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 mmol L 21 for DIN, , 0.02-0.10 mmol L 21 for DIP, and , 0.2-18.9 mmol L 21 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 mmol L 21 , , 0.02-0.03 mmol L 21, and , 2.0 mmol L 21, 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 D and defined by the deviation from conservative mixing. In the coastal upwelling zone and deep chlorophyll maximum layer, the nutrient uptake ratio DDIN : DDIP was 16.7, which is the classic Redfield ratio. In contrast, in the river plume the uptake ratio was 61.3 6 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.

Seasonal variations of sea–air CO<sub>2</sub> fluxes in the largest tropical marginal sea (South China Sea) based on multiple-year underway measurements

“…Plots of temperature-normalized pCO 2 versus salinity (Fig. 10a) revealed that many of those summer data were associated with an estuarine plume (with a salinity of less than 33), where biological productivity was enhanced in the surface water due to nutrient support from the eutrophicated PRE (Cao et al, 2011;Han et al, 2012). Dai et al (2008), Zhai et al (2009), andCao et al (2011) have shown that the Pearl River plume can serve as a CO 2 sink.…”

“…We considered four contrasting physical-biogeochemical domains, and were able to significantly improve our understanding regarding the spatial variability of CO 2 fluxes. Among these four domains, A was adjacent to the Pearl River estuary (PRE) and was thus influenced by the summer estuarine plume (Gan et al, 2009;Cao et al, 2010;Han et al, 2012). Domain B covered the slope and deep basin areas in the northern SCS, which is typically oligotrophic and low in productivity .…”

Distributions and sea-to-air fluxes of nitrous oxide in the South China Sea and the West Philippines Sea

“…1a; Table 1 Samples were collected close to the south of the Taiwan Strait, which is located in the northeastern part of the SCS, during the cruises in August 2003 (ORIII-896) and July 2004 (ORIII-983). ORI-695 was conducted near the Pearl River estuary and was influenced by the summer estuarine plume (Gan et al, 2009;Cao et al, 2011;Han et al, 2012;Zhai et al, 2013;Bai et al, 2015). The Pearl River (Zhujiang) is the 13th largest river in the world (Yin et al, 2004) and the second largest river in China (after the Yangtze River) with a water discharge of 326 Â 10 9 m 3 annually; it is also the second largest river (after the Mekong River) that enters the SCS (Chen et al, 2008).…”

Spatial structure of eukaryotic ultraplankton community in the northern South China Sea

“…The Pearl River, as the largest river draining into the nSCS, carries the considerable loads of anthropogenic nutrients (Han et al 2012). The nSCS has an extensive continental shelf with a variable offshore extension along the southern coasts of China.…”