Nutrient dynamics and biological consumption in a large continental shelf system under the influence of both a river plume and coastal upwelling
Limnol. Oceanogr., 57(2), 2012, 486-502 | DOI: 10.4319/lo.2012.57.2.0486
ABSTRACT: 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.