The impact and ecosystem scale consequences of dissimilatory nitrate reduction to ammonium (DNRA) on the nitrogen budget of a microtidal estuary

Background/Question/Methods

Increasing anthropogenic nutrient input to coastal waters is contributing to eutrophication of estuaries resulting in higher instances of hypoxia, harmful algal blooms, and fish kills. Ecosystem recovery from eutrophication and resilience is partly dependent on the ability of these ecosystems to assimilate or remove nutrients. We use an ecosystem approach to examine the retention and removal of nitrogen in Weeks Bay, a shallow microtidal estuary in Alabama receiving freshwater from two rivers with agricultural watersheds. Compared to other estuaries in the northern Gulf of Mexico, Weeks Bay receives the highest input of nitrogen, estimated at 10 mol N m^-2 yr^-1. We measured rates of denitrification and dissimilatory nitrate reduction to ammonium (DNRA) in intact sediment cores using the isotope pairing technique and from production of ¹⁵NH₄⁺ over an annual cycle at two stations (MidBay and Mouth). We hypothesized that rates of DNRA would exceed denitrification. DNRA by retaining bioavailable nitrogen in the system would reduce resilience of this highly impacted ecosystem.

Results/Conclusions

Denitrification capacity averaged 22.9 ± 15.0 μmol N m^-2 hr^-1 and was similar between MidBay (33.6 ± 10.8 μmol N m^-2 hr^-1) and the Mouth (21.6 ± 12.9 μmol N m^-2 hr^-1). DNRA ranged from a low of 8.8 ± 3.1 at the Mouth to a high of 89.7 ± 18.4 μmol NH₄⁺ m^-2 hr^-1 at MidBay. Consistent with our hypothesis, we found that on average DNRA (44.4 ± 5.5 μmol N m^-2 hr^-1) exceeded in situ denitrification (0.9 ± 2.3 μmol N m^-2 hr^-1) and that even in the presence of abundant water column nitrate DNRA was favored over denitrification by a factor of two. DNRA was a prominent pathway for NO₃^- reduction and contributed to a large fraction of the benthic NH₄⁺ flux. DNRA was estimated to provide nitrogen to the water column at a rate equivalent to 15% of the N input that is retained within the estuary and is a significant component of the nitrogen budget in this highly impacted estuary. DNRA by retaining N in the system and by providing a significant fraction of the nitrogen demand by primary producers can impact this estuary through enhanced rates of primary production. Future management efforts should focus on reducing nutrient input to this estuary without which the significant retention of nitrogen in this system through DRNA will contribute to the undesirable ecosystem attributes associated with eutrophication.