TAMEIKE (irrigation ponds) have been constructed for the last 1,300years. The size of these ponds ranges from small to large. The majority are small. Some of the small TAMEIKE have been abandoned because new water resources for irrigation were developed. High concentrations of nitrogen were observed not only in some of these abandoned ponds but also in ponds that are currently in use. Then authors conducted a survey about these ponds. The results show that the use of watershed for these TAMEIKE plays an important rolls in regard to the nitrogen concentrations of the TAMEIKE. Therefore, watershed management for upland fields is important in order to reduce the nitrogen concentrations of the TAMEIKE. To preserve forest between the TAMEIKE and the watershed is an another importance.

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A new enumeration method for nitrifying bacteria was developed using the 2-(p-indophenyl) -3-(p-nitrophenyl)-5-phenyltetrazolium chloride (INT) dehydrogenase assay with specific inhibitors for ammonia-and nitrite oxidizing bacteria. This technique was fistly applied to artificial mixed cultures of Nitrosomonas curopaea, Nitrobacter winogradskyi and Pseudomonas fluorescens and then to environmental mixed culture samples to evaluate the validity and sensitivity of this method. Detection efficiency of nitrifying bacteria by this method was more than 1 order of magnitude and 1-2 orders of magnitude higher than that of the most probable number (MPN) method for the pure culture samples and environmental mixed culture samples, respectively. Since the INT dehydrogenase assay counts only metabolically active bacteria, the numbers of NH₄- and NO₂-oxidizing bacteria determined by this method were directly proportional to ammonia and nitrite oxidation rates. Furthermore, this INT dehydrogenase method was applied to biofilm samples for in situ identification of nitrifying bacteria. Fractions of nitrifying bacteria in the biofilm were more than 1-3 orders of magnitude higher than those determined by the MPN method, whereas the fractions were comparable with those determined by the fluorescent in situ hybridization (FISH) with 16S rRNA-targeted oligonucleotide probes. Therefore, it could be summarized that this newly developed INT dehydrogenase method was more rapid, sensitive and reliable over the conventional MPN method for environmental samples and could be applied for in situ identification of nitrifying bacteria in biofilms.

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Nine metals (Cd, Cu, Co, Fe, Pb, Mn, Ni, Sn and Zn) were studied to determine their influence on sulfate reducing activity. There were three types of metal influences. Fe and Mn did not affect the sulfate reducing activity within 20mM. Co, Sn, Cd and Pb caused a lag in the sulfate reducing reaction that depended on the concentration of each metal. Zn, Ni and Cu had significant influence on the sulfate reducing bacteria compared the other metals. When their concentration exceeded 5mM, the sulfate reducing reaction was completely inhibited.
The model wastewater containing 1.0mM-Fe or 0.5 mM-Zn was continuously treated in an anaerobic reactor, and the metal was not detected in the treated water. It appeared that the metal was held in the form of a metal sulfide in the reactor.

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A countermeasure of preventing the eutrophication of lakes and marshes is to use filamentous algae to remove dissolved phosphorus and nitrogen from the waters of rivers flowing into them. In order to establish a procedure for harvesting filamentous algae growing in artificial waterways and a effective use of the harvested algae, it is necessary to clarify the quantity which can be harvested.
In the waterways, the quantity of algae separated from the substratum which are discharged outside the waterway or which accumulate in the interior of the waterways varies for each harvest. Cosequently, it is possible to predict the harvesting yield of algae which is separated from the substratum as the function of the phosphorus content, phosphorus absorbing ability, and the constant of non-harvesting algae.
The results of calculations based on field experiments with filamentous algae have confirmed that the harvesting yield is dependent upon the water temperature, concentration of nutrient in the raw water, and the stream regime. Typical harvesting yield values were obtained. When we treated raw water of dissolved orthophosphate concentration of 30μg·l^-1 at water temperature of 20°C using a gentle-sloped waterway, we found that the harvesting yield was roughly 20g dry weight·m^-2·d^-1.

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The free water method was applied to evaluate dissolved oxygen (DO) and dissolved inorganic carbon (DIC) dynamics and the metabolic characteristics by using continuously measured DO and pH data at the center of Lake Kasumigaura (L.K.) and the South Basin of Lake Biwa (S.B.). The rates of primary production and respiration were determined by subtracting the oxygen and C0₂ fluxes due to air-water gas exchange from the rates of change in the masses of DO and DIC in the Lake. The averaged ratios of air-water gas exchange to change in the masses were 14%(DO) and 20%(DIC) in L.K. and 10%(DO) and 9%(DIC) in S.B. and those ratios were affected by wind velocities. As the results of estimating gross primary production in the daytime (8am.-4pm.), we obtained the averaged DO production rates of 8.0 (S.D. 5.6) )μM·h^-1 in L.K. and 3.2(S.D. 2.0)μM·h^-1 in S.B. The averaged DIC consumption rates were 4.5(S.D. 2.9)μM·h^-1 in L.K. and 1.9(S.D. 1.0)μM·h^-1 in S.B.. The relationships between the metabolic quotients (MQ) and N0₃-N vs. NH₄-N ratios in the lake water were verified on the basis of the smoothed time series data by using moving average.

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A field research on occurrence of Cryptosporidium oocysts and Giardia cysts was conducted at Sagami river and its tributaries. In the first survey with presumptive test (screening survey), presumptive Cryptosporidium oocysts were found in 13 (81%) of 16 samples (9 (75%) of 12 sites), ranging from 1 to 2,500 oocysts·: 100l^-1 and presumptive Giardia cysts were found in 15 (94%) of 16 samples (11 (92%) of 12 sites), ranging from 1 to 2,400 cysts·100l^-1. In the second survey conducted at selected 6 sites with confirmed test, confirmed Cryptosporidium oocysts were found in 7 (70%) of 10 samples (4 (67%) of 6 sites), ranging from 3 to 4,500 oocysts·100l^-1 and confirmed Giardia cysts were found in 9 (90%) of 10 samples (5 (83%) of 6 sites), ranging from 1 to 1,200 cysts·100l^-1.

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