The effect of sediment resuspension on pelagic nitrogen fixation in a Danish Fjord

Sediment resuspension, driven by wind, waves, and tides, mixes benthic and pelagic environments by introducing sediment, nutrients, and microorganisms into the water column. While previous studies suggest resuspension enhances N 2 fixation, its impact remains unclear. Prior to our experiment in the microtidal fjord environment of Kærby Fed (Odense Fjord, Denmark), we experimentally determined the amount of resuspended material under various bed-shear-stress intensities, providing a novel approach to assessing its effects. Contrary to expectations, our data showed no significant increase in N 2 fixation rates under varying resuspension intensities. Instead, rates were influenced by geochemical dynamics, particularly phosphorus release during anoxic events. The diazotrophic community, primarily composed of Proteobacteria related to sulfate-reducing bacteria, was well-represented in the pelagic environment, likely sustaining observed N 2 fixation rates. These findings highlight the importance of environmental conditions in regulating pelagic N 2 fixation and contribute to nitrogen budget assessments for coastal systems. • Sediment resuspension had minimal impact on N 2 fixation near the benthic boundary layer. • Anoxic sediment conditions and release promoted elevated N 2 fixation rates. • Sulfate-reducing bacteria contributed significantly to N 2 fixation in eutrophic fjords. • Only the uppermost millimeter of sediment was resuspended under realistic conditions. • Strong diazotroph community overlap links sediment and pelagic environments.