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. Plain Language Summary Permeable sands cover almost half of the coastal continental shelf around the world. The permeability of sand sediments, means water can flow through these sediments in complex patterns driven by waves and currents. This flow through the sediment can control nitrous oxide and methane production which are poorly understood. This is particularly important in regions impacted by nitrogen pollution where production of these greenhouse gases can be high. Here we combine experiments and modeling to explore the factors controlling N2O production at a sandy site impacted by nitrogen pollution (Port Phillip Bay, Australia). Our results show the rate of nitrous oxide release to the water column is governed by the rate of seawater exchange with the bed. Shallower water with more intense waves (wave height exceeding 1 m) releases the most N2O, with up to 0.5% of dissolved nitrogen pumped into the sediment being released as N2O. Overall, these results suggest permeable sediments have relatively low rates of nitrous oxide release most of the time. Key Points Coastal hydrodynamics impose an important boundary condition on the production of N2O in permeable sediments Less than 0.5% of seawater dissolved inorganic nitrogen is converted to N2O due to forcing imposed by passing surface gravity waves Permeable sediments are a relatively minor source of N2O emission