Abstract
Population growth and economic development are driving changes in land use/land cover (LULC) of the transboundary Lower Mekong River Basin (LMB), posing a serious threat to the integrity of the river system. Using data collected on a monthly basis over 30 years (1985–2015) at 14 stations located along the Lower Mekong river, this study explores whether spatiotemporal relationships exist between LULC changes and instream concentrations of total suspended solids (TSS) and nitrate—as proxies of water quality. The results show seasonal influences where temporal patterns of instream TSS and nitrate concentrations mirror patterns detected for discharge. Changes in LULC influenced instream TSS and nitrate levels differently over time and space. The seasonal Mann–Kendall (SMK) confirmed significant reduction of instream TSS concentrations at six stations (p < 0.05), while nitrate levels increased at five stations (p < 0.05), predominantly in stations located in the upper section of the basin where forest areas and mountainous topography dominate the landscape. Temporal correlation analyses point to the conversion of grassland (r = −0.61, p < 0.01) to paddy fields (r = 0.63, p < 0.01) and urban areas (r = 0.44, p < 0.05) as the changes in LULC that mostly impact instream nitrate contents. The reduction of TSS appears influenced by increased forest land cover (r = −0.72, p < 0.01) and by the development and operation of hydropower projects in the upper Mekong River. Spatial correlation analyses showed positive associations between forest land cover and instream concentrations of TSS (r = 0.64, p = 0.01) and nitrate (r = 0.54, p < 0.05), indicating that this type of LULC was heavily disturbed and harvested, resulting in soil erosion and runoff of nitrate rich sediment during the Wet season. Our results show that enhanced understanding of how LULC changes influence instream water quality at spatial and temporal scales is vital for assessing potential impacts of future land and water resource development on freshwater resources of the LMB.
Highlights
Increasing development pressures have altered land use/land cover (LULC) patterns in many river basins around the world
The effects could be even more profound in areas upstream of the delta, with research by [22] projecting that the annual sediment load of the Tonle Sap Lake is likely to be reduced by nearly 60% as a direct result of the changing wet and dry season flow regimes due to hydropower operation. Understanding how these LULC change drivers influence instream water quality at spatial and temporal scales is vital for assessing potential impacts of future land and water resource development on freshwater sources. Addressing this knowledge gap, this paper presents a research undertaken in the Lower Mekong River Basin (LMB), a transboundary river basin undergoing major changes in LULC, with the objectives of (1) conducting a spatiotemporal exploratory analysis of how these changes affected water quality indicators (total suspended solids (TSS) and nitrate) using records gathered between 1985 and 2015 at 14 monitoring stations located along the Lower Mekong River (LMR), and (2) identifying trends and observed seasonality of historical TSS and nitrate concentrations
The Seasonal and Trend decomposition Loess algorithm (STL) decomposition of the water quality time series shows that seasonal factors strongly influence TSS and nitrate levels in the LMR (Figures 6 and 7 and Appendix D)
Summary
Increasing development pressures have altered land use/land cover (LULC) patterns in many river basins around the world. The interactions between LULC changes and stream integrity have been well documented [1,2,3,4,5]; previous studies have examined their impacts. Sustainability 2020, 12, 2992 examined their impacts spatially [6,7,8,9] and temporally [8,9,10]. Studies have shown the highest instream sediment concentrations in agricultural areas [11,12], while correlations between spatially andconcentrations temporally [8,9,10]. Nitrate and the proportion of have agricultural andhighest urbaninstream areas have been concentrations in agricultural areas [11,12], while correlations between instream nitrate concentrations documented [10,13,14].
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