Recovery of denitrification and dissimilatory nitrate reduction to ammonium following reoxygenation of sediments from a periodically hypoxic temperate lagoon

Eutrophication and stratification events have increased the incidences of hypoxia and anoxia in coastal environments. The depletion of oxygen in these environments can lead to a shift in biogeochemical processes such as denitrification and dissimilatory nitrate reduction to ammonium (DNRA). The balance between denitrification and DNRA is critical because denitrification leads to a loss of bioavailable nitrogen and DNRA maintains it within the system. This study examined the effects of reoxygenation on denitrification and DNRA in sediments collected from a periodically hypoxic system (Gippsland Lakes, Australia) and subjected to experimental hypoxia using a modified ¹⁵N-isotope pairing technique. For freshly collected sediments, the ratio of denitrification : DNRA was generally < 1 indicating a dominance of DNRA. After 3–4 weeks of oxygenation, denitrification increased relative to DNRA resulting in a denitrification : DNRA ratio > 1. Profiles of ¹⁵N-N₂ and (Formula presented.) accumulation in these experiments showed denitrification and DNRA generally only took place in the surface ~ 2 cm of sediment. Prolonged experiments with reoxygenated sediments showed a decrease in the denitrification : DNRA to < 1 after 56 d coincident with deep accumulation of (Formula presented.) within the sediment (2–5 cm) suggesting faunal irrigation enhanced DNRA deep in the sediment. An experiment with bromide tracer confirmed the occurrence of DNRA deep within the sediment at 2–6 cm was coincident with tracer penetration, and there was a significant positive relationship between bioirrigation and DNRA. We suggest DNRA deep within the sediment was enhanced by increased availability of organic carbon, reduced solutes, and possibly exposure of reworked sediment to (Formula presented.).