Nitrification and inorganic nitrogen distribution in a large perturbed river/estuarine system: the Pearl River Estuary, China

Abstract. We investigated the spatial distribution and seasonal variation of dissolved inorganic nitrogen in a large perturbed estuary, the Pearl River Estuary, based on three cruises conducted in winter (January 2005), summer (August 2005) and spring (March 2006). On-site incubation was also carried out for determining ammonium and nitrite oxidation rates (nitrification rates). We observed a year-round pattern of dramatic decrease in NH₄⁺, increase in NO₃⁻, but insignificant change in NO₂⁻ in the upper estuary at salinity ~0–5. However, species and concentrations of inorganic nitrogen at upper estuary significantly changed with season. In winter, with low runoff, the most upper reach of the Pearl River Estuary showed relatively low rates of ammonia oxidation (0–5.4 μmol N L⁻¹ d⁻¹) and nitrite oxidation (0–5.2 μmol N L⁻¹ d⁻¹), accompanied by extremely high concentrations of ammonia (up to >800 μmol L⁻¹) and nitrate (up to >300 μmol L⁻¹). In summer, the upper estuary showed higher nitrification rates (ammonia oxidation rate ~1.5–33.1 μmol N L⁻¹ d⁻¹, nitrite oxidation rate ~0.6–32.0 μmol N L⁻¹ d⁻¹) with lower concentrations of ammonia (<350 μmol L⁻¹) and nitrate (<120 μmol L⁻¹). The Most Probable Number test showed relatively lower nitrifier abundance in summer at most sampling stations, indicating a greater specific nitrification rate per cell in the warm season. Temperature appeared to control nitrification rates to a large degree in different seasons. Spatial variability of nitrification rates appeared to be controlled by a combination of many other factors such as nutrient concentrations, nitrifier abundance and dissolved oxygen (DO) concentrations. In addition to aerobic respiration, nitrification contributed significantly to the consumption of DO and production of free CO₂ at upper estuary. Nitrification-induced consumption accounted for up to approximately one third of the total water column community DO consumption in the upper estuary during the surveyed periods, boosting environmental stress on this large estuarine ecosystem.