RESEARCH ARTICLE Biological index based on epiphytic diatom assemblages is more restrictive than the physicochemical index in water assessment on an Amazon floodplain, Brazil Maria Tereza Morais Pereira Souza Lobo 1,2 & Paulo Sérgio Scalize 1,3 & Cleber Nunes Kraus 4 & Weliton José da Silva 5 & Jérémie Garnier 6,7 & David da Motta Marques 8,9 & Marie-Paule Bonnet 7,9 & Ina de Souza Nogueira 1,2,10 Received: 27 June 2019 /Accepted: 7 January 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020 Abstract Canadian Water Quality Index (CWQI) provides protection for freshwater life promoting healthy ecosystems and safeguarding human health. Biological Diatom Index (BDI) was developed to indicate the ecological status and water quality of freshwater systems. This paper evaluates the relations between the two different indices. During rising and falling, water samples were taken in the Curuai Floodplain, Brazil. CWQI was calculated using 14 physicochemical parameters and 1 microbiological parameter. The limits were established according to freshwater quality conditions and standards based on water use classes 1 and 2 determined in CONAMA 357 legislation and British Columbia. Canadian Water Quality Index categorization ranged from “marginal” to “excel- lent,” most sampling units were “good” (71%), followed by “fair” (12%) and “excellent” (12%) water quality. Total phosphorus (38 times), chlorophyll a (20), dissolved oxygen (10), and total organic carbon (10) were the parameters that presented the most non- compliance values. Encyonema silesiacum (14%), Gomphonema parvulum (13%), and Navicula cryptotenella (12%) were the main taxa in the rising period, while G. lagenula, E. silesiacum, and Fragilaria capucina were the main taxa during the falling period. BDI ranges from I to V water quality classes. We observed “poor” to “very good” ecological status, with most sampling units “moderate” (52%) and “good” (29%). Water quality for class 2 was better than water quality for class 1, as the limits of the parameters evaluated were more restrictive in class 1 than in class 2 and the predominant uses of water require a higher degree of water purity. The biological index based on diatoms was the most restrictive index whose water classes and categorizations have shown an ecological status that could threaten the protection of aquatic communities on the Curuai floodplain. We suggest the combined use of both indices—physicochemical and biological for water quality assessment in this type of environment. Keywords Canadian Council of Ministers of the Environment Water Quality Index . Water classes . Biological Diatom Index . Ecological status . Water Framework Directive Responsible editor: Philippe Garrigues * Maria Tereza Morais Pereira Souza Lobo mariatereza_lobo@yahoo.com.br 1 Programa de Pós-graduação Ciências Ambientais, Universidade Federal de Goiás, Goiânia, GO 74.690-900, Brazil 2 Laboratório de Análise e Gerenciamento Ambiental de Recursos Hídricos (LAMARH), Universidade Federal de Goiás, Campus Samambaia, Alameda Palmeiras - Quadra I - Lote i2, Goiânia, GO 74.690-900, Brazil 3 Escola de Engenharia Civil e Ambiental, Goiânia, GO 74.605-220, Brazil 4 Programa de Pós-graduação em Ciências Ambientais, Universidade de Brasília, Planaltina, DF 73.340-710, Brazil 5 Departamento de Biologia Animal e Vegetal, Universidade Estadual de Londrina, Londrina, PR 86.057-970, Brazil 6 Laboratório de Geoquímica, Instituto de Geociências, Universidade de Brasília, Brasília, DF 70.910-900, Brazil 7 Joint International Laboratory LMI OCE ‘Observatory of Environmental Change’, UnB/IRD, Brasília, DF 70.910-900, Brazil 8 Instituto de Pesquisas Hidráulicas, Universidade Federal do Rio Grande do Sul, Avenida Bento Gonçalves, Porto Alegre, RS 91.501-970, Brazil 9 Institut de Recherche pour le Développement, UMR Espace-DEV, Maison de la télédétection, 500 rue J.F. Breton, 34000 Montpellier, France 10 Departamento de Botânica, Universidade Federal de Goiás, Goiânia, GO 74.690-900, Brazil Environmental Science and Pollution Research https://doi.org/10.1007/s11356-020-07658-y