O GÊNERO Cosmarium CORDA EX RALFS (ZYGNEMAPHYCEAE, DESMIDIACEAE) DE UM LAGO DE INUNDAÇÃO DA BACIA DO RIO TAPAJÓS, AMAZONIA, BRASIL

Cosmarium is a genus of wide distribution in freshwater environments with great representation in Amazonian aquatic systems. The aim of this study was to determine the taxonomic composition and relative frequency of occurrence of the genus Cosmarium in a clearwater floodplain lake located in the municipality of Santarem, Para, Amazonia, Brazil. Twenty-eight samples were collected from the limnetic region during low water and high water periods. A total of 16 taxa were recorded of the genus Cosmarium and of this total, six are varieties that are not typical of their respective species. C. contractum var. contractum and C. bioculatum var. bioculatum were the best represented species in the environmented and were recorded in 100% of the samples in the low water period and in more than 50% in the high water period. C. contractum var. retusum and C. pseudopyramidatum var. rotundatum are new records for the state of Para. Keywords: desmids, taxonomy, amazon diversity, green algae, Amazon floodplain lake