The effect of a dam on the copper accumulation in estuarine sediment and associated nematodes in a Mekong estuary.

Dam construction across the main flow of an estuary can greatly contribute to a high accumulation of inorganic contaminants. However, it remains unknown to what extend externally available heavy metals are incorporated into biota living in those contaminated environments. In this study, the heavy metal copper was investigated both in the sediment and in the tissues of nematodes taken from the subtidal zone in the Ba Lai estuary where a dam is present, and compared with samples from the dam-free Ham Luong estuary, both part of the Mekong Delta. Samples were taken in the dry season of 2017 in four stations in the Ba Lai estuary with two stations in the downstream part from the dam and two upstream. Similar locations with respect to the distance were sampled in the dam-free estuary. The internal copper concentration in nematodes was measured by applying micro X-ray fluorescence. The results showed that both internal and sediment copper concentrations were different between the two estuaries and among estuarine sections. The highest copper concentration in nematodes was found in the upstream section of Ba Lai estuary where the greatest accumulation of sedimentary copper was observed, while the dammed downstream part was lowest in internal copper accumulation. Moreover, there was more variation in the copper levels between the two sections within the dammed estuary compared to those in Ham Luong. These observations might point to the contribution of the Ba Lai dam to the increase of copper contaminants in the benthic environment leading to accumulation in nematodes.

Seasonal changes in phytoplankton assemblages and environmental variables in highly turbid tropical estuaries of the Mekong River, Vietnam.

The distribution of phytoplankton assemblages in response to physicochemical variables was assessed using Spearman's correlation and canonical correspondence analysis in four highly turbid estuaries of the Mekong River (MKR) system in Ben Tre Province, Vietnam. During two surveys (September 2017 and April 2018) at 20 sampling sites, a total of 162 species were recorded, with a dominance of diatoms (> 50%). Phytoplankton abundance varied from 3.01 × 105 to 11.85 × 105 cells/L, with the highest cell densities found at the BL2 station in the rainy season, mainly attributed to cyanobacteria during the rainy period, whereas diatoms were dominant in the dry season. Chlorophyll-a concentration in this area was low and decreased from the mouth stations. Similarity analysis distinguished the freshwater and oligohaline regions (characterized by lower phytoplankton abundance in the oligohaline areas) from the mesohaline and polyhaline zones (characterized by higher phytoplankton density in the polyhaline sections), based on the salinity gradient, which mostly explained the spatio-temporal distribution of phytoplankton. In which, freshwater and oligohaline sites were dominated by the diatom Coscinodiscus, the euglenoids Lepocinclis, and the green alga Scenedesmus in the rainy season, while mesohaline and polyhaline stations were dominated only by diatom species (Thalassionema and Skeletonema) in the dry season. High salinity and total dissolved solutes, and low nutrients are the major factors affecting phytoplankton growth and lead to low density and low primary biomass in turbid estuarine systems like the MKR estuaries. Our results further emphasize the importance of taking into account of salinity in understanding seasonal changes of phytoplankton communities, which can provide a valuable baseline data for ecological management strategies in tropical estuarine ecosystems.

Identifying the impact of land use land cover change on streamflow and nitrate load following modeling approach: a case study in the upstream Dong Nai River basin, Vietnam.

Tri An Reservoir is a vital source of water for agriculture, industry, hydropower, and public usage in Southern Vietnam. Due to human activities, water eutrophication has become a serious problem in recent decades. This study investigated for the first time the impact of land use and land cover (LULC) change on streamflow and nitrate load from the upstream Dong Nai River basin, which is the largest watershed of the reservoir. The study utilized several LULC scenarios, including LULC 2000, 2010, and 2020. The SWAT model was applied to model the watershed during the period 1997-2009. Results showed that the hydrological model performed satisfactorily based on the Nash-Sutcliffe efficiency (NSE) coefficient, the root mean square error observations standard deviation ratio (RSR), and the percent bias (PBIAS). The average simulated values of monthly streamflow and nitrate load were 453.7, 450.0, 446.7 m3/s and 17,699.43, 17,869.13, 17,590.81 tonnes for the LULC 2000, 2010, and 2020 scenarios, respectively. There were no significant differences in streamflow and nitrate load at the basin level under the different LULC scenarios. However, when looking at the subbasin level, there were differences in nitrate load among the scenarios. This suggests that the impacts of LULC on nitrate load may be more pronounced at smaller scales. Overall, our finding underscores the importance of modeling techniques in predicting the impacts of LULC change on streamflow and water quality, which can ultimately aid in the sustainable management of water resources.

Ecological impact assessment of irrigation dam in the Mekong Delta using intertidal nematode communities as bioindicators.

The ecological response of nematode communities to dam construction has limited attention. In this study, the response of intertidal nematode communities in the Ba Lai River (Mekong Delta, Vietnam) to the construction of an irrigation dam was investigated. Nematode communities and environmental parameters were investigated during the rainy season of 2015. The obtained results showed that the Ba Lai dam had caused negative impacts on the local environment by disrupting longitudinal connectivity, the accumulation of nutrients (total organic carbon, total nitrogen, total phosphorus), and heavy metals (copper, iron, arsenic, lead) in the upstream and the lateral sides of the dam, consequently leading to changes in the nematode communities. The response of nematode communities to the dam's presence was clear based on their abundance, diversity, dominant genera, and community composition. Furthermore, changes in the abundance and diversity of nematodes in the Ba Lai River appeared to be controlled primarily by acidity (pH), total suspended solids (TSS), iron (Fe), clay, and salinity most responsible for changes to nematode communities. Because the nematode communities are well adapted to the physicochemical disturbances caused by dam construction, they are a potential tool for ecological monitoring and understanding the influence of dams on aquatic ecosystems.