Mekong River Plume Research Articles

Dynamical Impact of the Mekong River Plume in the South China Sea

AbstractNear the ocean surface, river plumes influence stratification, buoyancy and transport of tracers, nutrients and pollutants. The extent to which river plumes influence the overall circulation, however, is generally poorly constrained. This work focuses on the South China Sea (SCS) and quantifies the dynamical impacts of the Mekong River plume, which is bound to significantly change in strength and seasonality in the next 20 years if the construction of over hundred dams moves ahead as planned. The dynamic impact of the freshwater fluxes on the SCS circulation are quantified by comparing submesoscale permitting and mesoscale resolving simulations with and without riverine input between 2011 and 2016. In the summer and early fall, when the Mekong discharge is at its peak, the greater stratification causes a residual mesoscale circulation through enhanced baroclinic instability. The residual circulation is shaped as an eddy train of positive and negative vorticity. Submesoscale fronts are responsible for transporting the freshwater offshore, shifting eastward the development of the residual mesoscale circulation, and further strengthening the residual eddy train in the submesoscale permitting case. Overall, the northward transport near the surface is intensified in the presence of riverine input. The significance of the mesoscale‐induced and submesoscale‐induced transport associated with the river plume is especially important in the second half of the summer monsoon season, when primary productivity has a secondary maximum. Circulation changes, and therefore productivity changes, should be anticipated if human activities modify the intensity and seasonality of the Mekong River plume.

Environmental Regulation of the Nitrogen Supply, Mean Trophic Position, and Trophic Enrichment of Mesozooplankton in the Mekong River Plume and Southern South China Sea

AbstractThe mean trophic position (TP) of mesozooplankton largely determines how much mass and energy is available for higher trophic levels like fish. Unfortunately, the ratio of herbivores to carnivores in mesozooplankton is difficult to identify in field samples. Here, we investigated changes in the mean TP of mesozooplankton in a highly dynamic environment encompassing four distinct habitats in the southern South China Sea: the Mekong River plume, coastal upwelling region, shelf waters, and offshore oceanic waters. We used a set of variables derived from bulk and amino acid nitrogen stable isotopes from particulate organic matter and four mesozooplankton size fractions to identify changes in the nitrogen source and TP of mesozooplankton across these habitats. We found clear indications of a shift in N sources for biological production from nitrate in near‐coastal waters with shallow mixed layer depths toward an increase in diazotroph‐N inputs in oceanic waters with deep mixed layer depths where diazotrophs shaped the phytoplankton community. The N source shift was accompanied by a lengthening of the food chain (increase in the TP). This may provide further support for the connection between diazotrophy and the indirect routing of N through the marine food web. Our combined bulk and amino acid δ15N approach also allowed us to estimate the trophic enrichment (TE) of mesozooplankton across the entire regional ecosystem. When put in the context of literature values, a high TE of 5.1‰ suggested a link between ecosystem heterogeneity and the less efficient transfer of mass and energy across trophic levels.

Source and Fate of Dissolved Black Carbon in the Western South China Sea During the Southwest Monsoon Prevailing Season

AbstractDissolved black carbon (DBC) is of importance for understanding the marine carbon cycle especially on long time scales owing to its refractory nature in the dissolved organic carbon pool. However, its geochemical behavior is poorly understood in the open oceans due to limited DBC data. Here 86 seawater samples were determined using the benzene‐polycarboxylic acid method to investigate the source and fate of DBC in the western South China Sea. The DBC concentration varied from 0.49 to 1.60 μmol L−1, averaging 0.95 μmol L−1. Spatially, the Mekong River plume (i.e., MR plume) showed higher DBC concentrations and (B6CA + B5CA)/(B4CA + B3CA) ratios (i.e., RH/L) in the mixed layer than the distal regions. In addition, the DBC concentration positively correlated with salinity in the mixed layer within the MR plume, indicating the important DBC input from the Mekong River. In intermediate water (500–1500 m), DBC varied from 0.70 to 0.85 μmol L−1 and the RH/L value continually increased with depth, implying another DBC source with distinct RH/L ratio comparing with the euphotic zone. Based on a conservative mixing model, the Mekong River and atmospheric deposition collectively input 38–100 Gg DBC each year, and over one third of DBC were removed within the euphotic zone, probably via photodegradation and/or adsorption on particles. These results highlighted the different DBC behavior in the euphotic and intermediate zones and lent support to DBC as a proxy for tracing water mixing in the intermediate and deep oceans.

Microplankton biomass and diversity in the Vietnamese upwelling area during SW monsoon under normal conditions and after an ENSO event

Investigating microplankton biomass and diversity under different climatological conditions is key to the understanding of cascading effects of climate change on nutrient cycles and biological productivity. Here we have used data collected during two contrasting summers along the coast of Viet Nam to show how climatological-driven changes can have a significant influence on the distribution of microplankton communities and their biomass via its impact on nutrient concentrations in the water column. The first summer in July 2003 followed a weak El-Nino Southern Oscillation (ENSO) event and was characterized by weak coastal upwelling, in the second summer during July 2004, upwelling was normal.Very low silicate (SiO4) concentrations and SiO4:DIN ratios characterized the source water mass for upwelling in July 2004, and dynamic SiO4 to dissolved inorganic nitrogen ratios (SiO4:DIN) mainly below the Redfield-Brzezinski ratio and DIN to phosphate ratios (DIN:PO43−) below the Redfield ratio were a common feature off Viet Nam. Much higher particle concentrations and PSi/PC ratios during normal upwelling revealed major changes in the microplankton community structure among summers. Small dinoflagellates (10–20µm) prevailed ubiquitously during reduced upwelling. During normal upwelling, the diatom Rhizosolenia sp. dominated the cell-carbon biomass in the silicate poor upwelling waters. Trichodesmium erythraeum dominated in the Mekong-influenced and nutrient depleted offshore waters, where it co-occurred with Rhizosolenia sp. Both species were directly associated with the much higher primary production (PP) and N2 fixation rates that were quantified in earlier studies, as well as with much higher diversities at these offshore sites. Along the coast, the correlation between Rhizosolenia sp. and PP rates was less clear and the factors regulating the biomass of Rhizosolenia sp. in the upwelling waters are discussed. The very low silicate concentrations in the source water mass for upwelling and the offshore deflection of the Mekong river plume likely triggered the observed ecological differences in the microplankton communities off Viet Nam in the normal upwelling season compared to the post ENSO conditions.

Microbial eukaryotic diversity and distribution in a river plume and cyclonic eddy-influenced ecosystem in the South China Sea.

To evaluate microbial eukaryotic diversity and distribution in mesoscale processes, we investigated 18S rDNA diversity in a river plume and cyclonic eddy-influenced ecosystem in the southwestern South China Sea (SCS). Restriction fragment length polymorphism analysis was carried out using multiple primer sets. Relative to a wide range of previous similar studies, we observed a significantly higher proportion of sequences of pigmented taxa. Among the photosynthetic groups, Haptophyta accounted for 27.7% of the sequenced clones, which belonged primarily to Prymnesiophyceae. Unexpectedly, five operational taxonomic units of Cryptophyta were closely related to freshwater species. The Chlorophyta mostly fell within the Prasinophyceae, which was comprised of six clades, including Clade III, which is detected in the SCS for the first time in this study. Among the photosynthetic stramenopiles, Chrysophyceae was the most diverse taxon, which included seven clades. The majority of 18S rDNA sequences affiliated with the Dictyochophyceae, Eustigmatophyceae, and Pelagophyceae were closely related to those of pure cultures. The results of redundancy analysis and the permutation Mantel test based on unweighted UniFrac distances, conducted for spatial analyses of the Haptophyta subclades suggested that the Mekong River plume and cyclonic eddy play important roles in regulating microbial eukaryotic diversity and distribution in the southwestern SCS.

Phylogenetic composition of Prochlorococcus and Synechococcus in cold eddies of the South China Sea

Two mesoscale cyclonic eddies (CE I and CE II) occurred along the Vietnam coast in the South China Sea during summer 2007; CE I was in the decaying stage and CE II was in the intensifying stage. CE II was also influenced by the Mekong River plume. The upwelling effect of the 2 eddies on the phylogenetic diversity of Prochlorococcus and Synechococcus was investi- gated by clone library construction based on sequences of the internal transcribed spacer (ITS) region, with station SEATS in the oligotrophic basin as a comparison. Phylogenetic analysis revealed the presence of sequences affiliated with 3 Prochlorococcus clades (HL I, HL II & LL IV) and 7 Synechococcus clades (II, III, VII, X, XV, XVI, and sub-cluster 5.2) from the various water bodies, with HL II Prochlorococcus and clade II Synechococcus being the dominant phylotypes. Multivariate analysis demonstrated that spatial distribution of the 2 genera was mostly explained by temperature, but also affected by salinity and chlorophyll a concentration. Upwelling of the nutrient-rich subsurface water at the center of the cold eddy resulted in a significant increase in the abundance of Synechococcus in the young and intensifying CE II, whereas Prochlorococcus dominated in the old and decaying CE I and the surrounding oligotrophic waters. It is intriguing that Synechococcus clades III, XV and XVI, which are known to contain chromatically adapting members, were most numerous in eddy and river plume influenced waters, where light quality changes might be expected, compared to the oligotrophic basin waters. The disparity in cyanobacterial phylogenetic composition between the 2 eddies clearly reflected the differing hydrographic dynamics and developmental stages of these eddy systems.

Distribution of diazotrophic microorganisms and nifH gene expression in the Mekong River plume during intermonsoon

N2 fixation by marine pelagic prokaryotes plays a critical role in supplying new N to the ocean, and there is growing evidence that oceanic N2 fixation is generally enhanced in tropical river plumes, where N2 fixers (diazotrophs) benefit from riverine phosphorus and/or iron. Here we used nifH gene quantitative polymerase chain reaction (QPCR) and reverse transcription (RT) QPCR to investigate the horizontal distribution and activity of 9 diazotroph phylotypes in the Mekong River plume, South China Sea (SCS), in April 2007 (intermonsoon, lowest annual discharge). The nifH gene diversity was investigated by cloning and sequencing. Hydrodynamic modeling of the surface water advection revealed that the same water masses were sampled during the entire study, and helped to elucidate the physical forcing on diazotroph abundances and distributions. According to our esti- mates of nifH abundances and expression, the high N2 fixation rates detected in the Mekong plume can primarily be attributed to Trichodesmium spp. and the symbiotic diatom-diazotroph associations (DDAs) Rhizosolenia -Richelia and Hemiaulus -Richelia. NifH phylotypes from unicellular cyanobac- terial groups B and C and a gammaproteobacterial group (24774A11) were exclusively detected and expressed at oceanic stations. Clone libraries of nifH were dominated by Trichodesmium spp. but also included proteobacterial- and cluster III-like nifH sequences, including new lineages, possibly repre- senting other active diazotrophs in the Mekong plume and SCS offshore waters. This study extends the known distribution of quantitatively important oceanic diazotrophs, and shows that the Mekong River plume hosts a diverse diazotroph community, of which particularly Trichodesmium spp. and DDAs fix N2 at high rates.

Signature of the Mekong River plume in the western South China Sea revealed by radium isotopes

We investigated the distribution of 223Ra, 228Ra, and 226Ra in the surface water of the western South China Sea (SCS) during summer based on a 30 day cruise conducted in August–September 2007. The activities of 223Ra varied from almost undetectable to 0.74 disintegrations per minute (dpm)/100 L, and those of 228Ra varied from 12.2 to 61.5 dpm/100 L. Their spatial distribution was characterized by a jet of high 228Ra (>48 dpm/100 L) and 223Ra (>0.4 dpm/100 L) extending eastward from the Vietnam coast along ∼11°N, curling up in the vicinity of 112°E and swirling counterclockwise to form a cyclonic eddy with lower 228Ra (21–25 dpm/100 L) and 223Ra (0.04–0.14 dpm/100 L) at its center. High 226Ra (10–14 dpm/100 L) appeared in the eastward jet and decreased to 6.0–8.5 dpm/100 L along the track of the jet described above. The observed distribution of Ra isotopes was consistent with the pattern of the Southeast Vietnam Offshore Current in the western SCS in summer. The higher radium activities were in all likelihood derived from the Mekong River. Using a simple two‐end‐member mixing model based on the 228Ra/226Ra activity ratio and salinity, we calculated that approximately 53% of the surface water in the western SCS was originated from the Mekong River diluted water. Note that this estimation should be regarded as an upper limit due to the lack of sampling at its immediate source, the Mekong estuary. The data revealed that more than 2 weeks were required for the transportation of freshened water from the Mekong River's mouth several hundred kilometers to the western SCS.

The Mekong River plume fuels nitrogen fixation and determines phytoplankton species distribution in the South China Sea during low and high discharge season

The influence of the Mekong River (South China Sea) on N2 fixation and phytoplankton distribution was investigated during the lowest‐ and highest‐discharge seasons (April 2007 and September 2008, respectively). The river plays an essential role in providing nutrients (nitrate, phosphate, silicate) for the adjacent sea and creates different salinity and nutrient gradients over different seasons. River water (salinity 0), mesohaline waters (salinity 14–32), a transition zone with salinities between 32 and 33.5, and marine waters (salinity above 33.5) were sampled at different spatial resolutions in both cruises. High N2 fixation rates were measured during both seasons, with rates of up to 5.05 nmol N L−1 h−1 in surface waters under nitrogen‐replete conditions, increasing to 22.77 nmol N L−1 h−1 in nitrogen‐limited waters. Asymbiotic diatoms were found only close to the river mouth, and symbiotic diatoms, which potentially hosted diazotrophs, were most abundant in waters where N2 fixation rates were highest, nitrate concentrations were at the detection limit, and phosphate and silicate were still available. Filamentous cyanobacteria like Trichodesmium were present only in marine waters with salinities above 33.5. Overall, N2 fixation accounts for 1‐47% of the nitrogen demand of primary production.

Close coupling between phytoplankton growth and microzooplankton grazing in the western South China Sea

We conducted 28 dilution experiments during August‐September 2007 to investigate the coupling of growth and microzooplankton grazing rates among ultraphytoplankton populations and the phytoplankton community and their responses to habitat variability (open‐ocean oligotrophy, eddy‐induced upwelling, and the Mekong River plume) in the western South China Sea. At the community level, standing stocks, growth, and grazing rates were strongly and positively correlated, and were related to the higher abundance of larger phytoplankton cells (diatoms) at stations with elevated chlorophyll concentration. Phytoplankton growth rates were highest (>2 d−1) within an eastward offshore jet at 13°N and at a station influenced by the river plume. Among ultraphytoplankton populations, Prochlorococcus dominated the more oceanic and oligotrophic stations characterized by generally lower biomass and phytoplankton community growth, whereas Synechococcus became more important in mesotrophic areas (eddies, offshore jet, and river plume). The shift to Synechococcus dominance reflected, in part, its higher growth rates (0.87 ± 0.45 d−1) compared to Prochlorococcus (0.65 ± 0.29 d−1) or picophytoeukaryotes (0.54 ± 0.50 d−1). However, close coupling of microbial mortality rates via common predators is seen to play a major role in driving the dominance transition as a replacement of Prochlorococcus, rather than an overprinting of its steady‐state standing stock.

Barrier layer in the South China Sea during summer 2000

Using temperature–salinity profiles obtained from a cruise in summer 2000, the structure and formation of the barrier layer (BL) in the South China Sea (SCS) are investigated. Fresh water flux, ocean circulation, and wind stirring are important for BL formation, depending on regions. In the eastern SCS, Philippine mountains induce heavy rainfall, resulting in a fresh water cap at the surface and the formation of a thick wide spread BL. In the northwestern basin on the lee of the Annam Cordillera range, by contrast, a rain shadow reduces fresh water flux, which along with wind-induced upwelling, prevents the BL forming. Southeast of Vietnam, a thick BL forms as the Mekong River plume is advected by the northeastward western boundary current and its offshore extension. In the southeastern basin, the surface water is mixed deeply under the strong southwesterly monsoon, unfavorable for the BL formation despite heavy rainfall. In the Luzon Strait, the east/southeastward surface Ekman drift carries fresh SCS surface water, riding on the intruding Kuroshio meander that carries well-mixed, warm and saline water. The vertical overlapping of these two water masses gives rise to a thick BL.

Diversity and abundance of diazotrophic microorganisms in the South China Sea during intermonsoon

The spatial heterogeneity of diversity and abundance of diazotrophs were investigated off the Vietnamese coast in the South China Sea (SCS). The study area extended from the Mekong River plume to the shelf region and beyond to stations extending to 1700 m depth. The SCS diazotroph community, based on nifH gene diversity, had components closely related to sequences from open ocean, estuarine, saltmarsh and microbial mat communities. Rarefaction analysis suggested that by using a 97% similarity operational taxonomic unit definition, the majority of nifH sequence diversity in the samples was covered by the 384 nifH clones obtained. The majority of the nifH sequences recovered fell into two clusters: one comprised of Trichodesmium sequences and the other an alpha-proteobacterial group. Unicellular cyanobacterial groups A and B, and symbiotic filamentous cyanobacterial diazotrophs were detected sporadically. Trichodesmium was by far the most abundant diazotroph, with up to 6 x 10(5) nifH gene copies per liter. Quantitative PCR probe-primer sets were designed and used to quantify two proteobacterial groups, revealing abundances up to 10(3)-10(4) nifH gene copies per liter, with the highest abundances in the photic zone. Major components of the clone library were also revealed by a nifH microarray and multidimensional scaling (MDS) analysis. MDS showed that samples from the >10 microm size fraction from 0- to 5-m depths clustered separately from the rest of the samples, primarily due to the abundance of Trichodesmium sequences. The SCS diazotroph community has a relatively low diversity and is a mixture of both estuarine and oceanic fingerprints.

Simulating tropical river plumes, a set of parametrizations based on macroscale data: A test case in the Mekong Delta region

A year‐long simulation of the Mekong river plume is conducted. The purpose is to identify the main processes impacting the fate of tropical freshwater runoff onto the shelf for later inclusion into a global ocean circulation model. Factors influencing the river plume in a general case are listed and are included in a coastal ocean configuration of the Princeton Ocean Model. This is achieved either explicitly, as for wind forcing, or using parametrizations corresponding to the length and timescales of river plumes physics, as for tides or estuaries. Results exhibit a strong seasonal variability related to the monsoon wind and river flow regimes of the region. A comparison with in situ measurements made during a campaign carried out in 1997 shows a good agreement. Vertical salinity profiles extracted from the model configuration correspond to conductivity‐temperature‐depth salinity profiles behavior established during the campaign. This model configuration also provides a good estimate of sea surface salinity in the vicinity of the coast of the Mekong Delta.

Riverine influence on nitrogen fixation in the upwelling region off Vietnam, South China Sea

Nitrogen fixation is assumed to be most important in oligotrophic subtropical regions of the oceans. Here we show that nitrogen fixation can also be pronounced in the upwelling region off Vietnam. We compare nitrogen fixation rates and other hydro‐chemical variables from the monsoon season with the intermonsoon season and find that nitrogen fixation rates are app. 10 times higher during the monsoon season. However, this was not the case in the actual upwelling region – a 40–50 km wide strip along the coast – but further offshore, where the Mekong plume was noticeable. Therefore, we hypothesize that the stability of the water column, micronutrients and/or trace metals in the Mekong river plume may be responsible for enhanced cyanobacterial growth. These cyanobacteria may be either Trichodesmium, unicellular, or diatom‐associated symbionts, the latter are known from other river plumes like the Amazon.