The 500,000 inhabitants of Mayo Tsanaga River Basin are vulnerable to a "silent" fluorosis from groundwater consumption. For the first time, the groundwater is investigated for the purpose of identifying the provenance of fluoride and estimating an optimal dose of fluoride in the study area. Based on the fluoride content of groundwater, fluorine and major oxides abundances in rocks from the study area, mean annual atmospheric temperature, and on-site diagnosis of fluorosis in children, the following results and conclusions are obtained: Fluoride concentration in groundwater ranges from 0.19 to 15.2 mg/l. Samples with fluoride content of <1.5 mg/l show Ca-HCO(3) signatures, while those with fluoride >1.5 mg/l show a tendency towards Na-HCO(3) type. Fluor-apatite and micas in the granites were identified as the main provenance of fluoride in the groundwater through water-rock interactions in an alkaline medium. The optimal fluoride dose in drinking water of the study area should be 0.7 mg/l, and could be adjusted downward to a level of 0.6 mg/l due to the high consumption rate of groundwater, especially during drier periods.

Identifying nonpoint phosphorus (P) sources in a watershed is essential for addressing cultural eutrophication and for proposing best-management solutions. The oxygen isotope ratio of phosphate (δ18OPO4 ) can shed light on P sources and P cycling in ecosystems. This is the first assessment of the δ18OPO4 distribution in a whole catchment, namely, the Yasu River Watershed in Japan. The observed δ18OPO4 values in the river water varied spatially from 10.3‰ to 17.6‰. To identify P sources in the watershed, we used an isoscape approach involving a multiple-linear-regression model based on land use and lithological types. We constructed two isoscape models, one using data only from the whole watershed and the other using data from the small tributaries. The model results explain 69% and 96% of the spatial variation in the river water δ18OPO4 . The lower R 2 value for the whole watershed model is attributed to the relatively large travel time for P in the main stream of the lower catchment that can result in cumulative biological P recycling. Isoscape maps and a correlation analysis reveal the relative importance of P loading from paddy fields and bedrock. This work demonstrates the utility of δ18OPO4 isoscape models for assessing nonpoint P sources in watershed ecosystems.

In Lake Biwa, the largest freshwater lake in Japan, the lacustrine shrimp Palaemon paucidens has been reported to undertake seasonal migrations, living in deep waters from autumn to winter and in shallow waters from spring to summer. Some investigators have suggested that some individuals overwinter in shallow waters, but the ecology and life history of such nonmigratory populations are poorly understood. We developed a species-specific environmental DNA (eDNA) marker to detect P. paucidens in water samples and used it to examine the distribution and relative abundance of overwintering individuals in the shallow waters of Lake Biwa. Water samples were collected from 21 shore sites and 32 surrounding freshwater lagoons of Lake Biwa in November 2015 and February 2016. In November, 4 shore and 12 freshwater lagoon sites were positive for P. paucidens eDNA, and in February, 4 shore and 9 freshwater lagoon sites were positive. The relative abundance of eDNA copies was estimated by quantitative real-time polymerase chain reaction (PCR). Simultaneous sampling of P. paucidens and its eDNA, conducted at the inlet and outlet of 5 freshwater lagoons in December 2016, validated our method as a quantitative measure of the relative abundance of P. paucidens local populations. The eDNA approach used here confirmed that some P. paucidens individuals overwinter in the shallow waters, suggesting life-history diversity in Lake Biwa populations.

Abstract1. Isotope analysis has high potential for understanding fish ecology and food-web structure in aquatic ecosystems. The utility of isotope analysis will be greatly improved if we can reconstruct the chronology of several isotopes at multiple growth stages of individual fish. However, no practical methods exist for reconstructing the chronology of light-element isotopes (e.g. δ 13 C, δ 15 N, δ 34 S, andHere, we present and test a new analytical approach for reconstructing the isotopic ratios of light isotopes at multiple life-stages in teleost fishes.2. We sampled an anadromous salmon species, masu salmon Oncorhynchus masou (n = 3), along with water from its natal stream and from the ocean. We subdivided the vertebral centra of the salmon equally into 10 sections and extracted bone collagen from each sample. We then measured the stable sulphur isotope ratios of each vertebral section and compared them with δ 4. Our results show that the vertebral centra of teleost fishes record isotopic information from juvenile to adult life stages. We suggest that our method can provide reproducible isotopic chronology, even in teleost fishes smaller than 50 cm. This method can be used in isoscape studies and in studies of the ecology of marine teleost fishes. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. K E Y W O R D S

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Agriculture is one of the most critical anthropogenic disturbances to freshwater ecosystems globally.In monsoon Asia, rice paddies provide habitats for aquatic species, but how their associated irrigation systems can affect downstream ecosystems is poorly understood. Here, we used structural equation modeling (SEM) to estimate seasonal variations in benthic macroinvertebrate diversity and environments and quantified indirect effects of land use, especially of rice paddy areas, on benthic macroinvertebrate diversity through local environmental alteration. Our study site was the Yasu River tributary of Lake Biwa, Japan, and we performed our investigation during both irrigation and nonirrigation periods, representing different seasons. Seasonal variations in physical and chemical variables and diversity were observed. Seasonal particulate phosphorus concentrations to a large extent determined the abundance and community composition of benthic macroinvertebrates. SEM revealed that the proportions of forest, urban, and rice paddy areas in the catchment had significant indirect effects on biodiversity indices of local benthic macroinvertebrate communities. The proportion of forest area had robust negative effects on water temperature, but the overall indirect effects on the macroinvertebrate diversity contrasted between the two seasonal periods. The proportion of rice paddy area had a strongly negative indirect effect on the diversity through increased particulate phosphorous loading during the irrigation period, while the proportion of urban area had a significantly negative indirect effect during the nonirrigation period only. The seasonal negative impacts of rice paddy irrigation on benthic macroinvertebrate communities were possibly due to siltation arising from rice paddy soils. Our results have implications for environmental restoration and biodiversity conservation in catchment management.