Denitrification and oxygen consumption in the sediments of four New Zealand estuaries

Rates of sediment denitrification were determined under illuminated conditions by the ¹⁵N Isotope Pairing Method (IPM) in four New Zealand estuaries (Avon-Heathcote, Waikouaiti, Tokomairiro, and Tautuku), which ranged in size, nutrient loading, and catchment land use. The proportion of the gaseous end products (N₂O produced relative to N₂) and differentiation of denitrification supported by NO₃⁻ from the water column (D(w)) or NO₃⁻ produced via nitrification (D(n)) were determined with the overall aims of quantifying the rates of denitrification and identifying which environmental factors influenced the rates. Denitrification rates ranged from 0-12 μmol m⁻² h⁻¹ in the Waikouaiti River estuary, from 0-118 μmol m⁻² h⁻¹ in the Avon-Heathcote and Tautuku River estuaries, and from 43-347 μmol m⁻² h⁻¹ in the Tokomairiro River estuary. The size of the sediment particles, temperature, water column pH, chlorophyll a content of the sediment, and biomass of annelids and arthropods were related to the rates of denitrification. At most sites, the concentration of NO₃⁻ and NO₂⁻ in the water column was 86 % of the total denitrification. The rate of D(w) and the ratio of N₂O:N₂ produced increased with higher DIN concentration in the water column. Denitrification was measured temporally in the Avon-Heathcote estuary. Although there were some significant differences in the rates of denitrification among the sampling events, no seasonal differences in the rates of denitrification were detected. The IPM is based on several assumptions that must be met in order to achieve accurate rates of denitrification; thus, these assumptions were tested by adding three concentrations of ¹⁵NO₃⁻ to the incubation chambers and measuring the isotope production ratios of N₂O and N₂ over time. Fluxes of oxygen across the sediment-water interface (determined under dark and illuminated conditions by sediment core incubations and microelectrode profiles) and the potential for anaerobic ammonium oxidation (anammox) in the sediment were quantified and discussed in the context of their influences on sediment denitrification. Ladderane lipids, a bioindicator of bacteria capable of anammox, were not detected at any of the study sites where it was measured. Oxygen uptake by the sediments ranged from 146-1597 μmol m⁻² h⁻¹ and was much higher than oxygen release, indicating low benthic primary production and a high oxygen demand by these sediments. The sediment oxygen penetration depth at the sites ranged from 2.7-9.6 mm. The results indicate that maintaining an oxic layer in the sediment is essential for the maintenance of denitrification