Abstract
We have investigated the effect of eddies (cold and warm eddies, CEs and WEs) on the nutrient supply to the euphotic zone and the organic carbon export from the euphotic zone to deeper parts of the water column in the northern South China Sea. Besides basic hydrographic and biogeochemical parameters, the flux of particulate organic carbon (POC), a critical index of the strength of the oceanic biological pump, was also measured at several locations within two CEs and one WE using floating sediment traps deployed below the euphotic zone. The POC flux associated with the CEs (85 ± 55 mg-C m−2 d−1) was significantly higher than that associated with the WE (20 ± 7 mg-C m−2 d−1). This was related to differences in the density structure of the water column between the two types of eddies. Within the core of the WE, downwelling created intense stratification which hindered the upward mixing of nutrients and favored the growth of small phytoplankton species. Near the periphery of the WE, nutrient replenishment from below did take place, but only to a limited extent. By far the strongest upwelling was associated with the CEs, bringing nutrients into the lower portion (∼50 m) of the euphotic zone and fueling the growth of larger-cell phytoplankton such as centric diatoms (e.g., Chaetoceros, Coscinodiscus) and dinoflagellates (e.g., Ceratium). A significant finding that emerged from all the results was the positive relationship between the phytoplankton carbon content in the subsurface layer (where the chlorophyll a maximum occurs) and the POC flux to the deep sea.
Highlights
Eddies are ubiquitous in oligotrophic regions of the open ocean
Our study provided in situ and direct measurement of particulate organic carbon (POC) flux associated with CEs and WE in the northern SCS (NSCS)
The enhanced POC export was primarily due to the greater abundance of large cell phytoplankton at depths ranging between 30 and 90 m
Summary
Eddies are ubiquitous in oligotrophic regions of the open ocean. They are believed to induce an upward mixing of deep, nutrient-enriched waters, alleviating the nutrient limitation of the euphotic zone, and resulting in an increase in primary production (PP, as known as biological carbon fixation), which in turn enhances its prospect for export out of the euphotic zone (Hung et al, 2004; Hung et al, 2010a; Zhou et al, 2013; Shih et al, 2015; Boyd et al, 2019). Warm eddies can further be split into two types depending on whether surface water sinks, depressing the thermocline and taking nutrients away from the photic zone or, on the contrary, the thermocline forms a dome which brings nutrients closer to the sea surface and results high phytoplankton biomass (Sweeney et al, 2003; McGillicuddy et al, 2007) Despite this broad classification, recent research has produced inconsistent results among studies conducted in various marine environments when it comes to the intensity of vertical mixing and/or the stimulation of phytoplankton blooms (Hung et al, 2004; Hung et al, 2010a; Zhou et al, 2013; Chen et al, 2015; Shih et al, 2015). Field observations can provide detailed spatial variations regarding thermocline/ pycnocline and nutrients in the water column of the euphotic zone, but only in selected regions and on an episodic basis
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