The Mekong River is one of the world’s largest rivers, unparalleled in terms of its biodiversity and ecosystem services. As in other regions, sufficient water quality is required to support diverse organisms, habitats, and ecosystems, but in the Mekong region, water quality has not been well studied. Based on biological and physical-chemical data collected over the last two decades, we evaluated spatial-temporal water quality of the Lower Mekong Basin (LMB) using biotic and abiotic assessment metrics. We found that during the 2000s, water quality in the LMB was unpolluted, with “very good” metrics for tributary rivers and “good” status for mainstem rivers. However, during the last decade, water quality has been degraded in the LMB, particularly near Vientiane City; the Sekong, Sesan, and Srepok (3S) Rivers; the Tonle Sap Lake system; and the Mekong Delta. Water quality degradation likely corresponds to flow alteration, erosion, sediment trapping, and point and non-point wastewater, which have occurred from rapid hydropower development, deforestation, intensive agriculture, plastic pollution, and urbanization. Regular biomonitoring, physical-chemical water quality assessment, transparent data sharing, and basin-wide water quality standards or management are needed to sustain water quality to support biodiversity and ecosystem function in the LMB.

Establishing reference conditions in rivers is important to understand environmental change and protect ecosystem integrity. Ranked third globally for fish biodiversity, the Mekong River has the world’s largest inland fishery providing livelihoods, food security, and protein to the local population. It is therefore of paramount importance to maintain the water quality and biotic integrity of this ecosystem. We analyzed land use impacts on water quality constituents (TSS, TN, TP, DO, NO₃⁻, NH₄⁺, PO₄³⁻) in the Lower Mekong Basin. We then used a best-model regression approach with anthropogenic land-use as independent variables and water quality parameters as the dependent variables, to define reference conditions in the absence of human activities (corresponding to the intercept value). From 2000–2017, the population and the percentage of crop, rice, and plantation land cover increased, while there was a decrease in upland forest and flooded forest. Agriculture, urbanization, and population density were associated with decreasing water quality health in the Lower Mekong Basin. In several sites, Thailand and Laos had higher TN, NO₃⁻, and NH₄⁺ concentrations compared to reference conditions, while Cambodia had higher TP values than reference conditions, showing water quality degradation. TSS was higher than reference conditions in the dry season in Cambodia, but was lower than reference values in the wet season in Thailand and Laos. This study shows how deforestation from agriculture conversion and increasing urbanization pressure causes water quality decline in the Lower Mekong Basin, and provides a first characterization of reference water quality conditions for the Lower Mekong River and its tributaries.

Tonle Sap lake-river floodplain ecosystem (TSE) is one of the world’s most productive freshwater systems. Changes in hydrology, climate, population density, and land use influence water quality in this system. We investigated long term water quality dynamics (22 years) in space and time and identified potential changes in nutrient limitation based on nutrient ratios of inorganic nitrogen and phosphorus. Water quality was assessed at five sites highlighting the dynamics in wet and dry seasons. Predictors of water quality included watershed land use, climate, population, and water level. Most water quality parameters varied across TSE, except pH and nitrate that remained constant at all sites. In the last decade, there is a change in the chemical nutrient ratio suggesting that nitrogen may be the primary limiting nutrient across the system. Water quality was strongly affected by development in the watershed i.e., flooded forest loss, climatic variation, population growth, and change in water level. Seasonal variations of water quality constituents were driven by precipitation and hydrology, notably the Mekong’s distinct seasonal flood pulse.

Study region: Tonle Sap System, CambodiaStudy focus: We investigated how hydroclimatic variability, watershed connectivity, and small-scale river regulation influence river discharge and sediment and nutrient delivery to Tonle Sap Lake. Monthly discharge and water-quality data from the Stung Sen, Pursat, and Sangke Rivers, together with daily discharge from the Tonle Sap River in 2022, were analyzed to quantify hydrological and water quality fluxes.New Hydrological Insights for the Region: Lake volume fluctuated, ranging from 1.17 to 45.86 km³ over 334 days. Inflow lasted 53 days, contributing 20.3% (9.3 km3) of the lake’s maximum volume, below historical averages (29.8 km3 for 1997-2005; 31.74 km3 for 2010-2019). The Stung Sen River was the dominant tributary (31.3%), while Pursat and Sangke contributed 6% and 1.6%, respectively. Average inflow (53 days) and outflow (281 days) discharges at the Tonle Sap River were 1,561 m³ s⁻¹ and 2,525 m³ s⁻¹, with tributary discharges varying by river and dam presence. River discharge was the primary driver of sediment and nutrient transport, with wet-season rainfall generating peak flows and loads. Late dry-season delays in discharge and nutrient delivery were attributed to irrigation withdrawals and reduced rainfall. These findings demonstrate that climatic variability and anthropogenic pressures, including dams, land-use change, and water extraction, modulate hydrology and water quality fluxes. An integrated, basin-wide management approach is essential to preserve the lake’s ecological integrity.