Hydrodynamics Control Nitrous Oxide Production in Eutrophic Coastal Permeable Sediments

Permeable sediments, which make up almost half of the continental shelf worldwide, are potential sources of the important greenhouse gas N₂O from coastal regions. Yet, the extent to which interactions between these sediments and anthropogenic pollution produce N₂O is still unknown. Here we use laboratory experiments and modeling to explore the factors controlling N₂O production at a eutrophic site in a temperate shallow marine embayment (Port Phillip Bay, Australia). Our results show that denitrification is the main source of N₂O production within permeable sediments, but the extent to which N₂O is actually released is determined by the rate of seawater exchange with the sediment bed (which governs solute residence time within the bed). In wave-dominated coastal areas, shallower water with more intense waves (wave height > 1 m) release the most N₂O, with up to 0.5% of dissolved inorganic nitrogen pumped into biologically active eutrophic sediment being released as N₂O. Our results suggest rates of N₂O production in coastal permeable sediments are generally low compared to other environments.