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.

We examined the dynamics of dissolved inorganic nitrogen (DIN, nitrate + nitrite), dissolved inorganic phosphorus (DIP), and silicate (Si(OH)₄) in the northern shelf of the South China Sea in summer, which is under a complex hydrodynamic scheme largely shaped by river plume and coastal upwelling, along with the enhanced biological consumption of nutrients therein. The Pearl River plume, with high nutrient concentrations (∼ 0.1–14.2 µmol L⁻¹ for DIN, ∼ 0.02–0.10 µmol L⁻¹ for DIP, and ∼ 0.2–18.9 µmol L⁻¹ for Si(OH)₄), occupied a large area of the middle shelf (salinity < 33.5). The nearshore area had high nutrient concentrations apparently sourced from subsurface nutrient‐replete waters through wind‐driven coastal upwelling. These nutrient levels were significantly elevated relative to those on the oligotrophic outer shelf where DIN, DIP, and Si(OH)₄ concentrations dropped to < 0.1 µmol L⁻¹, ∼ 0.02–0.03 µmol L⁻¹, and ∼ 2.0 µmol L⁻¹, respectively. A three end‐member mixing model was constructed based on potential temperature and salinity conservation to assess biological consumption of inorganic nutrients, which was denoted by Δ and defined by the deviation from conservative mixing. In the coastal upwelling zone and deep chlorophyll maximum layer, the nutrient uptake ratio ΔDIN : ΔDIP was 16.7, which is the classic Redfield ratio. In contrast, in the river plume the uptake ratio was 61.3 ± 8.7. We believed that an alternative non‐DIP source likely contributed to this higher DIN : DIP consumption ratio in the river plume regime. Meanwhile, Si(OH)₄ showed predominant consumption in the river plume and a combination of regeneration and consumption along the path of the coastal upwelling current.