Nutrient dynamics and biological consumption in a large continental shelf system under the influence of both a river plume and coastal upwelling

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 mol 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. © 2012, by the Association for the Sciences of Limnology and Oceanography, Inc.