Abstract
Centric diatoms of an assemblage from Hashie-numa (Hashie Pond) in Isesaki, Gunma, Japan, were investigated over a period of 26 years. Hashie-numa is a man-made pond was built somewhere between the late Heian Period (ca. 1100 A.D.) and the middle Kamakura Period (ca. 1300 A.D.). The pond is used to irrigate rice fields and to farm carp and smelt. The latter caused the pond to become eutrophic in quality. A total of 16 samples taken from 1979 to 2005 yielded 22 taxa of centric diatoms belonging to 10 genera. The assemblage is characteristically abundant in Aulacoseira taxa in fall to winter and Stephanodiscus taxa in spring. Dominant taxa are Aulacoseira ambigua (Grunow) Simonsen, Aulacoseira granulata (Ehrenb.) Simonsen, Cyclostephanos cf. invistatus (M.H.Hohn et Hellerman) E.C.Ther., Stoermer et Håk., Cyclotella meneghiniana Kütz., Stephanodtiscus hantzschii f. tenuis (Hust.) Håk. & Stoermer and Stephanodiscus minutulus (Kütz.) A.Cieve et J.D.Möll. Microstructure and/or taxonomic information of the 14 prevalent taxa is presented here.
