An investigation of genera Euastrum Ehrenberg ex Ralfs and Micrasterias C.Agardh from an Amazon floodplain lake of the Negro river Basin was undertaken. The samples were collected using a plankton net in four stations on the Tupe Lake and one station near a lake on the Negro river. The species were identified and described on the basis of morphological and morphometrical characteristics. A total of 12 species were identified, five species of Euastrum genus (E. evolutum var. perornatum; E. gemmatum var. monocyclum; E. ornans; E. sinuosum; E. spinulosum), and seven of Micrasterias genus (M. borgei; M. radiata var. brasiliensis; M. torreyi; M. laticeps var. acuminata; M. mahabuleshwarensis var. amazonensis; M. rotata var. rotata; M. siolii). All species, except M. radiata Hassal var. brasiliensis Grönblad, were found in the Negro river; on the other hand, only seven species were found in the Tupé lake, three of Euastrum and four of Micrasterias.
Desmids; Black water; Floodplain lakes
