Lake budgetary study on nutrients in water and sediment : Long-term change in sediment nutrients, nutrient budget, and modeling of sediments

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Water quality has not improved in many lakes in Japan, regardless of the apparent inflow load reduction; thus, sediment-water exchange of nutrients was suspected to be the reason, but its long-term influence on lake nutrient dynamics has not clarified. Sediment cores were taken in Lakes Kasumigaura, Suwa, Kizaki and Shinji several times during 2005-2007; then, vertical profiles of nutrients (P, N, C), PAHs, Pb-210_ex, Cs-137, etc. were determined to estimate the changes in mass sedimentation rates and nutrient accumulation during several decades and nutrient budget was calculated.1. Mass sedimentation rates were higher at inflow river mouths in the lakes and several peaks of sedimentation rates were observed during heavy rainfall events.2. In Lake Kasumigaura, release of several of TP contents in sediments could account for TP release rates estimated by laboratory experiment and increase of lake water TP concentration in summer; the decrease in sediment TP concentration was observed near river mouth, but not observed at the center of the lake, probably due to redeposit.3. Apparent decrease of TP contents in sediments, which could nearly account for the increase in lake water TP during summer, was observed in Lake Suwa.4. The TP accumulation in sediments for more than 20 years corresponded nearly to the sum residuals of TP budget in Lakes Kasumigaura and Suwa if we could estimate the accurate depositional area. Nutrient sedimentation rates were overestimated for the difference in TP vertical profiles between 2003 and 2007, probably due to low resolution of sediment dating.5. PAHs vertical profiles were also determined for the sediments to discuss the long-them change in change in relation to watershed development. The recent increase in turbidity in Lake Kasumigaura was analyzed from the view of the change in physical properties of sediments.