In Lake Biwa, annual changes in species composition and abundance of the phytoplankton community have been frequently examined. However, it is not well understood how the seasonality of phytoplankton has changed over the past several decades. In this study, we analyzed the monitoring data of the phytoplankton community collected twice a month from 1978 to 2014 in the north basin of Lake Biwa and categorized seasonal change patterns of the phytoplankton community using the Bray-Curtis similarity index. Then, the seasonal patterns were compared with those of phytoplankton communities described in the Plankton Ecology Group (PEG) model and examined in relation to the long-term changes in the water quality and nutritional state of Lake Biwa. The results showed that the seasonality of the phytoplankton community in Lake Biwa changed from one typical of a eutrophic condition to one typical of an oligotrophic condition and was found, in specific years, to deviate from these patterns. Thus, the seasonal pattern of the phytoplankton community is a useful and sensitive gauge for evaluating the water quality, nutritional state, and trophic condition of Lake Biwa.

The aim of the present study is to develop a simple modified method for scope for growth (SFG) assay of Corbicula japonica using the carbon mass balance estimated from three experiments. In the clearance experiment, it was effective for putting clams into a plankton net in order to estimate the net carbon-based clearance rate by considering pseudofeces production. In the excretion experiment, it was desirable to assess cumulative POC with attention given to carbon elusion from feces to estimate the carbon-based excretion rate. In the respiration experiment, it was desirable to put a paraffin liquid on the sample surface in order to prevent CO₂ contamination prior to DIC analysis for the estimation of the carbon-based respiration rate. The modified method was applied to clams exposed to different salinities. Clams exposed to 5 psu salinity showed relatively high SFG, whereas those exposed to 20 psu salinity exhibited low SFG. The results correspond to previous findings that high salinity inhibits the growth of clams.

General and precise environmental exposure models are required for exposure evaluations in environmental risk assessments of chemical substances. In this study, to understand the practical use of exposure models, the performance of the model AIST-SHANEL ver. 3.0 was evaluated by estimating the concentrations of five surfactants with different physicochemical properties [linear alkylbenzene sulfonate (LAS) , amine oxide (AO) , alcohol ethoxylate (AE) , ester-amide-type dialkyl amine salt (EA) , and triethanolamine quaternary salt (TEAQ) ] in rivers. Furthermore, the relationship between the organic carbon-water partition coefficient (K_oc) , which is related to adsorption onto soil particles, and the traction coefficient in mathematical models, which represents parameters of wet-weather effluent load in the sediment, was investigated. Good estimation performances were confirmed using a traction coefficient of 30 m^-1 for chemical substances with relatively small K_oc values (LAS, AE, and AO; in the order of 1×10³ L kg^-1) , and traction coefficients of 300-1000 m^-1 for chemical substances with high K_oc values (EA and TEAQ; in the order of 1×10^5-6 L kg^-1) . By selecting the appropriate traction coefficient on the basis of the K_oc value of chemical substances, the AIST-SHANEL model can be used as an exposure analysis tool to precisely estimate concentrations of various chemical substances.

Understanding the nitrogen dynamics in the atmosphere and nitrogen deposition into the forest is required for appropriate management of forested and headwater areas. In this study, we measured nitrate-nitrogen from rainfall from May 2012 to March 2013 at 4 points inside Kofu basin and from April 2010 to March 2013 at 3 points at the northern boundary of the basin. The amount of nitrate-nitrogen deposited in summer was larger than that in winter. Nitrogen and oxygen isotope values of nitrate were in the range of the level of atmospheric nitrate. At two sampling points in the apex of the Arakawa alluvial fan and upstream in the northern boundary of the basin, even though they are covered by mountain forest and dry field, the amounts of nitrate-nitrogen deposited were larger than that in the basin including the urban area and exceeded the standard of nitrogen saturation. In contrast, such a large deposition was not detected at another point on the northern boundary north of Kofu station and approximately 5 km southeast of the above points. Therefore, we conclude that these differences are associated with the atmospheric stream such as prevailing wind and mountain-valley wind, caused by the micrometeorology and topography.

To elucidate the historical trend of eutrophication in Lake Nakaumi over the last 100 years, we measured the concentrations of total organic carbon (TOC) , total nitrogen (TN) , total sulfur (TS) , and molybdenum (Mo) in sediment cores collected at two sites in the lake. TOC and TN in the sediments were derived mainly from planktonic organisms, and their concentrations increased abruptly after the 1950s. Most of the TS in the surface layer of 0–2 cm was present as sulfides, indicating that large amounts of sulfides are deposited around the sediment surface. Assuming that sulfides were formed mostly in the upper layer of 0–10 cm, the vertical profile of TS concentration in layers deeper than 10 cm suggests that the amount of sulfides formed in the sediment cores increased after the 1950s. This increase may be attributable to the expansion of the area of anoxic water around the lake bottom owing to the increase in primary production after the 1950s. The vertical profile of Mo concentration in the cores was similar to that of TS concentration. It is likely that the vertical profile of Mo concentration is also related to the historical trend of primary production in Lake Nakaumi.