Abstract
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.
Highlights
Lower Mekong have noted a decline of water quality [14,15,16], we found only one study that assessed the effects of land use change on total suspended solids (TSS) and dissolved inorganic nitrogen (NO3 − ) [17]
Land use changed from 2000 to 2017 in the 30 watersheds that we analyzed for this study (Figure 1)
In this study we show that deforestation, agriculture, and urbanization pressure have increased in the last 17 years in the Lower Mekong Basin, and these changes in land use have led to the degradation of water quality, with respect of TSS, TN, TP, DO, NO3 −, NH4 +, and PO4 3− concentrations
Summary
Water quality is becoming degraded worldwide [1] due to increasing sediment loads and nutrient concentrations of nitrogen and phosphorus into rivers and downstream coastal areas [2]. Rivers provide a wide range of ecosystem services [3] which are valuable in tropical developing countries where the livelihoods depend directly on freshwater resources [4]. In Southeast Asia’s Mekong Basin, food security and economic well-being of nearly 70 million people depend on about 2.3 million tonnes of fish caught from the Mekong Basin annually. Fish production in this region represents around 20%
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