Widearea correlation of the Sekiya Tephra Bed in the Ogawa Formation in the western Iiyama City to the Niigata region and Kakegawa Group, central Japana

Limnological studies of Lake Mitarasegata (N37°49′, E138°53′, altitude: 5m, area: 9ha, length: 1km, width: 0.08km) were conducted from April 2000 to February 2002 with special reference to nutrient dynamics and phytoplankton growth. This lake is belonging to one of sand dune lakes, which are rare from the viewpoint of lake origin in Japan. This lake is fed by spring water and rainwater directly poured to the lake. High concentrations of nitrogen, especially nitrate in spring water, probably originated from fertilizers used around the lake, were determined in St.2, where seemed to be spring area. Nitrate decreased toward the outlet of lake, whereas very high concentrations of chlorophyll-a, up to 300 (μg/L), were detected at the lake center (St.3). A half of the lake was covered by vegetation of Nelumbo nucifera that reached a maximum growth in the end of August. In this area (St.4), growth of phytoplankton was suppressed by the growth of Nelumbo (Fig.16), suggesting a nutrient uptake competition. A high concentrations of Cl^- equally equivalent to Na^+ in lake water (Fig.14) suggest the addition of NaCl from the out side of the lake. This lake should be given urgently a conservation plan from the point of protection of waterfowls and reduction of eutrophication