Seasonal Changes in Water Quality of a Sand Dune Lake, Lake Kamisagata, Niigata, Japan : Limnologiccall Studies of Lakes in Niigata Prefecture XIII

Limnological studies of Lake Kamisagata (N37° 48' 33", E138° 51' 43", altitude: 4.5m, area : 0.18km^2, length : 830m, width : 260m,) were conducted seasonally at two stations from April 2002 to October 2004 with special reference to seasonal changes in nutrient dynamics including some inorganic ions and macrophyte standing crops. This lake is beloging 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 pured to the lake. Spring water containing high concentration of nitrate, probably originated from fertilizers used around the lake, flows into the lake through a wide reed belt of 20-120m width. So high concentrations of nitrate ranging from 0.5 to 5mg N/L are detected in lake water, especially in winter. Inorganic phosphate concentration showed high value in June, after lowering of water table from about 80cm to 40cm. The standing crops of Trapa spp. and Nelumbo nucifera, dominating in macrophytes, reached in maximum during the end of August and the beginning of September, and declinedd toward fall with increasing of nitrate concentration. Some inorganic ions such as Na^+, K^+, Mg^<2+> indicated a stable seasonal changes during a year. Concentration of chlorophyll-a reached to 400µg/L(Station LB) in June 2002. Water quality of a sand dune lake might be influenced largely by the agricultural activities around the lake