Relationships between environmental conditions and the morphological variability of planktonic testate amoeba in four neotropical floodplains

•We analysed how the environmental variables influenced the morphometric variation on 27 planktonic testate amoebae.•Environmental conditions play substantial role under great environmental range to each floodplain.•Shell morphometry varies according to environmental factors.•Environmental variables related to the nutrients (ammonia, nitrate, and phosphate) influenced most testate amoeba species.•Temperature could influence indirectly on some testate amoebae variation.•Greater chlorophyll-a concentration could influence on morphological variation of the testate amoebae. Planktonic testate amoebae in floodplains exhibit a broad-range of morphological variability. The variation size is already known, but it is necessary to know how this is for morphological variables. This study aimed to identify the relationships between testate amoebae morphology and environmental factors in four neotropical floodplains. We conducted detailed morphometric analyses on 27 common species of planktonic testate amoebae from genera Arcella, Centropyxis, Cucurbitella, Suiadifflugia, Difflugia, Lesquereusia and Netzelia. We sampled subsurface water from each lake in 72 lakes in four Brazilian floodplain lakes. Our goals were to assess: (1) the range of their morphological variability (a) over space within each floodplain, and (b) among the four floodplains, and (c) over time, and (2) which environmental factors explained this variation. Mean shell height and breadth varied considerably among the different floodplain lakes, especially in the Pantanal and Amazonian floodplains. The morphological variability of testate amoeba was correlated to environmental conditions (ammonia, nitrate, phosphate, chlorophyll-a, turbidity, temperature, and depth). Thus, understanding the morphological variation of the testate amoeba species can elucidate many questions involving the ecology of these organisms. Furthermore, could help molecular studies, bioindicator role of these organisations, environmental reconstruction, among others.