Background and aims – Phenotypic plasticity is the morphological response of a single genotype to produce several different physiological and/or morphological types under changing environmental conditions. Desmodesmus, a green alga, has been well documented for its phenotypic responses (e.g. colony transformation to unicells, and spiny transformation to spineless). Two oxbow lakes (R1 and R2) and one artificial dam reservoir (R3) were used to evaluate the quantity and diversity range of Desmodesmus species occurring during four seasons of one-year. Methods – Water samples were quantified for Desmodesmus species and identified using the Scanning Electron Microscope (SEM). To better understand the morphological responses of Desmodesmus, an axenic culture of Desmodesmus abundans, CCAP 258/299, was cultured in sterile-filtered water from each water body collected from each season. Key results – Not surprisingly, different Desmodesmus species inhabited each water body and exhibited different densities of growth, contributing 77% of the total density in R1, 22% in R2 and only 1% in R3. Summer and autumn were the seasons when Desmodesmus showed the highest density in the shallow and polymictic oxbow lakes (R1 and R2). In the deep and dimictic reservoir (R3), the highest density of Desmodesmus was during autumn. Phenotypic plasticity was observed in the experiments, with unicells being formed from colonies, some with shorter spines. Desmodesmus grown in a high concentration of nitrogen (SE, Medium 7 +3N and BBM) had a higher percentage of unicells compared to the other culture conditions. Conclusions – Our studies are important from both taxonomical and ecological points of view, since our results showed that there may be new possibilities for using the phenotypic plasticity of Desmodesmus to assess water quality and as a potential bioindicator of nutrient availability in natural ecosystems.