Phytoplankton dynamics and morphological plasticity were studied in an oligo-mesotrophic lake from late spring to early autumn in 2004 on a weekly to bi-weekly basis. This study aimed to evaluate the effect of environmental constraints on morphological plasticity and size structure of dominant species through the stratified period of the lake. While centric diatoms developed mainly in the epilimnion, Fragilaria crotonensis, Synedra sp. and Synedra acus were very well distributed through the whole water column, as was the cryptophyte Plagioselmis nannoplanctica. A filamentous cyanobacterium, Planktothrix rubescens, was stratified in the metalimnion throughout the sampling period, while Mougeotia sp. was entrained through the whole water column during the early autumn. Surface area / volume (S/V) ratios were calculated for the dominant species on each sampling date. The minimum S/V ratio was found with centric diatoms, while maximal values measured were for Synedra sp. and Asterionella formosa. The S/V ratio of the only dominant cryptophyte, Plagioselmis nannoplanctica was also high; variations were negatively correlated with NO3-N and PO4-P (-0.62 and -0.61, respectively) and positively correlated with euphotic depth (r=0.57) and temperature (r=0.82). On the other hand, the S/V ratios of Fragilaria crotonensis and Cyclotella ocellata each showed strong negative correlation with silica concentrations (-0.55 and -0.62, respectively). It is concluded that allometric relationships showed significant differences among species according to depth and season.