Impacts of mesoscale eddies in the South China Sea on biogeochemical cycles (original) (raw)
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Modeled biogeochemical responses to mesoscale eddies in the South China Sea
Journal of Geophysical Research, 2011
Mesoscale eddies are observed each year in the South China Sea (SCS); however, their contributions to the biogeochemical cycles have never been systematically quantified. Here, we use a coupled three-dimensional physical-biogeochemical model to evaluate the eddy impact. We first track the modeled mesoscale eddies in the SCS and then analyze the biogeochemical responses to these eddies individually. Compared with the SCS basin mean, modeled depth-integrated (0-125 m) chlorophyll, zooplankton, new production, and silicate uptake are significantly enhanced in the cyclonic eddies and reduced in the anticyclonic eddies. Following the movements of the eddy center, temporal variations of phytoplankton community structure suggest that diatoms respond to cyclonic eddies strongly first and the responses last longer; then picoplankton grow after the diatoms. In the cyclonic eddies, modeled new production is 1.87 ± 0.37 mmol N m −2 d −1 , which is 28% higher than the SCS basin-averaged value, while in the anticyclonic eddies, modeled new production is about 32% lower than the SCS basin mean. As a consequence, modeled detrital nitrogen export for cyclonic eddies is 41% higher than the SCS basin mean, and that for anticyclonic eddies is 31% lower than the SCS basin mean. These values experience strong interannual variations with anomalously low magnitudes found during El Niño conditions for both of the eddies and the SCS basin mean. Our results indicate that cyclonic eddies in the SCS are important sources of nutrients to the euphotic zone and therefore play a significant role in regulating biological productivity and the carbon cycle.
Mesoscale eddies are observed each year in the South China Sea (SCS); however, their contributions to the biogeochemical cycles have never been systematically quantified. Here, we use a coupled three-dimensional physical-biogeochemical model to evaluate the eddy impact. We first track the modeled mesoscale eddies in the SCS and then analyze the biogeochemical responses to these eddies individually. Compared with the SCS basin mean, modeled depth-integrated (0-125 m) chlorophyll, zooplankton, new production, and silicate uptake are significantly enhanced in the cyclonic eddies and reduced in the anticyclonic eddies. Following the movements of the eddy center, temporal variations of phytoplankton community structure suggest that diatoms respond to cyclonic eddies strongly first and the responses last longer; then picoplankton grow after the diatoms. In the cyclonic eddies, modeled new production is 1.87 ± 0.37 mmol N m −2 d −1 , which is 28% higher than the SCS basin-averaged value, while in the anticyclonic eddies, modeled new production is about 32% lower than the SCS basin mean. As a consequence, modeled detrital nitrogen export for cyclonic eddies is 41% higher than the SCS basin mean, and that for anticyclonic eddies is 31% lower than the SCS basin mean. These values experience strong interannual variations with anomalously low magnitudes found during El Niño conditions for both of the eddies and the SCS basin mean. Our results indicate that cyclonic eddies in the SCS are important sources of nutrients to the euphotic zone and therefore play a significant role in regulating biological productivity and the carbon cycle.
The asymmetric distribution of phytoplankton in anticyclonic eddies in the western South China Sea
Deep Sea Research Part I: Oceanographic Research Papers, 2017
An anticyclonic eddy's periphery is characterized by large horizontal density gradients, strain and vertical velocity. In this paper we document the asymmetric distribution of phytoplankton around the periphery of anticyclonic eddies in the western South China Sea based on 432 eddies detected from satellite data. The high level of phytoplankton occurs consistently at the northwestern periphery of eddies, with a maximum positive chlorophyll anomaly greater than 0.01 mg m −3. The asymmetric distribution of phytoplankton primarily tags the non-uniform surface velocity field varying from 0.15 m s −1 to 0.3 m s −1 along the eddy's periphery. The coastal boundary and off-coast jet may be the primary cause of the non-uniform flow. Associated with the nonuniform flow speed, the combined effects of the velocity convergence and steepened northwestern part of the eddies act to sharpen the density fronts and tend to result in an ageostrophic secondary circulation at the northwestern edge of the eddy. The upward component of the ageostrophic secondary circulation can enhance the nutrient flux into the euphotic layer, thereby increasing phytoplankton productivity. Anticyclonic eddyinduced ageostrophic secondary circulation appears to be an important mechanism for increasing phytoplankton productivity in the oligotrophic waters of the South China Sea.
Phytoplankton community at warm eddies in the northern South China Sea in winter 2003/2004
Deep Sea Research Part II: Topical Studies in Oceanography, 2010
Two anti-cyclonic (warm) eddies were located in the northern South China Sea (SCS) during winter 2003/2004. They were monitored using sea-level anomaly data provided by satellite altimetry, surface drifters and in situ hydrographic data. Phytoplankton Chl a biomass as well as community structure were determined using high-performance liquid chromatography. Both warm eddies displayed positive sea-level anomaly, high sea-surface temperature and similar phytoplankton Chl a biomass compared with the reference area. However, phytoplankton communities we observed in these two eddies were significantly different. In one warm eddy which was formed by the Kuroshio intrusion, the phytoplankton community was dominated by prochlorophyceae within the euphotic zone. In the other warm eddy, which might be formed in coastal waters of northern SCS, haptophyceae dominated in the euphotic zone. This difference in the phytoplankton community was due to the different origins and ages of the two warm eddies.
Aquatic Microbial Ecology, 2009
We examined the effect of 2 cold-core cyclonic eddies on the activity of the prokaryotic plankton community in the South China Sea. The abundance of bulk prokaryotes and major prokaryotic groups (Bacteria, marine Crenarchaeota Group I and marine Euryarchaeota Group II) and the number of cells taking up D-vs. L-aspartic acid (Asp) were determined using microautoradiography combined with catalyzed reporter deposition fluorescence in situ hybridization (MICRO-CARD-FISH). At all sites, the bulk D-Asp:L-Asp uptake ratio by the prokaryotic community increased with depth. Concurrently, the contribution of marine Crenarchaeota Group I to total prokaryotic abundance and total active cells also increased with depth, while an opposite pattern was observed for Bacteria. Marine Euryarchaeota Group II were generally more dominant in near-surface than in deep waters. Significant differences were observed between sites inside and outside the cyclonic eddies in terms of distribution and activity of prokaryotic communities and the concentration of fluorescent dissolved organic matter (FDOM, an important refractory fraction of DOM). Generally, higher bulk D-Asp:L-Asp uptake ratios by the prokaryotic community and a greater crenarchaeotal contribution were found in the upper mesopelagic water column inside the cold-core eddies as compared to the outside sites. Taken together, the MICRO-CARD-FISH data and the pattern of FDOM indicate that the higher contribution of refractory DOM induced by upwelled water in the cyclonic eddy may have led to a more prominent role of Crenarchaeota in the organic carbon cycling in the mesopelagic realm of the cold-core eddy than outside the eddy in the South China Sea.
Deep Sea Research Part II: Topical Studies in Oceanography, 2008
Mesoscale currents, fronts, and eddies are ubiquitous and energetic features of ocean circulation. These phenomena, sometimes referred to as the "internal weather of the sea," accommodate a diverse set of physical, chemical, and biological interactions that influence marine biogeochemistry on a wide range of timescales. These biogeochemical processes include the "biological pump", i.e. the transfer or flux of biologically produced organic matter and associated elements from the surface ocean to depth . Within ~ 80% of the world's oceans, the productivity and species composition of the autotrophic organisms that contribute to the biological pump are typically limited by major nutrients (e.g. nitrogen, phosphorus, and silica), or trace metals (e.g. iron). Primary production in such oligotrophic regions therefore depends mostly on intense recycling of nutrients within the surface sunlit waters, with only a small fraction supported by that entering from the atmosphere, or from the physical transport of nutrients from nutrient-rich deep waters below. Evidence that mesoscale and submesoscale phenomena play a role in the latter process dates back more than two decades
Frontiers in Microbiology
To examine the influence of mesoscale eddy on the natural phytoplankton community and its sinking rate changes, a comprehensive investigation cruise was carried out in the western South China Sea in autumn 2016. A total of 108 phytoplankton species were found, which belong to 54 phytoplankton genera; most of them were dominated by Dinophyta (54 genera), followed by Bacillariophyta (50 genera), Cyanophyta (3 genera), and Chrysophyta (1 genus). Bacillariophyta and Dinophyta were the main phytoplankton communities in the investigated sea area. The sinking rate of phytoplankton ranged from 0.12 to 3.17 m day–1, determined by the SETCOL method. The highest phytoplankton sinking rate was found in the 200-m water layer, followed by the DCM layer. No significant correlation was found between phytoplankton sinking rates and most of the environmental parameters during this cruise. At a similar time, we have carried out the estimation of carbon flux in the investigated sea area by using the si...
Pathways of mesoscale variability in the South China Sea
Chinese Journal of Oceanology and Limnology, 2010
The propagation of oceanic mesoscale signals in the South China Sea (SCS) is mapped from satellite altimetric observations and an eddy-resolving, global ocean general circulation model by using the maximum cross-correlation (MCC) method. Significant mesoscale signals propagate along two major bands of high variability. The northern band is located west of the Luzon Strait, characterized by southwestward eddy propagation. Though eddies are the most active in winter, their southwestward migrations exist throughout the year, steered by bathymetry. Advection by the mean flow plays a secondary role in modulating the propagating speed. The southern eddy band lies in the southwest part of the SCS deep basin and is oriented in an approximately meridional direction. Mesoscale variability propagates southward along the band in autumn. This southward eddy pathway could not be explained by mean flow advection and is likely related to the eddy detachments from the western boundary current due to nonlinear effects. Our mapping of eddy propagation velocities provides important information for further understanding eddy dynamics in the SCS.
PloS one, 2015
Eddies play a critical role in regulating the biological pump by pumping new nutrients to the euphotic zone. However, the effects of cyclonic eddies on particle export are not well understood. Here, biogenic silica (BSi) and particulate organic carbon (POC) exports were examined inside and outside a decaying cyclonic eddy using 234Th-238U disequilibria in the tropical South China Sea. For the eddy and outside stations, the average concentrations of BSi in the euphotic zone were 0.17±0.09 μmol L-1 (mean±sd, n = 20) and 0.21±0.06 μmol L-1 (n = 34). The POC concentrations were 1.42±0.56 μmol L-1 (n = 34) and 1.30±0.46 μmol L-1 (n = 51). Both BSi and POC abundances did not show change at the 95% confidence level. Based on the 234Th-238U model, BSi export fluxes in the eddy averaged 0.18±0.15 mmol Si m-2 d-1, which was comparable with the 0.40±0.20 mmol Si m-2 d-1 outside the eddy. Similarly, the average POC export fluxes were 1.5±1.4 mmol C m-2 d-1 and 1.9±1.3 mmol C m-2 d-1 for the edd...
Mesoscale Eddies Drive Increased Silica Export in the Subtropical Pacific Ocean
Science, 2007
Mesoscale eddies may play a critical role in ocean biogeochemistry by increasing nutrient supply, primary production, and efficiency of the biological pump, that is, the ratio of carbon export to primary production in otherwise nutrient-deficient waters. We examined a diatom bloom within a cold-core cyclonic eddy off Hawai`i. Eddy primary production, community biomass, and size composition were markedly enhanced but had little effect on the carbon export ratio. Instead, the system functioned as a selective silica pump. Strong trophic coupling and inefficient organic export may be general characteristics of community perturbation responses in the warm waters of the Pacific Ocean.