Spatiotemporal patterns of N₂ fixation in coastal waters derived from rate measurements and remote sensing

Abstract. Coastal lagoons are important sites for nitrogen (N) removal via sediment burial and denitrification. Blooms of heterocystous cyanobacteria may diminish N retention as dinitrogen (N₂) fixation offsets atmospheric losses via denitrification. We measured N₂ fixation in the Curonian Lagoon, Europe's largest coastal lagoon, to better understand the factors controlling N₂ fixation in the context of seasonal changes in phytoplankton community composition and external N inputs. Temporal patterns in N₂ fixation were primarily determined by the abundance of heterocystous cyanobacteria, mainly Aphanizomenon flosaquae, which became abundant after the decline in riverine nitrate inputs associated with snowmelt. Heterocystous cyanobacteria dominated the summer phytoplankton community resulting in strong correlations between chlorophyll-a (Chl-a) and N₂ fixation. We used regression models relating N₂ fixation to Chl-a, along with remote sensing-based estimates of Chl-a to derive lagoon-scale estimates of N₂ fixation. N₂ fixation by pelagic cyanobacteria was found to be a significant component of the lagoon's N budget based on comparisons to previously derived fluxes associated with riverine inputs, sediment-water exchange and losses via denitrification. To our knowledge, this is the first study to derive ecosystem-scale estimates of N₂ fixation by combining remote sensing of Chl-a with empirical models relating N₂ fixation rates to Chl-a.