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.