters. This trophic status is similar to that of Lake Hibara, while Lake Onogawa is on the boundary between meso- and eutrophic. The difference in maximum depth (33, 31 and 21 m for Lakes Akimoto, Hibara and Onogawa, respectively) probably caused these trophic differences, which have resulted in some limnological differences among the lakes. Apparent hypolimnetic ammonia regeneration is 6-14 times greater in Lake Onogawa than in Lakes Akimoto and Hibara. Hypolimnetic dissolved oxygen was not exhausted in Lakes Akimoto and Hibara, while an anoxic condition prevailed for two and a half months in Lake Onogawa. Hypolimnetic nitrate was not depleted by nitrate reduction in Lakes Akimoto and Hibara, while it was completely exhausted for three months in Lake Onogawa. Apparent nitrification is indicated from the depth-time diagrams of nitrate in Lakes Akimoto and Hibara, which is not the case in Lake Onogawa.
These three lakes are peculiar in that their lake histories are clear, they have the same age and they lie in a similar environment, but their trophic status is different. Thus, these lakes serve as good fields for comparative studies of many aspects of limnology.

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The rates of methane production and sulfate reduction were investigated in the profundal sediments (a water depth of 29 m) of Lake Kizaki from April 1992 to July 1994. The rate of methane production on an area basis ranged from 0.81 to 5.55 mmol m^-2 day^-1 (average 1.89 mmol m^-2 day^-1.) The most active layer. of methane production was often found in the deepest layer (9-12 cm) examined. Relatively high activities of methane production were observed from summer to early autumn.
The sulfate reduction rate on an area basis was from 0.028 to 0.26 mmol m^-2 day^-1 (average 0.119 mmol m^-2 day^-1). The rate was highest in the surface layer and decreased with depth, in contrast to methane production. The rate of sulfate reduction was relatively high in spring and summer and low in late autumn and winter.
The contribution of methane production to anaerobic mineralization was much greater than that of sulfate reduction due to a low level of sulfate. Methane production seemed to be mainly limited by low temperature, but not by substrate supply.

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Pesticide residues in rainwater were investigated for the estimation of pesticide flux into a lake in Ibaraki Prefecture during the 1990 rainy season. The maximum concentration of oxiadiazon in the rainwater was 4.2μg l^-1. The maximum concentrations of fenobucarb, fenitrothion and edifenphos in the rainwater were 0.58, 0.53 and 0.36μg l^-1, respectively. The annual ratio of fenitrothion redeposition by rainfall was estimated at 1.5% of the total shipping amount in Ibaraki Prefecture. There was no significant difference between redepositions of pesticides by rainfall and shipping amounts in the prefecture (α= 0.05). Pesticide residues in lake water were investigated in Lake Nakanuma, Ibaraki Prefecture, from March to December 1990. The concentrations of molinate, simetryn, iprobenfos, and thiobencarb in irrigation canal water were in the high range (>1.2μg l^-1). The concentrations of diazinon, malathion, and fenthion in the water were at a lower level (<0.2μg l^-1). The maximum concentration of simetryn in the lake was 0.91μg l^-1. Except for chlornitrofen, the maximum concentration of pesticide in the lake (<1.0μg l^-1) was about one-tenths in that of irrigation canals. The amount of pesticide from rainfall was a major factor from the viewpoint of estimating the environmental viability of the lake.

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The origins of freshwater red tides of Peridinium bipes f. occultatum which occur in many reservoirs in western Japan were analyzed from the view point of physiological characteristics for growth.
The optimum light intensity of P. bipes was 130μEm^-2 sec^-1, and this value was relatively high among freshwater phytoplankton. The optimum concentrations of phosphorus, nitrogen, calcium and magnesium for P. bipes growth were compared with those for other blooming freshwater phytoplankton. The optimum concentrations of phosphorus and calcium for P. bipes were lower than those of blooming phytoplankton except for Uroglena americana. These characteristics indicated that P. bipes is capable of forming freshwater red tides in many reservoirs in western Japan.
The maximum specific growth rate (μmax.) and half saturation constant (ks) calculated from the MONOD equation were compared with those of P. gatunense and 4 species of diatom. The maximum specific growth rate of P. bipes was lower, and its ks was higher than those of the phytoplankton. The maximum value of the measured specific growth rate was 0.171d^-1, which was lower than those of other known phytoplankton. The characteristics of these growth parameters of P. bipes are not necessarily advantageous for growth when compared with other coexisting phytoplankton.

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