Dominance of heterotrophic N2 fixation in a temperate estuary

Mar Benavides1, Søren Hallstrøm2, Clayton Woodward Evans3, Lindsey Janel Potts4, Julie Granger5, Craig R Tobias4, Colleen B Mouw6, Lasse Riemann7 and Pia Moisander8, (1)Aix Marseille University, Mediterranean Institute of Oceanography, Marseille, France, (2)Copenhaguen University, Denmark, (3)Texas State University San Marcos, San Marcos, TX, United States, (4)University of Connecticut, Department of Marine Sciences, Groton, CT, United States, (5)University of Connecticut, Marine Sciences, Groton, United States, (6)University of Rhode Island, Narragansett, RI, United States, (7)University of Copenhagen, Marine Biological Section, Helsingor, DE, Denmark, (8)University of Massachusetts Dartmouth, Department of Biology, North Dartmouth, MA, United States
Abstract:
Recent studies suggest that marine N2 fixation is not constrained to open ocean oligotrophic waters, but may be also important in nutrient-rich coastal environments. We surveyed N2 fixation rates and diazotroph diversity at biweekly intervals from May through September 2017 in Narragansett Bay. The Bay is a shallow estuary that receives freshwater from seven river sub-drainage basins, and holds a semidiurnal tidal regime. We observed a consistent decrease in N2 fixation rates from the head to the mouth of the estuary, but considerable intraseasonal variability. The diazotrophic community was predominantly composed of heterotrophic bacteria, highlighting the putative importance of non-cyanobacterial diazotrophs in nutrient-rich environments. Here we explore the spatiotemporal variability of diazotrophic activity and composition in the context of tidal currents and associated sediment resuspension. Our results reinforce the need to measure coastal N2 fixation to constrain global nitrogen budgets and decipher the importance of non-cyanobacterial diazotrophs.
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