Spatiotemporal patterns of N<sub>2</sub> fixation in coastal waters derived from rate measurements and remote sensing

Abstract

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.

Denitrification, Nitrogen Uptake, and Organic Matter Quality Undergo Different Seasonality in Sandy and Muddy Sediments of a Turbid Estuary

“…At sandy sites, there were no seasonal differences in terms of quantity and macromolecular quality of the OM sedimentary pool, so this factor was likely not affecting denitrification. TNU by microphytobenthos was much larger than denitrification rates and measured DIN fluxes and remained the main sink of inorganic N. At Kairiai and Vente, the discrepancy between theoretical assimilation rates and net DIN fluxes suggests that pore water NH 4 + or N-fixation represented an important source of N for the benthic microalgal communities ( Zilius et al, 2020 ).…”

“…A similar amount of N is likely produced during summer by the fluctuating population of tourists visiting the Curonian Spit (150,000–250,000 units). If N loads from civil sources represent minor N inputs, the rates of summer fixation of N 2 recently estimated by Zilius et al (2020) in 42.4 ± 6.4 tons N day −1 represent an important N input term to the Curonian Lagoon. Such net import of N via fixation nearly doubles summer riverine loads and offset rates of N abatement via denitrification.…”