Zygnemaphyceae, better known as demydiae, is undoubtedly the best represented group of continental algae in terms of species numbers, varieties and taxonomic forms, besides being one of the most studied algae classes in Brazil. In Lago Verde (clear water flooding system) scientific articles of this nature are relatively scarce or nonexistent. This lake is located in the Alter do Chão Bathing Village, Santarém Municipality, Pará State, Amazon, Brazil. In this context, the present work had the objective of conducting a floristic survey of the desmidia genus: Cosmarium (Chapter I) and Staurastrum (Chapter II), describe morphologically, illustrate, analyze frequency of occurrence based on the methodology of Mateucci & Colma (1982) and report the geographic distribution of these genera in the State of Pará. Twentyeight samples were collected from plankton net. (20 μm mesh) in the limnetic region in the periods of low water (November and December 2013) and high water (May and June 2014), in seven sampling stations and analyzed by Optical Microscopy. A total of 39 taxa were recorded, of which 23 Staurastrum and 16 Cosmarium taxa. Of these, six desmídias taxa are being added for the first time to the State of Pará ficoclower and are illustrated in the present study (C. contactum var. retusum e C. peseudopyramidatum var. rotumdatum, St. punctulatum var. punctulatum f. minor; St. pachyrhynchum var. pachyrychum f. pachyrychum; St. furcatum e furcatum e St. jurucuiense). In general, the occurrence of the species can be attributed to some characteristics of the environment, such as slightly acidic waters and reduced electrical conductivity, conditions cited in the literature as favorable to the development of media. Therefore, the studied genera were well represented in green lake, when compared to the richness registered with other regions of the Amazon and Brazil, demonstrating the relevance of the work developed for the taxonomy and the knowledge of gender diversity in the State of Pará. In addition to the detailed descriptions, it provided cellular measurements based on population variation and figures for all taxa. Additionally, it provides subsidies for future ecological and conservation studies of this Amazonian flood lake, considering the high level of tourism activity and urban expansion in the region without environmental planning.
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