Speciation and dynamics of dissolved inorganic nitrogen export in the Danshui River, Taiwan

Abstract

Abstract. Dissolved inorganic nitrogen (DIN, including ammonium, nitrite and nitrate) export from land to ocean is becoming dominated by anthropogenic activities and severely altering the aquatic ecosystem. However, rare observational analyses have been conducted in the Oceania, the hotspot of global DIN export. In this study a whole watershed monitoring network (20 stations) was conducted in 2003 to investigate the controlling factors of DIN export in the Danshui River of Taiwan. The results showed that DIN concentration ranged from ∼16 μM in the headwater and up to ∼430 μM in the estuary. However, the dominating DIN species transformed gradually from NO3− in the headwater (∼97%) to NH4+ in the estuary (∼70%), which well followed the descending dissolved oxygen (DO) distribution (from ∼8 mg L−1 to ∼1 mg L−1). NO2− was observed in the transition zone from high to low DO. DIN yield was increasing downstream, ranging from ∼160 to ∼6000 kg N km−2 yr−1 as population density increases toward the estuary, from ∼15 pop km−2 to ∼2600 pop km−2. Although the individual DIN export, ∼2.40 kg N person−1 yr−1, was comparable to the global average, the close-to-top DIN yield was observed owing to abundant rainfall, dense population, and the sensitive response to population increase. The Danshui River occupies 1.8 × 10−3% of the land surface area of the Earth but discharges disproportionately high percentage, ∼60 × 10−3% (∼14 000 t N yr−1) of the annual global DIN export to the ocean. Through this study, regulating factors and the significance of human population on DIN export were identified, and the regional databases were supplemented to promote the completeness of global models.