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

日本许多湖泊的水质没有得到改善，尽管入湖负荷明显减少；因此，沉积物-水中营养物质的交换被认为是原因，但其对湖泊营养物质动态的长期影响尚未阐明。2005-2007年间，在鹿儿湖、苏瓦、奇崎湖和新知湖多次采集沉积物岩心；然后测定营养盐(P、N、C)、多环芳烃、铅-210EX、Cs-137等的垂直剖面，以估计几十年来质量沉积速率和营养物质积累的变化，并计算营养物质收支。结果表明：1.入湖河口的泥沙淤积速率较高，在暴雨过程中出现了几个泥沙淤积高峰。在Kasumigaura湖，沉积物中几种TP含量的释放可以解释实验室实验估计的TP释放速率和夏季湖水TP浓度的增加；沉积物TP浓度的下降在河口附近观察到，但在湖中心没有观察到，可能是由于再沉积。苏瓦湖沉积物中的总磷含量明显下降，这可能是夏季湖水总磷含量增加的主要原因。如果能准确估算沉积面积，20多年来沉积物中总磷的积累量几乎相当于鹿儿湖和苏瓦湖总磷收支的残差值。2003年和2007年TP垂直剖面的差异被高估了营养盐沉积速率，这可能是由于沉积物数据的低分辨率所致。还测定了沉积物中多环芳烃的垂直剖面，以讨论与流域发育有关的长程变化。从沉积物物性变化的角度分析了近期鹿儿湖浑浊度的上升。

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.