What Drives Seasonal Mat Dynamics in Eutrophic Lake: Allelopathy or Environmental Gradients?

Eutrophication of aquatic ecosystems promotes the occurrence of filamentous macroalgae and free-floating macrophytes that form dense surface mat in the littoral zone of shallow water bodies. The structure of these communities is shaped by environmental gradients, species features, and tolerance to competition-related stressors. While laboratory studies show allelopathic potential of these primary producers, field-based evidence integrating chemical interactions with environmental gradients is still poorly understood. We conducted a two-season field study in eutrophic Lake Oporzyńskie to assess the relative importance of allelopathy and environmental gradients in regulating community dynamics of C. glomerata and L. minor. We measured the phenolic content in water and species extracts, physicochemical water parameters and mat size, analyzing the associations using RDA, variance partitioning, and GAM. Variance partitioning showed that phenolics and physicochemical parameters jointly explained more variation in mat size (adjusted R ² = 0.446) than either fraction alone, with phenolics contributing a markedly larger unique fraction (R ² = 0.337) than physicochemical parameters (R ² = 0.060). RDA identified phenolic content in water as the strongest predictor of mat size, with an asymmetric relationship, positive for C. glomerata and negative for L. minor, consistent with an allelopathic advantage for the macroalga, which also had higher phenolic content in its own extract. GAM analyses confirmed this pattern and revealed contrasting, taxon-specific response curves to phenolic content, pH, dissolved oxygen, and TDS. This was associated with temporal niche partitioning, with C. glomerata dominating mid-summer and L. minor expanding in late summer and autumn. Our findings suggest that coexistence of these species reflects the combined action of allelopathic interactions and environmental gradients generating species-specific temporal windows of dominance. This integrated approach highlights the need to jointly consider chemical and environmental drivers when studying plant community dynamics in aquatic ecosystems.