Variability of the Red River Plume in the Gulf of Tonkin as Revealed by Numerical Modeling and Clustering Analysis

We study the daily to interannual variability of the Red River plume in the Gulf of Tonkin from numerical simulations at high resolution over 6 years (2011–2016). Compared with observational data, the model results show good performance. To identify the plume, passive tracers are used in order to (1) help distinguish the freshwater coming from different continental sources, including the Red River branches, and (2) avoid the low salinity effect due to precipitation. We first consider the buoyant plume formed by the Red River waters and three other nearby rivers along the Vietnamese coast. We show that the temporal evolution of the surface coverage of the plume is correlated with the runoff (within a lag), but that the runoff only cannot explain the variability of the river plume; other processes, such as winds and tides, are involved. Using a K-means unsupervised machine learning algorithm, the main patterns of the plume and their evolution in time are analyzed and linked to different environmental conditions. In winter, the plume is narrow and sticks along the coast most of the time due to the downcoast current and northeasterly wind. In early summer, the southwesterly monsoon wind makes the plume flow offshore. The plume reaches its highest coverage in September after the peak of runoff. Vertically, the plume thickness also shows seasonal variations. In winter, the plume is narrow and mixed over the whole water depth, while in summer, the plume can be detached both from the bottom and the coast. The plume can deepen offshore in summer, due to strong wind (in May, June) or specifically to a recurrent eddy occurring near 19°N (in August). This first analysis of the variability of the Red River plume can be used to provide a general picture of the transport of materials from the river to the ocean, for example in case of anthropogenic chemical substances leaked to the river. For this purpose, we provide maps of the receiving basins for the different river systems in the Gulf of Tonkin.

Heavy Metals In Surface Sediments Of The Intertidal Thai Binh Coast, Gulf of Tonkin, East Sea, Vietnam: Distribution, Accumulation, And Contamination Assessment

Heavy metals contamination in sediments may endanger ecosystems and human health via the food chain. In fact, there is little to no understanding about heavy metal accumulation in surface sediment of one of the most economically important marine bodies for Vietnam, the Thai Binh Coast, where five large rivers co-discharge into the Gulf of Tonkin. Twenty-seven surface sediment samples were collected from the intertidal regions and analyzed for: arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), mercury (Hg), nickel (Ni), and zinc (Zn) using inductively coupled plasma mass spectroscopy (ICP-MS). The studied area exhibited a large spatial variation in the concentration of heavy metals, e.g., the dry sediment concentration of Cd was the least (0.05–0.49 mg.kg−1), whereas that of Zn was the greatest (45.4–252 mg.kg−1). Based on the geoaccumulation index (Igeo), most of the studied heavy metals were accumulated at low pollution levels, except four locations exhibited moderately and highly polluted levels of Hg with Igeo Hg values from 1.92 to 2.66. Whereas the high contamination factor value implicated that not only Hg but also all other detected heavy metals in this area resulted from anthropogenic activities along the coast and the river upstream. This implied the need for quick action from the government.

The first step of application of water quality index (WQI) to assessment of sea water quality in the Gulf of Tonkin in 2018

The study to calculate the water quality index was conducted in the Gulf of Tonkin at the surface water in August 2018. The calculation results showed that out of 48 surveyed points, there was 1 point in the Northeast area at poor water quality, 15 points in the Northeast and coastal areas from Thanh Hoa to Thua Thien Hue at medium water quality, 14 points in the Gulf of Tonkin, Northeast and Con Co Island at good water quality, the remaining 18 points in the Gulf of Tonkin and Bach Long Vi Island at excellent water quality. Overall, the average water quality of the whole region was good (average WQI = 79). Considering each area, the Gulf of Tonkin area had good and excellent water quality, the Northeast had water quality from poor to good level, the coastal areas from Thanh Hoa to Thua Thien Hue had medium water quality, Con Co Island area had good water quality, Bach Long Vi Island area had excellent water quality.

Determining seasonal dominant fish eggs and larvae species groups in the west of Tonkin Gulf

From 2003 to 2016, 1,649 samples were collected, covering both the time and space of the Gulf of Tonkin. The results have identified groups of seasonal dominant fish eggs and larvae: Seven species groups in Spring, nine species groups in the Summer, six species groups in Autumn, and four dominant species groups in the Winter. The dominant index (Yi) ranges from 0.02 to 0.26 depending on the species group and each season of the year, the highest in the Goby group - Gobiidae (Yi = 0.26) achieved in the Spring, followed by Herringgroup - Clupeidae reaches Yi = 0.20 in the Summer and the Anchovy group - Engraulidae reaches Yi = 0.16 in the Summer. The highest advantage index is only Yi = 0.09 in the Winter for Unicorn cod species - Bregmaceros macclelandi. During this period, the number of taxa and dominant indexes tended to decrease from Spring to Winter slightly.

Multi-scale variability of surface currents in the Gulf of Tonkin derived from HF radar observations

<p>The surface circulation in the Gulf of Tonkin (GoT) was analyzed using 2.5 year-long dataset from the High-Frequency radar (from April 2014 to October 2016). High temporal resolution of the measurements and large coverage from HFR dataset enable us to characterize the variability of surface circulation in the GoT in a wide range of scales: from tidal to annual scale. A number of techniques of data including rotary spectral analysis (RSA), principal component analysis (PCA), harmonic tidal analysis, coherent analysis, etc. were used to identify the dominant modes of variability. The tidal motions, accounting for approximately 62% of the total variability, revealed the dominance of diurnal components (K1 and O1) with 4 times larger magnitude than that of semi-diurnal constituent (M2). At seasonal scale, the monsoon wind plays an important role in driving the surface circulation in the GoT. This was supported by a tight correlation (0.7) between the wind stress and current velocities and by a large contribution (more than 50%) of the Ekman-driven component to the total variability of currents in the offshore area. Along the shore, large seasonal variability of circulation was highlighted. During the year,<strong> </strong>the<strong> </strong>seaward extension of the coastal current is primarily controlled by the cross-shore wind stress while the flow intensity is modulated by the Red River discharge.</p>

Variability of river plume in the Gulf of Tonkin

<p>The quality of estuarine, coastal and marine environment in the Gulf of Tonkin, in the South China Sea, is an essential issue to the ecosystems’ health and to the living conditions and economy of the Viet Nam population. The stakes are particularly high since the demographic density in the Red River delta is one of the highest in the world. Understanding the physical processes that drive the ocean circulation and its response to anthropic pressure there is therefore of primarily importance for enlightened resource management, as well as for designing adequate monitoring and forecasting systems.</p><p>As a first step toward a better understanding of the physical coastal and marine environment, we present here a study on the Red river plume variability in the Gulf of Tonkin over the period 2011-2016. The study is based on a numerical simulation, under realistic conditions, using the SYMPHONIE coastal model developed at LEGOS (Marsaleix et al., 2008). Compared with various data sources, the model results show good performances. The river plume is then identified and examined at different time scales. In general, the surface coverage of the river plume is strongly correlated with the runoff but with a 1-month lag. However, in some years, a higher peak in runoff does not create a higher peak of the plume area, suggesting that other forcings need to be taken into account to explain the variability of the river plume.</p><p>Using K-mean clustering, the main patterns of the plume are identified. The result shows that the plume has a large variability at both seasonal and interannual scales. Each pattern shows the plume under different forcing conditions.  Most of the time, the plume is narrow and sticks along the coast due to the downcoast current and northeasterly wind. In the summer, due to monsoon, the wind direction changes to southwesterly and helps the plume to spread offshore. The plume reaches its highest coverage in September after the peak of runoff; then its coverage decreases again when the monsoon reverses.</p><p>We also analyze events of offshore export of freshwater at daily time scales and show that they can be associated with recurrent coastal eddies during the summer monsoon. We investigate the respective role of wind and runoff in the eddies formation. Comparison with a run without river allows to identify the main impacts of the plume on the ocean states, for example in the current and sea surface elevation.</p>