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

摘要

We investigated the spatial distribution and seasonal variation of dissolvedinorganic 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 fordetermining ammonium and nitrite oxidation rates (nitrification rates). Weobserved a year-round pattern of dramatic decrease in NH₄+,increase in NO₃−, but insignificant change in NO₂− inthe upper estuary at salinity ~0–5. However, species andconcentrations of inorganic nitrogen at upper estuary significantly changed withseason. In winter, with low runoff, the most upper reach of the Pearl RiverEstuary showed relatively low rates of ammonia oxidation (0–5.4 μmolN 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). Insummer, the upper estuary showed higher nitrification rates (ammoniaoxidation rate ~1.5–33.1 μmol N L−1 d−1, nitriteoxidation rate ~0.6–32.0 μmol N L−1 d−1) with lowerconcentrations of ammonia (<350 μmol L−1) and nitrate (<120 μmol L−1).The Most Probable Number test showed relatively lowernitrifier abundance in summer at most sampling stations, indicating agreater specific nitrification rate per cell in the warm season. Temperatureappeared to control nitrification rates to a large degree in differentseasons. Spatial variability of nitrification rates appeared to becontrolled by a combination of many other factors such as nutrientconcentrations, nitrifier abundance and dissolved oxygen (DO)concentrations. In addition to aerobic respiration, nitrificationcontributed significantly to the consumption of DO and production offree CO₂ at upper estuary. Nitrification-induced consumption accounted for up toapproximately one third of the total water column community DO consumptionin the upper estuary during the surveyed periods, boosting environmentalstress on this large estuarine ecosystem.