Nitrogen fixation rate and phosphorus enrichment effects on diazotrophic cyanobacteria in the Gulf of Riga

Abstract In eutrophied marine systems such as the Baltic Sea, the diazotrophic cyanobacteria has a capacity to further enhance eutrophication or negate mitigation measures by converting atmospheric nitrogen into bioavailable forms through N₂-fixation. This study investigates the response of two cyanobacteria species, Aphanizomenon flosaquae and Nodularia spumigena, collected from the Gulf of Riga under different environmental conditions to a short-period phosphorus enrichment. The samples were collected during the summer cyanobacterial bloom of 2022 in the central region of the Gulf of Riga. Contrary to expectations, neither species demonstrated a significant increase in biomass. The study also established that N₂-fixation rates did not correlate directly with the total diazotrophic cyanobacteria biomass, but showed a significant correlation with heterocyst presence in both species addressed during this study. The findings of this study highlights challenges that can be expected in using chlorophyll a or even cyanobacteria biomass as a proxy for N₂-fixation rates.