Abstract
Coastal downwelling is generally considered to have a limited biological effect compared with coastal upwelling. In this study, downslope transport of nearshore, nutrient-enriched waters during downwelling is found to induce distinct biological productivity in the water column over the northeastern South China Sea (NSCS). By conducting a process-driven study over a widened shelf with intensified downwelling in the NSCS, we investigated the biophysical processes associated with strong nutrient enrichment in the water column of downwelled waters. These processes and underlying mechanisms are largely unreported and remain unclear. Field measurements and a three-dimensional coupled physical-biological model incorporating nitrate (N), phytoplankton (P), zooplankton (Z), and detritus (D) were utilized to investigate distinct cross-shore nutrient transport over the uniquely widened NSCS shelf. We revealed that intensified downwelling circulation, dynamically induced by the widened shelf topography, enhanced chlorophyll a and biological productivity in a strip of well-mixed water over the inner shelf as well as in the downwelled water over the mid-shelf. Strong time lags and spatial differences existed among N, P, and Z because of the physical transport and the ensuing biogeochemical response. The intensified downslope transport of nutrient-rich coastal water formed distinct cross-shore wedge-shaped P, Z, and D structures, while N was rapidly consumed in the water column. This study illustrates the underlying coupled physical-biogeochemical processes associated with the observed biogeochemical response to wind-driven downwelling circulation over the variable shelf, which are commonly found in coastal oceans worldwide.
Highlights
Nutrient-rich waters over continental shelves are mainly provided by river input, coastal upwelling, and submarine groundwater discharge, and these waters have high biological productivity (Walsh, 1991; Jickells, 1998; Gan et al, 2010; Han et al, 2013)
A northeasterly monsoon drives the flow of the China Coastal Current (CCC) southwestward along the coast, supplying the northeastern South China Sea (NSCS) shelf with water masses that contain nutrient-rich waters discharged from the Changjiang River (Chen, 2008; Han et al, 2013)
We showed that inter-shelf nutrient transport from the East China Sea (ECS) via the CCC contributes a major nutrient source to the NSCS shelf (Han et al, 2013)
Summary
Nutrient-rich waters over continental shelves are mainly provided by river input, coastal upwelling, and submarine groundwater discharge, and these waters have high biological productivity (Walsh, 1991; Jickells, 1998; Gan et al, 2010; Han et al, 2013). In this study, based on our previous in situ observations (Han et al, 2013), we construct a three-dimensional, coupled physical-biological, nitrogen-based ecosystem model to investigate time-dependent, three-dimensional biological responses to the intensified coastal downwelling circulation over a unique widened shelf in the NSCS. Since the intensified downslope transport occurred between Shantou and Shanwei where the distinctly widened shelf is located (Gan et al, 2013), our study concentrated on Transects 2 and 4 (Figure 1B) to illustrate the biophysical characteristics over the NSCS shelf during downwelling. Observed at a different time (1 week later), the variables along Transect 4 (Figures 3C–F) exhibited similar downwelling features to those of Transect 2 These were associated with the alongshore transport of the CCC and with intensified downslope cross-shore transport over the widened shelf. The enhanced nutrient and Chl a concentrations with increasing depth at these two stations provide further evidence of the strong downslope nutrient and Chl a supply from nearshore waters
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