Abstract
Spatio-temporal dynamics of riverine nitrogen (N) and phosphorus (P) in karst regions are closely linked to hydrological conditions, human activities and karst features in upstream catchments. From October 2017 to September 2019, we undertook 22 sampling campaigns in 11 nested catchments ranging from 21.00 to 373.37 km2 in Huixian karst wetland to quantify forms, concentrations, and fluxes of riverine total nitrogen (TN) and total phosphorus (TP), and to identify spatial and temporal variations of nutrients transfer from upstream to downstream, tributaries (Mudong River and Huixian River) to the main stem (Xiangsi River) in the dry and wet seasons. Considering the hydrological conditions, human activities and karst features within upstream catchments, the following three spatial and temporal variations of riverine nutrients were found over the monitoring period: (1) the dynamics of riverine nitrogen and phosphorus varied seasonally with hydrological conditions; (2) the spatial disparities of riverine nitrogen and phosphorus were induced by different human activities within catchment scales; (3) the dynamics of riverine nitrogen and phosphorus varied similarly at spatial scale restricted by karst features. The findings from this study may improve our understanding of the influence of hydrological conditions, human activities and karst features on nitrogen and phosphorus variations in river waters at different spatial and temporal scales in the Huixian karst wetland basin, and will help managers to protect and restore river water environments in karst basin from a catchment-scale perspective.
Highlights
The eutrophication of surface waters has become an increasingly significant global water quality concern [1,2]
The mean contribution of total dissolved phosphate (TDP) to total phosphorus (TP) ranged from 49.2% to 75.1% in the whole monitoring period at the eleven monitoring sections (Figure 6), so TDP was the main form of TP in different periods, and the correlation between them was more significant in the dry season than wet season (Table 2)
The nutrient concentration and forms within 11 nested catchments ranging in scale from 21.00 km2 up to a maximum of 373.37 km2 in Huixian karst wetland have been monitored over 2 years
Summary
The eutrophication of surface waters has become an increasingly significant global water quality concern [1,2]. River water is suffering from nitrogen and phosphorus pollution in karst areas [5] where water resources are valuable [6,7]. Characterized by gullies, gaps, skylights and funnels in the surface and conduits, fractures and matrix flow under the surface [12], the karstic aquifer is vulnerable to anthropogenic contamination due to its rapid transport from surface to groundwater, and to lowland rivers fast through fracture networks within the karst architecture. The rapid flow pathways through the unique karst surface–subsurface binary spatial structure [13] shorten contaminant residence time, which reduces capacity for attenuation, affects surface receiving water quality and poses a serious threat to karst water ecosystems [14,15]. Nutrient loading in surface water has been predicted to increase through the middle of the century for more precipitation in both frequency and magnitude [16]
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