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 flos-aquae, 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.
A novel septal protein of multicellular heterocystous cyanobacteria is associated with the divisome
