Short-term hypoxia alters the balance of the nitrogen cycle in coastal sediments
Limnol. Oceanogr., 56(2), 2011, 651-665 | DOI: 10.4319/lo.2011.56.2.0651
ABSTRACT: We measured denitrification, anaerobic ammonium oxidation (anammox), oxygen uptake, nutrient exchange, and pore-water profiles of oxygen in intact sediments at three sites in the southern North Sea, which we experimentally exposed to different oxygen saturations (ambient and ∼ 33% of air-saturation for oxygen [i.e., our hypoxic treatment]) over 14 months. Denitrification ranged from 1 µmol N m−2 h−1 to 21 µmol N m−2 h−1, anammox 0.2 µmol N m−2 h−1 to 5.7 µmol N m−2 h−1, and oxygen uptake 47 µmol O2 m−2 h−1 to 631 µmol O2 m−2 h−1. The seasonal patterns under ambient oxygen were correlated with those in the hypoxic treatment; though, on the whole, the magnitude of flux was different. On average, under hypoxia, both the penetration and consumption of oxygen decreased by ∼ 50%, denitrification increased by 32%, and anammox remained constant. Anammox accounted for between 10% and 20% of the total N2 production, which agrees with expectations for waters of these depths (30–80 m). Under ambient oxygen the sediments were strong sources of nitrate to the overlying water, 12 µmol NO−3 m−2 h−1 on average, but under hypoxia total N mineralization decreased by 46% and nitrate exchanged ceased. Short-term hypoxia alters the balance between available N returned to the overlying water, primarily as NO−3, and that removed from the ecosystem as N2 gas.