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.

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.

Toxic cyanobacteria and microcystin dynamics in a tropical reservoir: assessing the influence of environmental variables.

Toxic cyanobacterial blooms (TCBs) have become a growing concern worldwide. The present study investigated the dynamic of toxic cyanobacteria and microcystin (MC) concentrations in the Tri An Reservoir (TAR), a tropical system in Vietnam, with quantitative real-time polymerase chain reaction (qPCR) and high-performance liquid chromatography (HPLC), respectively. The results of the qPCR quantification revealed that Microcystis was the dominant group and the primary MC producer in the TAR. Potentially toxigenic cyanobacteria varied from 1.2 × 104 to 1.58 × 107 cells/mL, and the mean proportion of toxic Microcystis to that of the total toxic cyanobacteria varied from 21 to 88%. Microcystin concentrations in raw water and sediment samples often peaked during June to October as blooms occurred and varied from 0.27 to 6.59 µg/L and from 1.79 to 544.9 ng/g in wet weight, respectively. The results of this study indicated that conditions favoring Microcystis proliferation lead to the selection of more toxic genotypes. Water temperature and light availability were not driving factor in the formation of TCBs in the TAR. However, the high loads of total nitrogen (TN), phosphate, and total phosphorus (TP) into the water via rainfall runoff in combination with a high total suspended solid (TSS) and decreased water level during the early months of the rainy seasons did lead to a shift in Microcystis blooms and higher proportions of toxic genotypes of Microcystis in the TAR. This research may provide more insight into the occurrence mechanism of TCBs in tropical waters. The strategy to control TCB problems in tropical regions should be focused on these limnological and hydrological parameters, in addition to a reduction in nitrogen and phosphorus loading.