We investigated the major chemical and stable isotopic compositions of the Sugao Marsh and pore waters in the sediment under the Marsh in Ibaraki Prefecture, central Japan. We also examined the mineral assemblage of the sediment to verify anthropogenic pollutant sources and the formation mechanism of the water quality.
The high concentrations of T-P, T-N and COD in the river waters indicate its ongoing eutrophication. The increase in Na⁺ concentrations together with arise in the concentrations of Cl^- in the waters can be attributed to sodium chloride from domestic sewage. A linear correlation between Ca²⁺ and HCO₃^- concentrations in the waters can be explained by the dissolution of calcite being supported by the chemical equilibrium calculation results showing that the water is undersaturated with calcite. The δ¹⁵N values of nitrate and δ³⁴S values of sulfate in the waters ranging mainly from + 11.2 to + 15.4 ‰ and from + 4.0 to + 7.0 ‰, respectively, confirm that both the nitrogen and sulfate contamination originated from domestic sewage. Additionally, the oxidation of pyrite in the sediment acts as a source of sulfate and iron in the Upper Marsh. This interpretation is in close agreement with supersaturation of pyrite confirmed by the chemical equilibrium calculations.
In the pore waters of the Marsh sediment, the NO₃^- and SO₄^2- contents are rapidly reduced, while the Fe content rapidly increases from the bottom of the Marsh up to a depth of 24cm. This appears to be due to the organic matter decomposition processes by which once oxygen has been depleted in order of energy efficiency by nitrate, iron and sulfate reductors. Moreover, the rapid increase of Ca²⁺ content and the equally rapid decrease of Na⁺ content in the pore waters between 24 and 100cm in depth can be explained by the cation exchange between Ca²⁺ in smectite of the sediment and Na⁺ in the pore water that originated from the river water.

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Spatial and temporal variability of fish assemblages in Nakaumi Lagoon along the Sea of Japan were studied using the seasonal catch data acquired by small set-net sampling from February to December 2008. We compared species composition and assemblage structures among different seasons and areas such as the sub-enclosed Honjo District and the outer Honjo District in the lagoon. A total of 10649 individuals representing 39 species were recorded, with Engraulis japonicus, Sardinella zunasi, Nuchequula nuchalis, Lateolabrax japonicus, Plecoglossus altivelis altivelis, Trachurus japonicas, Plotosus japonicus, and Konosirus punctatus being numerically dominant in the Lagoon. Species richness was greatest in summer due to the seasonal occurrence of temporary resident species including Sardinops melanostictus and Acanthopagrus schlegelii. In addition, the fact that the number of benthic species in summer was higher in the Honjo District than in the outer Honjo District, may have been a response to the relatively low level of hypoxia in the former.

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In order to elucidate the effects of cormorant colonization on the water quality of small ponds, we investigated the water quality of a small cormorant-colonized pond; (Yamadaonuma-kaminuma Pond) and smaller nearby non-colonized ponds. We also investigated the monthly changes in the water quality and the size of the cormorant population in the pond. The concentrations of nitrogen (N), phosphorus (P), total organic carbon (TOC), chlorophyll a, suspended solids and the chemical oxygen demand were found to be very much higher in the Yamadaonuma-kaminuma Pond than in the nearby non-colonized ponds. These data indicated that the latter was in an extreme by eutrophic state. During the breeding period of the cormorants from April to October when their population increased, the N/P ratio declined, probably due to inputs into the pond of cormorant excreta with low N/P ratios. Higher levels of N, P, and TOC concentrations were detected in the particulate fractions from Yamadaonuma-kaminuma Pond. Those dissolved fractions exhibited higher levels of N concentrations. However, we could find no significant Pearson’s correlation between the monthly changes in the cormorant population and the water quality. This might be because, in addition to the input of cormorant excreta directly into the pond, there were also indirect inputs of cormorant excreta through the forest during and after a rain. Our observation of extreme eutrophication and low N/P ratios indicated that cormorant colonization considerably affected the water quality of the adjacent small ponds.

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In order to investigate the impact of global warming on the inland waters of Japan, we examined the rates of rises in water temperature in nine dam reservoirs that had kept long-term records of vertical temperature observations. Water temperatures at the shallowest and deepest layers were selected as being representative for our investigation, and their trend lines were obtained using time-series analysis. Our first expectation was that the deepest water temperatures would be little-affected by meteorological disturbances and might even show clearer temperature rises than those in the shallowest water. However, all the nine reservoir trend lines from 1993 to 2006 in the shallowest layers showed an upward tendency probably attributable to recent rises in air temperature. In some reservoirs, the ratio of water temperature rises exceeded the ratio of rises in air temperature. In contrast, trend lines of the deepest layers during the same period revealed both upward and downward tendencies. Those results suggested that water temperatures in the deepest layers were affected by a reduction in volume due to accretions of sand, enhanced stratification, or diminished vertical mixing, rather than to a rise in temperature due to a concomitant rise in air temperature. To determine the impacts of these effects will be an important issue in future investigation.

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We cultured larvae of Chironomus yoshimatsui (Diptera: Chilonomidae) in the laboratory and determined their lipid amounts and fatty acid compositions at all life stages. Lipid amounts were in the range of 12-35 % dry weight of the midges, with its highest value at the egg stage. Saturated and monounsaturated fatty acids accounted for about 50 % of total fatty acids, and the most abundant saturated and monounsaturated fatty acids were C16: 0 (13-27 mol %) and C18: 1 (8-13 mol %) , respectively, while major polyunsaturated fatty acids were C18: 2n-6 (29-40 mol %) , and C20: 5n-3 (4-10 mol %) , and the n-3/n-6 ratio was lower than 1 at all stages. This n-3/n-6 ratio differed from those of Tanypodinae and Orthocladiinae chironomids.

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Numerous studies in biological and chemical processes have been conducted in Lake Suwa, only a few of which were concerned with physical processes that strongly affect a lake’s biochemical dynamics. To gain a better understanding of the corresponding physical dynamics (e. g., current pattern and discharge), we deployed an updated current instrument (i. e., an Acoustic Doppler Current Profiler, ADCP), which is accepted world·wide for its accuracy and high resolution as well as for its ability to resolve the spatial and temporal variations of currents. Using the ADCP, we measured discharges of the dominant inflows and current patterns of a submerged macrophytic area near the shore together with an outflowing area (nearby Kamaguchi floodgate). Our results show that (1) the discharges observed in the dominant inflows (the Kami, Miya and To rivers) are different from those reported by Nagano Prefecture, whereas the outflowing discharge at Kamaguchi Floodgate is similar to that in the Nagano Prefecture report, and (2) the vertical distributions of velocity outside the submerged macrophytic area differ from those inside the area, whose horizontal velocities were reduced by macrophyte-induced turbulence, and (3) the current patterns near the outflow showed a strong velocity in the northeast, rather than in the southwest region.

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The occurrence of water lettuce (Pistia stratiotes L.), an introduced floating waterweed, was investigated in Shiga prefecture, Japan, during October-December, 2007, largely through the efforts of Field Reporters at the Lake Biwa Museum. P. stratiotes L. was found in 15 out of 205 examined meshes in a 1/50,000 scale issued by the Ministry of the Environment. The distribution range of P. stratiotes L. was limited to the southeast coast of the southern basin of Lake Biwa, its inflowing river, and a pond in the upper reaches of that river. The largest accumulation was found in Akanoi Bay, with 300 m along the shore and 40 m wide toward offshore. Overwintering of P. stratiotes L. was investigated at Akanoi Bay and its inflowing river during January-April, 2008. They were disappeared in the bay by the beginning of March, but 30 individuals were found on 11 April in the inflowing river. Such a successful overwintering of P. stratiotes L. in the inflowing river is probably due to high water temperature associated with thermal effluent from a Factory.

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