Spatial and temporal variability of sedimentary nutrients in relation to regional development in the urbanizing lower Chao Phraya watersheds of Thailand

Abstract

Excess nutrients pose profound aquatic environmental impacts besides well-known essential elements for cropping. The enrichment of excess nutrients in riverine systems requires more investigation for various scenarios under different culture and governance regimes in the world. A survey on sedimentary nutrients (nitrogen and phosphorus) was conducted covering the lower Chao Phraya watershed in Thailand to explore their spatial and temporal patterns under various anthropogenic influences. Surface sediment and sediment core samples were collected from three subwatersheds (i.e., Pasak, Thachin, and Chao Phraya) of the lower Chao Phraya watershed in 2017. Various nutrient species (total nitrogen, ammonium nitrogen, nitrate nitrogen, total phosphorus, and phosphate phosphorus) were analyzed. The sediment cores were dated using a computerized multichannel gamma spectrometry. Multivariate statistical analyses were conducted to examine the spatial and temporal patterns of sedimentary nutrients, and to interpret their correlations with catchment land use pattern and socioeconomic development, respectively. Results identified spatial hot spot in downstream sections of Thachin and Chao Phraya subwatersheds and temporal hot moment between 1990 and 2000 for the three subwatersheds and an extra hot moment of 1960s for Chao Phraya subwatershed. Both spatial and temporal patterns of sedimentary nutrients were due to a shift from agriculture to urban land uses in the catchments. The catchment land use pattern explained approximate 33% spatial variation of sedimentary nutrient loadings with the top contributions of urban land use and water surface ratios in the studied watershed. With Thailand’s rapid industrialization, the nutrient sedimentation flux showed a temporal change in response to socioeconomic development with a turning point in the 1990s. This study demonstrates that anthropogenic sources strongly influence sedimentary nutrient loadings following an order of urbanization > aquaculture > agriculture. Catchment land use pattern and socioeconomic development are efficient indicators to explain the variations of riverine environmental quality, both spatially and temporally.