Denitrification, anammox, and dissimilatory nitrate reduction to ammonium across a mosaic of estuarine benthic habitats

Estuaries play a key role in moderating the flow of nitrogen (N) to marine ecosystems. However, the magnitude of this N removal can vary dramatically both within and between estuaries due to the benthic habitats present. Here, we compare denitrification, coupled nitrification–denitrification, anammox, and dissimilatory nitrate reduction to ammonium (DNRA) across a mosaic of benthic habitats in the subtropical Noosa River Estuary, Australia. Using ¹⁵N tracer techniques and passive pore-water samplers, we show that coupled nitrification–denitrification was the dominant pathway for N₂ production across all habitats, with higher rates in vegetated habitats (10–70 μmol N m¯²h¯¹) compared to bare sediments (0.9–2 μmol N m¯²h¯¹). Unusual pore-water profiles in the macroalgal sediments suggest the presence of sulfur-driven anoxic nitrification of NH₄⁺ to NO₃¯ and N₂. A benthic N budget showed that combined denitrification and coupled nitrification–denitrification accounted approximately 96% of the N₂ production, while DNRA accounted for 9% of total NO₃¯ reduction pathways in the Noosa River Estuary. The macroalgae habitat contributed 76% of total N removal via N₂ production and 65% of N retention via DNRA, despite accounting for only 25% of the total surface area. We show a strong relationship between seagrass and macroalgae area and N₂ production (r² = 0.8; p < 0.01), and as such, the capacity to mitigate reactive N loads in estuaries may decrease with the large-scale loss of seagrass and other vegetated habitats