The deterioration signs of water quality in the Hau River are apparent. The present study analyzed the surface water quality of the Hau River using multivariate statistical techniques, including principal component analysis (PCA) and Cluster Analysis (CA). Eleven water quality parameters were analyzed at 19 different sites in An Giang and Can Tho Provinces for 12 months from January to December 2019. The findings show high levels of Biological Oxygen Demand (BOD), Total Soluble Solids (TSS), and total coliform, all year round. The PCA revealed that all the water quality parameters influenced the water quality of the Hau River, hence the relevance for water sample scrutiny. The dendrogram of similarity between sampling sites showed a maximum similarity of 95.6%. The Accumulation Factor (AF) trend showed that the concentrations/values of TSS, BOD, and phosphate (PO43−) in the downstream were 1.29, 1.53, and 1.52 times, respectively, greater than the upstream levels. Despite most of the parameters analyzed supporting aquaculture production, caution is needed in the regulation of pollution point sources to undertake sustainable aquaculture production.

This study assessed phytoplankton community composition variation under two discrete salinity intrusion rate experiments. Experiment 1 simulated salinization under short-term, natural tidally induced rates of salinity change typical of the Mekong Delta, Vietnam (0–30‰ in 6 h), while Experiment 2 simulated salinization under a longer, more gradual rate of salinity change (0–30‰ at 5‰ increase per week). Phytoplankton community composition was, overall, more abundant following prolonged salinity change (Experiment 2) than rapid, tidally-induced salinity change (Experiment 1). Under both experimental conditions phytoplankton species number and density varied with changes in salinity, indicating a strong effect of salinity regardless of its rate of change. At the end of the salinity acclimation, Cyanobacteria, Chlorophyta, Euglenophyta and Bacillariophyta were all most abundant at a salinity of 5‰ after a short-term tidally simulated change in salinity, but with a longer, more gradual change in salinity, the abundance of different groups varied with salinity; With a longer, more gradual change in salinity, Chlorophyta was most abundant at 5–10‰ salinity, while Cyanobacteria, Euglenophyta and Bacillariophyta were all most abundant at a salinity of 5‰. The species composition in treatments of less than 10‰ had high similarity with the freshwater environment while treatments of 30‰ (Experiment 1) and 25‰ (Experiment 2) had high similarity with the natural seawater environment. This study demonstrates the utility of mesocosm experiments for understanding phytoplankton community composition variation in natural aquatic ecosystems under rapid and prolonged saline intrusion rates and predicting associated impacts on food webs in natural water bodies and aquaculture systems.