Biological response to intensified upwelling and to a river plume in the northeastern South China Sea: A modeling study

Abstract

[1] A coupled three-dimensional physical model and a nitrogen-based dissolved inorganic nitrogen, phytoplankton, zooplankton, and detritus (NPZD) ecosystem model was used to study the ecosystem responses to the wind-driven summer upwelling and to the Pearl River plume over a distinctly widened shelf in the northeastern South China Sea (NSCS). Forced with an idealized, but representative, upwelling-favorable wind and the river discharge for the purpose of process-oriented study, we identified two high chlorophyll centers that are typically observed over the NSCS shelf and stimulated by nutrient enrichment from intensified upwelling over the widened shelf and from the river plume. The nutrient enrichment has strong along-shore variability involving the variable cross-isobath nutrient transport between the middle and the inner widened shelf during the upwelling and an eastward expansion of the nutrient-rich plume. About 20% of the upwelled nutrientrich deep water from the outer shelf reaches the inner shelf where algal blooms occur. Nutrient enrichment in the plume stretches over a broad extent of the shelf and produces significant biomass on the NSCS shelf. The plume is physically governed by intensified surface Ekman dynamics that leads to a strong offshore nutrient transport and eventually offsets the shoreward transport caused by the upwelling in the NSCS. Biological forcing and circulation dynamics of the surface Ekman layer jointly form the spatial dislocation and temporal variation of NO 3 , phytoplankton, and zooplankton biomasses in the upwelled and plume waters. The simulated results qualitatively resemble field and satellite measurements and demonstrate the physically modulated biological responses to the intensified upwelling and plume-influenced NSCS shelf.

Phytoplankton dynamics driven by vertical nutrient fluxes during the spring inter-monsoon period in the northeastern South China Sea

“…Vertical velocity (w) at the base of the euphotic zone is assumed equal to the curl-driven upwelling/downwelling velocity (w e ) by Ekman pumping. We have neglected Ekman transport as its effect is restricted only to the near coast (Gan et al, 2010). Variations of w during the transect study is consistent with the isopycnal oscillation along the section (Fig.…”

“…These three different zones were influenced by a diverse set of physical processes. The coastal ocean zone, which can be further separated into two subregions including the nearshore area (stations C 1−2 ) and the continental shelf (stations C 3−6 ), was strongly affected by wind-driven upwelling processes including Ekman transport and Ekman pumping (Gan et al, 2010). The nearshore area was characterized by low sea surface temperature (Fig.…”

“…There are typically high nutrients in the coastal regions of the nSCS due to river discharge, inter-shelf transport, and upwelling and mixing (Gan et al, 2010), in contrast to the oligotrophic low-latitude offshore regions with strong stratification. The nSCS is also strongly influenced by Kuroshio intrusion through the Luzon Strait (Farris and Wimbush, 1996).…”

“…Wind-driven coastal upwelling, river discharge and inter-shelf nutrient transport were important mechanisms supplying nutrients to the euphotic zone of the nSCS (Liu et al, 2002;Gan et al, 2010;Han et al, 2013), while their contributions to primary production were mostly limited to coastal regions as these nutrients would be mostly utilized in the coastal waters before reaching the large area of the nSCS. Kuroshio intrusion would dilute the nSCS waters with the low-nutrient North Pacific waters (Farris and Wimbush, 1996), which appeared to be much weaker during April-September (Centurioni et al, 2004).…”

On the consistency of MODIS chlorophyll $a$ products in the northern South China Sea

“…Assuming that +25 % higher than climatology indicated a positive anomaly, a unique positive anomaly was found in 2009 for C_OC3M and C_GSM, while a > 25 % anomaly was found in 2008 for C_GIOP. Based on these observations, it could be inferred that summer blooms associated with river plumes and upwelling (e.g., Gan et al, 2010;Dai et al, 2008) were relatively weak in 2004. The bloom would however be inferred to be strong in 2009 if it was based on C_OC3M and C_GSM, or in 2008 if it was based on C_GIOP.…”