Seasonal associations between

Harmful cyanobacterial blooms are an increasing threat to freshwater ecosystems worldwide; however, the role of macroalgae in regulating these blooms remains poorly understood. This study investigated the effects of Cladophora glomerata and its allelopathic potential on the biomass of Planktothrix agardhii and microcystin concentrations across two growing seasons in the shallow hypertrophic Lake Łekneńskie, Poland. Phenolic content in water strongly correlated with phenols in Cladophora extracts and showed the strongest negative association with cyanobacterial biomass among all tested predictors, although this effect appeared most visible early in the season. Water temperature and P. agardhii biomass were the primary predictors of extracellular microcystins in water. Intracellular microcystins were negatively associated with phenols and positively with chlorides. All effects showed significant seasonal variation. These findings suggest that C. glomerata may be associated with reduced cyanobacterial biomass in an aquatic ecosystem through allelopathic mechanisms, although this potential appears to be dependent on environmental conditions and the phenological stage of development. However, cause-and-effect associations still require experimental confirmation.