Spatiotemporal patterns of N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; fixation in coastal waters derived from rate measurements and remote sensing

Mindaugas Zilius,Evelina Grinienė,Paul A Bukaveckas,Diana Vaiciute,Anastasija Zaiko,Tobia Politi,Irma Vybernaite-Lubiene,Iris Liskow,Maren Voss,Sonia Brugel,Agneta Andersson,Stefano Bonaglia,Donata Overlingė

doi:10.5194/bg-18-1857-2021

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 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.

Highlights

Biological dinitrogen (N2) fixation plays an important role in the nitrogen (N) budget of aquatic ecosystems as it transforms gaseous N2 into reactive forms, which are available for assimilation by microorganisms, algae, and plants (Gruber, 2004; Hayes et al, 2019)
The occurrence of elevated NO−3 concentrations and high DIN : dissolved inorganic phosphorus (DIP) after spring runoff was followed by an extended period (8 months) of persistent low N availability, creating a temporal niche for heterocystous cyanobacteria (Supplement, Fig. S1)
We used molecular techniques to document the diversity of diazotrophs of the Curonian Lagoon and found that the community shifted from N2-fixing heterotrophic bacteria in spring to photosynthetic heterocystous cyanobacteria in summer to fall (Zilius et al, 2020)

Summary

Introduction

Biological dinitrogen (N2) fixation plays an important role in the nitrogen (N) budget of aquatic ecosystems as it transforms gaseous N2 into reactive forms, which are available for assimilation by microorganisms, algae, and plants (Gruber, 2004; Hayes et al, 2019). Coastal ecosystems contain diverse diazotrophic communities, which are comprised of unicellular cyanobacteria, colonial heterocystous cyanobacteria, and heterotrophic bacteria (Riemann et al, 2010; BentzonTilia et al, 2015; Zilius et al, 2020). Unicellular cyanobacteria and heterotrophic diazotrophs dominate in tropical and oligotrophic marine systems (Zehr et al, 2003; Riemann et al, 2010; Farnelid et al, 2016), whereas colonial heterocystous cyanobacteria dominate N2 fixation in temperate systems (Klawonn et al, 2016). The dominant colonial heterocystous cyanobacteria in the Baltic Sea and its coastal areas are Aphanizomenon, Nodularia, and Dolichospermum (formerly Anabaena) (Olofsson et al, 2020a). Their proliferation has the potential to alter N cycling and thereby influence N export to coastal waters.

Methods

Results

Conclusion