Diverging temporal trends and environmental drivers of dominant cyanobacteria in the Gulf of Riga, 1976–2024

Cyanobacteria are major contributors to summer primary production in the Baltic Sea, where extensive blooms of diazotrophic taxa such as Aphanizomenon flosaquae and Nodularia spumigena shape ecosystem functioning. However, the environmental drivers regulating their biomass dynamics remain incompletely understood. Using long-term monitoring data (1976–2024) from the Gulf of Riga, this study examined how nutrient concentrations, salinity and river runoff influenced cyanobacterial bloom composition and intensity. While both taxa co-occurred, A. flosaquae contributed substantially more biomass, allowing for a detailed statistical analysis. GAM (Generalized Additive Model) revealed that winter dissolved inorganic phosphorus (DIP) was the strongest and most consistent predictor of subsequent summer biomass, suggesting a ‘legacy effect’ where early-season nutrient availability governs bloom magnitude months later. Summer dissolved inorganic nitrogen (DIN) also contributed significantly, which is consistent with the ability of A. flosaquae to meet nitrogen requirements through N-fixation. In contrast, integrated salinity and winter river runoff exhibited limited effect, although co-plot analyses indicated that higher-salinity regions, naturally poorer in DIP, amplified the role of phosphorus in shaping biomass responses. Understanding how these factors interact is essential for predicting bloom development in the Baltic Sea.