We analysed diel cycles of total protein, lipid and carbohydrate content of seston in four seasonal sampling campaigns in a tropical hypereutrophic reservoir. Physical and water chemical variables and taxonomic composition of phytoplankton were measured in parallel. Seston lipid and carbohydrate contents exhibited highest values during the day, while protein content was highest at night. Carbohydrate content was negatively correlated with nitrate and nitrite concentrations. Lipid content was negatively correlated with temperature and positively with soluble reactive phosphorus (SRP) concentrations. Protein content was positively correlated with temperature and negatively with SRP concentrations. In all sampling campaigns, the biochemical composition was correlated to the biovolume of phytoplankton. However, relationships differed between seasons. In the dry season, the relative biovolume of non-dominant groups such as cyanobacteria and bacillariophyceae was strongly correlated with all biochemical compounds of seston. During the rainy season, significant correlations were detected between biochemical compounds of seston and biovolumes of the dominant groups, chlorophyceae and euglenophyceae. We conclude that diel fluctuations in environmental conditions as well as changes in the community composition of phytoplankton affected seston biochemical composition during the diel cycles investigated. However, the type of interaction was clearly influenced by seasonality in this tropical reservoir.