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 N-15 tracer techniques and passive pore-water samplers, we show that coupled nitrification-denitrification was the dominant pathway for N-2 production across all habitats, with higher rates in vegetated habitats (10-70 mu mol N m(-2) h(-1)) compared to bare sediments (0.9-2 mu mol N m(-2) h(-1)). Unusual pore-water profiles in the macroalgal sediments suggest the presence of sulfur-driven anoxic nitrification of NH4+ to NO3- and N-2. A benthic N budget showed that combined denitrification and coupled nitrification-denitrification accounted approximately 96% of the N-2 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 N-2 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 N-2 production (r(2) = 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.

Automated summary

Estuaries function as key modulators of nitrogen flow to marine ecosystems, with the dominant pathway for N-2 production being coupled nitrification-denitrification, especially in vegetated habitats. Macroalgae habitats play a significant role in N removal and retention, contributing to a large portion of N-2 production and DNRA despite their smaller surface area compared to other habitats. The presence of seagrass and macroalgae is strongly correlated with N-2 production, indicating a potential decrease in the capacity of estuaries to mitigate reactive N loads with the loss of these vegetated habitats.