The Characteristics of Diatom Species and Their Distribution in Gun Nuur,Northern Mongolian Plateau

Diatom species from the surface sediments of the Gun Nuur in the north of the Mongolian Plateau were tested in order to discuss the distribution characteristics and their responses to water environment.The results showed that diatom species in the Gun Nuur include 2 classes,7 families,12 genera,and 21 species.Pennates is a predominant diatom species in this lake.The dominant species are Cymbella cistula,Amphora ovalis,Amphora baltica and Amphora ovalis var.pediculus,with the average percentages of 24.4%,23.8%,16.9%,15.6%,respectively,while all other species are no more than 10%.The predominant diatom species Cymbella cistulaindicates that the Gun Nuur is a fresh water body with slight alkalinity(pH=8.0),and blooms a plenty of aquatic living organisms in the lakeshore.Diatoms Anomoeoneis costata,Anomoeoneis sphaerophora var.sphaerophoraand,Navicula oblongareflect that the lake waterbody is a Na-Ca/Cl-SO4 type with an increasing tendency of water salinity and mineralization,which is caused by evaporation in the summer and autumn.The diatom species Anomoeoneis sphaerophora var.sphaerophora and Navicula oblongaindicate the feature that the water body is SO42- -rich.Under the extreme cold weather conditions,the growing of diatoms has been restrained.And the small and fat-liked diatoms will float in order to adapt to the temperature change.Under the ice-closed lake,the lake water is quiet with higher temperature than that in the open air in the sunshine days,which is good to the growth of diatoms.Meanwhile,the euryhaline and lenific diatomEpithemia intermedia can also be taken as evidence that the water body is static under the ice-closed lake.