Spatio-Temporal Dynamics of Riverine Nitrogen and Phosphorus at Different Catchment Scales in Huixian Karst Wetland, Southwest China

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.


Introduction
The eutrophication of surface waters has become an increasingly significant global water quality concern [1,2]. Direct and indirect effects from human activities, such as the excessive use of fertilizers in agriculture and the indiscriminate discharge of untreated sewage, have dramatically increased the nitrogen (N) and phosphorus (P) loading to aquatic ecosystems [3,4]. River water is suffering from nitrogen and phosphorus pollution in karst areas [5] where water resources are particularly valuable [6,7]. It is estimated that carbonate karst covers approximately 15% of the Earth's ice-free helpful for the government to establish long-term monitoring of nitrogen and phosphorus in the Huixian karst wetlands in the future. The objectives of this study were as follows: (1) quantify the spatial and seasonal variations of riverine nutrient forms, concentrations and mass flux in Huixian karst wetland basin during the monitoring period; (2) identify the effects of hydrological conditions, human activities and karst features on such variations; (3) provide suggestions for controlling nitrogen and phosphorus pollution of the river system in Huixian Karst Wetland basin to further improve the quality of river waters.

Study Area
Huixian karst wetland basin (E 110 • 04 -110 • 16 , N 24 • 55 ~25 • 18 , elevation 59-974 m, 373.37 km 2 ) is located in the southwest of Guilin City, Guangxi Province, China ( Figure 1). The region has a typical subtropical humid monsoon climate with a mean annual precipitation of 1853.7 mm, a mean annual temperature of 20 • C. The landform is characterized by low hills and hilly in the high north and south, karst peak cluster-depression and peak forest plain in the low center. The soil is classified as red-yellow soil, paddy soil, limestone soil and swamp soil [37]. The soil overlying the carbonate rock varies from 0.0 to 40 m in depth. The catchment contains one vice-city (Lingui district) and 15 administrative villages that consist of approximately 350,000 permanent urban population, 25,000 inhabitants, several fish farms on scale, and livestock farms on scale or decentralized with several to hundreds of animals. The karst landforms are classified as non-karst, bare karst and covered karst areas. Paddy field, forest and brush are the dominant land-use types in the basin (Table 1). Forests are generally distributed in the non-karst areas, while brushes are on the bare karst areas. Paddy fields are the largest share of land-use and generally distributed in the covered karst areas. Paddy rice is planted twice a year in mid-April and mid-July, and the annual fertilizer application rates for paddy rice are approximately 700 kg N and 225 kg P ha −1 year −1 , respectively. Table 1. Distribution of the eleven monitoring sites (M 1 -M 3 , H 1 -H 4 and X 1 -X 4 ), catchments (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11), and the share of karst and land use area in Huixian karst wetland basin.

Rivers
Monitoring Sites (Index in Figure 2  The Xiangsi River main stem and tributary Huixian River run down from the non-carbonate highlands in north and south respectively and both flow into the carbonate lowland of the center where they merge with tributary Mudong River which originates from karst springs in the middle east, and then flow into the lower reaches of the Xiangsi River ( Figure 1a). The center of the basin is dominated by karst landforms, and the eastern boundary of the basin is the water divided area of the Lijiang (Gui) river and Luoqing (Liu) river. According to the field survey, there are five small dams storing water in the approximately 3 km long channel between the M 2 and M 3 sections of the Mudong River, which are used to pump water to fish ponds and rice fields on the river banks. In addition, there are five small dams in the approximately 8 km long channel between the H 3 and H 4 sections of the Huixian River, which are used to raise the water level of the river so that the water can flow into canals to irrigate farmland ( Figure 1).

Catchments Delineation
Eleven monitoring sections of three rivers were selected as sampling sites for discharge and riverine N and P levels after investigating the spatial distribution of the river network systems in the studied basin. Each river section was defined as the outlet of upstream catchment, and the boundary of each catchment was delineated based on the digital elevation model (DEM) of the studied area in ArcGIS™ 10.2 software (ESRI, Redlands, CA, USA). Then, land use areas in each catchment were determined based on the DEM. Detailed information for the delineated catchments is provided in Table 1 and Figure 1. In addition, daily rainfall data were obtained from the Guilin weather station (about 10 km northeast from the Huixian karst wetland basin) run by the national weather service.

Data Collection and Analysis
From October 2017 to September 2019, water depth and velocity at the outlet of each catchment scale was monitored monthly, and the water samples were collected for N, P concentration and form analysis. The flow velocity of the river water was measured in field by hand-held Electric Wave Flow Meter (Stalker II SVR, Applied Concept Inc., Brannon, TX, USA), and the cross-section flow area of the river was measured by channel geometry and water depth using a ruler. The cross-section discharge (m 3 /s) of each catchment outlet was estimated by combining the measured cross-section flow area (m 2 ) with the observed flow velocity (m/s).
Two methods were used to collect water samples depending on the nature river channel at the sampling site. A sampler with 2.5 L volume was used to collect water samples over the bridge from non-wade able river sections where the water depth in the river center was greater than 1.5 m. At these sites, the samples were collected from 0.5 m water depth in the center and sides of the channel. Then samples were mixed in the collection and stored in acid-washed 1000 mL polyethylene bottles. For wade able river sections, water samples were collected at the water surface on both sides of the river. Once collected, the water samples were transported to the laboratory at ambient temperature on the same day, then preserved in fridge at 4 • C and they were processed and analyzed within 48 h of collection.
All water samples were split into two before analysis. One part was unfiltered and analyzed for total nitrogen (TN) and total phosphorus (TP). The other part was passed through a 0.45 µm filter, and then the filtrates were analyzed for nitrate nitrogen (NO − 3 -N), ammonium nitrogen (NH + 4 -N), and total soluble phosphate (TDP). All samples were analyzed following the National Standard Methods (Standard Methods for the Examination of Water and Wastewater, 2002) [38]. TN concentrations were measured by alkaline potassium persulfate digestion ultraviolet spectrophotometric method (HJ636-2012) [39], NO − 3 -N concentrations were measured by ultraviolet spectrophotometric method (HJ/T 346-2007) [40], and NH + 4 -N concentrations were measured by Nessler's reagent spectrophotometric method (HJ535-2009) [41]. Total phosphorus (TP) and total dissolved phosphate (TDP) were determined by potassium persulfate digestion ammonium molybdate spectrophotometric method (GB11893-89) [42].
The area of the landscape type was calculated with

Meteorological and Hydrological Conditions over the Study Period
The precipitation from October 2018 to September 2019 was approximately 2245.8 mm, which was 71.11% more than the 1312.5 mm from October 2017 to September 2018. The precipitation from April to September contributed about 75% of the annual precipitation. Thus, April 2018-September 2018 and April 2019-September 2019 were defined as the wet seasons, while October 2017-March 2018 and October 2018-March 2019 as the dry seasons for the monitoring periods. Overall, there was a distinct seasonal variation in riverine discharges which were greater in the wet season than dry season influenced by rainfall ( Figure 2).
In spatial distribution, discharges decreased significantly both at the outlet sections (M 3 and H 4 ) of tributaries (Mudong River and Huixian River), while increased from upstream to downstream sections of the Xiangsi River main stem basin with greater values than those of tributaries during the same period (Figures 2 and 3). X 2 catchment covers 88.4% of X 3 catchment, and its share of discharge in X 3 section ranged from 64.5% to 96.7% with a mean value of 82.3%. M 3 catchment covers 11.4% of X 3 catchment, and its share of discharge in X 3 section ranged from 1.4% to 34.9% with a mean value of 7.9% during the wet season and 17.3% during the dry season. H 4 catchment (tributary Huixian River sub-basin) covers 34.1% of X 4 catchment, and its share of discharge ranged from 0.2% to 17.4% with a mean value of 6.2%. Overall, the mean discharge per unit catchment area in basin of the Xiangsi River main stem ranged between tributaries Mudong River and Huixian River sub-basins. Due to the increased quantity of agricultural water intake from the lower reaches of tributaries Mudong River and Huixian River by damming, discharge decreased in both M 2 and H 4 sections, resulting in the decreases of discharge per unit catchment area of the corresponding catchments ( Figure 3).

Spatio-Temporal Variations of Nutrient Concentration
TN and TP concentrations at each monitoring section are shown in Figure 4. TN and TP concentrations ranged from 0.48~27.55 mg/L and 0.03~2.49 mg/L respectively. Based on the "Chinese Environmental Quality Standards for Surface Water" (GB3838-2002) [43] and the requirements of Guilin Surface Water Environmental Functional Zone Division, TN and TP pollution occur when the concentrations exceed 1.0 mg/L and 0.2 mg/L respectively. As shown in Figure 4, 90.9% and 50.0% of 110 collected water samples in the dry season were categorized as having TN and TP pollution respectively, while 83.6% and 49.2% of 132 collected water samples in the wet season, indicating the more serious pollution of TN than TP in river water. Moreover, 100% and 81.8% of 88 collected water samples in Xiangsi River basin were categorized as having TN and TP pollution respectively, while 72.7% and 39.4% of 66 samples collected in Mudong River sub-basin and 88.6% and 25.0% of 88 samples collected in Huixian River sub-basin, meaning that TN pollution was more serious than TP in the Xiangsi River.
In the main stem Xiangsi River, the concentration of TN and TP showed a decreasing trend with increasing flow, with the highest nutrient concentrations under low flow conditions in the dry season and lower nutrient concentrations under high flow conditions in the wet season. In the tributaries Mudong and Huixian rivers, the trend of TN and TP concentrations with flow was not obvious, and the difference between the dry and wet seasons was also not obvious ( Figure 4).  The forms of TN at each monitoring section showed obvious seasonal variations as well as concentration, with NO − 3 -N as the main form in the dry season and the increased contribution of NH + 4 -N in the wet season. As shown in Figure 6, the mean contribution of NO − 3 -N to TN ranged from 44.8% to 78.3% in the dry season and from 18.2% to 48.0% in the wet season, the mean contribution of NH + 4 -N) to TN in the wet season were 12.9% to 209.8% higher than the mean contribution in the dry season at the eleven monitoring sections. It was also confirmed that the Pearson correlation between NO − 3 -N and NH 4 + -N was not significant either in the dry season or wet season ( Table 2). The mean contribution of TDP to 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 correlations between TN, TP concentrations and discharge were relatively stable at each monitoring section in the Xiangsi River main stem ( Table 2).

Spatio-Temporal Variations of Nutrient Fluxes
Nutrient fluxes (g·s −1 ) in the rivers were estimated by combining measured nutrient concentration data (mg·L −1 ) with observed discharges data (m 3 ·s −1 ). Figure 7 shows TN and TP fluxes for each spatial catchment scale in the dry and wet seasons. Nutrient fluxes showed significant seasonal variations, with greater fluxes in the wet season than dry season during the same period, which was not consistent with the seasonal variations of nutrient concentration. Nutrient fluxes of the Xiangsi River were the largest among the three rivers and tended to increase from the upstream to downstream, while decrease at the outlet of the Mudong River and Huixian River sub-basin, which was also not consistent with the spatial variations of nutrient concentration. Results suggested that the seasonal variations of nutrient fluxes were mainly controlled by hydrological conditions. The Person correlation analysis also confirmed that there was a significant positive relation between TN, TP fluxes and discharge at different spatial catchment scales ( Table 3). The power function relationships between TN, TP fluxes and discharges at different spatial catchment scales of the three rivers were verified further using regression analysis. Regression models were developed using TN and TP fluxes (g·s −1 ) as a dependent variable and observed discharge (m 3 ·s −1 ) as an explanatory variable. Six models all had significant power function relationships which were maintained not only in different periods (dry and wet seasons) but also in different hydrological years with different rainfall frequencies (Figure 8), further indicating that hydrological conditions played a dominant role in nitrogen and phosphorus nutrient fluxes in river water.  TN and TP fluxes of the upstream catchment can be calculated by these power function equations (Table 4) based on the observed discharge at the catchment outlet. Table 5 shows the relative contributions of area, discharge, and nutrient flux in the upstream X 2 , M 3 , H 4 catchments to the downstream X 3 and X 4 catchments. In M 3 upstream catchment (tributaries Mutong River sub-basin) which covered 11.4% of X 3 catchment, the share of TN and TP fluxes ranged between 5.5% and 8.9%, and in H 4 upstream catchment (tributaries Huixian River sub-basin) which covered 34.1% of X 4 catchment, the share of TN and TP fluxes ranged between 3.7% and 6.1%, which were both less than the contribution of area and discharge in the dry and wet seasons respectively. Therefore, TN and TP fluxes in the lower reaches of the Xiangsi River mainly came from the upper X 2 catchment, which confirmed the more serious nitrogen and phosphorus pollution in the upper reaches of the Xiangsi River.  Nutrient export coefficients (g·km −2 ·s −1 ) are estimates of the nutrient fluxes (g·s −1 ) normalized by area (km 2 ). As shown in Figure 9, the mean nutrient export coefficients of TN and TP in tributary Mudong River sub-basin were generally larger than those in Huixian River sub-basin during the same season, and those in the Xiangsi River basin were estimated between them. Since nutrient fluxes at different spatial catchment scales were significantly positively correlated with discharges in this study, the nutrient export coefficients were significantly positively correlated with discharges per unit area accordingly (p ≤ 0.05, correlation coefficient ranged from 0.569 to 0.953). The mean nutrient export coefficients of TN and TP were significantly positively correlated with the share of agricultural land and covered karst land over the monitoring periods ( Table 6). The spatial variations of TN and TP fluxes at small catchment scales in tributaries Mudong River and Huixian River sub-basin were greater than that at large catchment scales in the Xiangsi River main stem (Table 7). Table 6. The Pearson correlation coefficient between the average nutrient export coefficients and the share of agricultural land in the dry seasons and wet seasons.

Spatio-Temporal Variations of Riverine Nutrient Controlled by Hydrological Conditions
Analysis revealed that NO − 3 -N was the main form of TN in the dry season and the contribution of NH + 4 -N to TN increased in the wet season. In the dry season, because of the reduced discharge and slow flow of rivers, NO − 3 -N was easily transformed from NH + 4 -N in human and animal manure sewage drained into the rivers, which was soluble in water and does not easily adsorb to sediment, and could be transported to surface water through various pathways (including surface runoff, subsurface seepage and groundwater recharge) [44,45], ultimately becoming the main form of riverine TN. In the wet season, because of the fertilization and irrigation of agricultural land and the conditions of weak nitrification and abundance available nitrogen, excess NH + 4 -N which above the soil's adsorption capacity was carried by plenty of surface runoff directly flowing into the river water or infiltrating from the soil to groundwater and ultimately recharging to surface rivers, directly or indirectly lead to an increase in the contribution of NH + 4 -N to TN in rivers. Thus, the seasonal hydrological conditions played a controlling role in determining the contributions of NO − 3 -N and NH + 4 -N to TN. Hydrological processes of rainfall-runoff in surface play an important role in nitrogen and phosphorus transfer in the basin. Large amounts of suspended and adsorbed nutrients are transported to the river when runoff increases caused by rainfall [46,47], especially phosphorus, as TP is mainly carried by surface runoff in the forms of dissolved material and suspended particulate. In this study, TDP was the main form of TP at different spatial catchment scales in dry and wet seasons. Although the mean contribution of TDP to TP in the dry season was higher than wet season, the contribution of TDP to TP was not significant in seasonal disparities because our study did not target rainfall events, periods of high flow are underrepresented in our assessment. Analysis showed that the correlation between TP and TDP was more significant in the dry season than the wet season, and was most significant in Huixian River than other two rivers ( Table 2). This may be mostly explained by hydrological conditions with lower-flow and slower flow velocity during the dry season as well as in Huixian River, helping to enhance the correlations between TP and TDP. Altering flow conditions by damming Mudong River and Huixian River for water storage may also result in reduced particulate phosphorus (PP) fluxes. High flow conditions during the wet season increase the proportion of PP in TP and re-suspend the sediments stored at the river bed, providing an additional potential for P release.
The controlling of hydrological conditions on the nutrient export in Huixian karst wetland rivers not only in terms of the different forms of TN and TP, but also in terms of concentration and flux. The results in this study show that the riverine nutrient concentrations were generally higher under low-flow conditions, while fluxes were relatively higher under high flow conditions during the wet season, indicating that the controlling of hydrological conditions on the nutrient export was stronger than other factors. Thus, TN, TP concentrations and fluxes in the Xiangsi River showed significant seasonal variations as discharge, benefited from the significant correlation with discharge ( Table 3). The role of hydrological conditions in controlling nitrogen and phosphorus export in surface water systems has been verified in several studies [48,49]. In our study, the TN and TP fluxes of the upstream catchment can be calculated by power function equations (Table 4). Similar empirical equations have been presented in other areas of China, showing a significant exponential or logarithmic relationship between nutrient fluxes and discharges [50,51]. However, since riverine nitrogen and phosphorus are related to various factors such as natural attributes and human activities in the study area, there are regional limitations of the empirical equations presented in each study area. Therefore, the empirical equations proposed in our study are only applicable to similar basins.

Spatio-Temporal Variations of Riverine Nutrient Affected by Human Activities
It is generally accepted that human-influenced landscapes such as urban and agricultural land are nutrient sources in river water [52,53]. The correlation between nutrient concentrations and discharges was not significant in the sub-basins of tributaries Mudong River and Huixian River, where agriculture was the main type of landscape in addition to forest (Table 1), so the rivers were mainly polluted by nitrogen and phosphorus from agricultural non-point sources. The sewage from poultry, aquaculture farming and living drained nitrogen and phosphorus pollutants into river water in the dry season, and the application of fertilizers increased augmenting nitrogen and phosphorus pollution in the wet season [32]. Seasonal variations in nitrogen sources caused by human activities in the Mudong River and Huixian River sub-basins, resulted in the difference in TN concentrations (observation A and B in Figure 4) and fluxes (observation A and B in Figure 8) for the similar flow conditions after rain events in the dry season (observation A in Figure 2) or during low-flow period in the wet season (observation B in Figure 2). It is also confirmed by the disparities in the contribution ratios between NO − 3 -N and NH + 4 -N in the wet and dry seasons ( Figure 6). In this study, the contributions of NO − 3 -N to TN were generally greater than 50% in the dry seasons (the contributions of NO − 3 -N was 68.7% at observation A in Figure 4, while NH + 4 -N was only 9.7%), and NH + 4 -N were generally greater than 50% in the wet seasons (the contributions of NH + 4 -N to TN was 89.5% at observation B in Figure 4, while NO − 3 -N was only 7.0%). Among the three rivers, TN and TP pollution of the Xiangsi River was the most serious, with the nutrient pollution mainly coming from the X 2 upstream catchment, where urban was the main landscape type (Table 1), so the Xiangsi River was mainly affected by nitrogen and phosphorus point source pollution. In X 1 and X 2 sections, the nutrient concentrations were high and the discharge-concentration relationships ( Figure 4) were likely to display dilution pattern [54] due to the stationary source with large and stable discharge of sewage drained from urban residential and industrial activities.
In the Xiangsi River main stem, TN and TP fluxes increased with increasing catchment area scale, while in M 3 and H 4 sections, the tributaries outlets of Mudong River and Huixian River, TN and TP fluxes decreased when compared with that of upstream catchments. Discharge was the dominant factor since the nutrient fluxes were positively correlated with it. The decrease in discharge is due to the damming of water by agricultural producers. However, since TN and TP fluxes in the lower reaches of the Xiangsi River mainly came from the X 2 upstream catchment (Table 5), the decreased quantity of water flowing from tributaries had less impact on TN and TP fluxes in the Xiangsi River main stem, indicating that the spatial variations of TN and TP fluxes at small catchment scales in lower-order tributaries were larger than that at large catchment scales in higher-order main stem, due to the human activities in sub-basin.

Spatio-Temporal Variations of Riverine Nutrient Restricted by Karst Features
It is generally accepted that there is a strong positive correlation between nutrient export and the proportion of available and arable land within a catchment [55]. This was confirmed also in this study that there was a significant positive correlation between the mean nutrient export coefficients and the share of agricultural land in each catchment area over different periods (Table 6). Further studies indicated that the share of covered karsts was significantly positively correlated with the mean of nutrient export coefficients as well as the share of agricultural land in each catchment area (Figure 10), with the correlation coefficients of 0.837 and 0.976, respectively. In karst region, bare-karst areas are extremely difficult to exploit within no or little soil which is thin, patchy, and slow to form, while cover-karst areas are usually the central and areas for intensive human living and productive activities, as well as the runoff-drainage areas of catchments. As a result, catchment with high shares of cover-karst area are generally characterized by high shares of agricultural land, high discharge per unit area, and correspondingly high TN, TP nutrient export coefficients. In addition to constraining the share of available and arable land within the catchment that indirectly affect the nitrogen and phosphorus flux, karst features also have strong control on water movement within the catchment and indirectly affect nitrogen and phosphorus concentrations in surface water systems, causing the range of mean TN and TP concentrations at the outlet of each catchment to converge to smaller in downstream reaches during the same season ( Figure 5). The aquifer of the Huixian karst wetland basin is characterized by the high altitude and impermeable non-karst aquifers in the north and south, and karst water storage tectonic basin in the central lowland ( Figure 11). The tectonic basin receives precipitation and surface water recharge from the oblique flanks, as well as recharge from upstream or adjacent karst groundwater, and from external sources (e.g., Qingshitang Reservoir water). The main drainage directions of surface water and groundwater systems flow from the north and south to the central tectonic lowlands, and these surface water and groundwater have rapidly influenced the development of the central karst landscape, developing and forming numerous wetland lakes and swamps-Huixian karst wetland, where the unique hydrogeology is characterized by primarily decentralization runoff with slow flow exchanging between surface water and groundwater in the central tectonic lowlands. In downstream reaches of three rivers, a rising groundwater table can intersect the near-river catchment surface, especially in heavy rainfall events during the wet season, when the river level raised so high that the lowlands were almost inundated and the range of nutrient concentrations converged to smaller for the connections of all the surface water. With the increasing catchment scale, the hydraulic connections were stronger in the confluent area of the lower reaches of three rivers, suggested that the effect of hydrological connectivity formed under the hydrogeological constraints of karst on nutrient concentrations prevails over the effect of other factors, especially in the wet season. This convergence effect is common in ecological data and depends on the spatial scale of observation [56] and hydrological characteristics of basin. Identification of the effects of karst features on nutrient export may not be possible without complete understanding of the restrictive relation between karst features and hydrological conditions, as well as landscape which was closely related to human activities in basin.

Measures to Control Nitrogen and Phosphorus Pollution in Karst River Waters
Results from riverine nutrients monitoring for the whole study period show that the situation of nitrogen and phosphorus in the rivers of the Huixian karst wetland was not optimistic. TN and TP concentrations were wider than the observed values in other karst systems in Europe, North America and southwest China, higher than those in other wetland systems and in rivers of Lijiang River basin (Table 8), indicating the high concentrations of TN and TP in the rivers of study area. During the wet season, when nitrogen and phosphorus concentrations were relatively low, up to 83.6% and 49.2% of water samples were still categorized as having TN and TP pollution, respectively, even in the wet season when the nitrogen and phosphorus concentrations were relatively lower. The complexity of the dual surface-subsurface hydrological structure of the Huixian karst wetland basin means that diffuse nutrient pollution is particularly difficult to control. Complex relationships between nutrient sources, forms and transfer in river waters also mean the necessity of long-term implementation of various measures. Temporality and spatiality must be taken into account together in setting and implementing measures for providing measures to control nitrogen and phosphorus diffuse pollution. This implies that, when solving water quality problems on the local scale, we should focus on the effects of catchment-scale variables, such as land use mainly affected by human activities or hydrogeological features on river water quality from a long-term perspective [24,57]. In our study, results demonstrated that high concentrations of nitrogen and phosphorus in the river generally occurred during low flows, which were consistent with the findings of others that nitrogen and phosphorus were retained within the river during low flows [65,66]. More precipitation and thus more water discharge can dilute the nutrients in the water of the mainstem Xiangsi River, then reduce the nutrient concentrations. However, the nutrient concentrations can also increase in the tributaries Mudong and Huixian River, namely when nutrients are washed out or when wastewater tanks overflow. Results confirmed that human-influenced land-use types such as urban and agricultural land are nutrient sources [52,53]. Since the current situation of nutrient pollutions in Xiangsi River basin was mainly contributed from the upstream urban dominant catchment, with 100% and 81.8% of 88 collected water samples being categorized as having TN and TP pollution respectively. Therefore, in terms of setting up river nitrogen and phosphorus monitoring stations, it is more reasonable for the monitoring stations of the mainstem Xiangsi River to be close to the downstream of the urban outfall, mainly to monitor the point source of pollution in urban areas. The monitoring station for comparison and reference should be set up in the upstream of the urban area. The monitoring stations of tributaries Mudong River and Huixian River are more reasonable setting at downstream of irrigation canals, farms and domestic sewage outfalls, to focus on monitoring agricultural nitrogen and phosphorus pollution. Additionallly, with regard to nitrogen and phosphorus pollution control measures, some measures of restricting during agricultural water peak in the wet season or recharging during low flows in the dry season are necessary, to maintain ecological flows in the river and to reduce the frequency of high concentrations of nitrogen and phosphorus. It is clear that policy measures should be taken as soon as possible by the local government to limit the nutrient pollution emissions from the upstream Lingui District for reducing the nitrogen and phosphorus of the lower reaches. For agricultural land, ecological farming with central collection of aquaculture wastewater and treatment before discharge, and ecological cultivation with environmentally friendly fertilization should be advocated and supported by local managers to reduce agricultural nitrogen and phosphorus pollution.

Conclusions
The nutrient concentration and forms within 11 nested catchments ranging in scale from 21.00 km 2 up to a maximum of 373.37 km 2 in Huixian karst wetland have been monitored over 2 years (from October 2017 to September 2019) to identify the spatial and temporal variations of riverine nutrients affected by hydrological conditions, human activities and karst features. The following conclusions can be made based on the results of this study: (1) In the dry season, NO − 3 -N was the main form of TN with a mean contribution ranged from 44.8% to 78.3% at the eleven monitoring sections, while in the wet season, the mean contribution of NH + 4 -N to TN were 12.9% to 209.8% higher than dry season. TDP was the main form of TP in different periods with a mean contribution ranged from 49.2% to 75.1% in the whole monitoring period and a more significant correlation between them in the dry season than the wet season. The concentrations of TN and TP were generally higher during low flow, while the fluxes were higher during high flow events in the wet season and maintained a power function relationship with discharges over the sampling periods. Results demonstrated the seasonal hydrological condition was a decisive factor controlling the seasonal variations of in-stream TN and TP.
(2) The different nitrogen and phosphorus sources from human activities in different spatial catchments induced dynamic disparities of riverine nitrogen and phosphorus. In agriculturally dominant catchments of tributaries Mudong and Huixian River, the forms, concentrations, and fluxes of TN and TP were different even if the discharge from the same monitoring section was the same in different seasons. While in urban dominant catchments of the Xiangsi River main stem, the nutrient concentrations were generally higher (up to 100% and 81.8% of water samples were categorized as having TN and TP pollution, respectively) and maintained a relatively stable correlation with discharge over the sampling periods (the Pearson correlation coefficient between discharge and TN, TP concentration were −0.476~−0.632 and −0.402~−0.776, respectively). On the other hand, the significant values of discharge, TN and TP flux in Xiangsi River (0.496, 0.394, 0.646, respectively) were larger than that in tributaries Mudong River (0.023, 0.115, 0.281, respectively) and Huixian River (0.256, 0.303, 0.481, respectively), indicating that the spatial variations of TN and TP fluxes were larger at small catchment scales in lower-order tributaries than that at large catchment scales in higher-order main stem for the increased quantity of agricultural water taking from tributaries. These results suggested that the catchment-scale human activities play an important role in inducing spatial disparities in riverine nitrogen and phosphorus. (3) There was a significant positive correlation between the share of covered karst land and the mean TN and TP nutrient export coefficients (p ≤ 0.05, the correlation coefficient ranged from 0.607 to 0.906). Because of the dispersed discharge of hydrological system in Huixian Karst Wetland basin, the range of mean TN and TP concentrations in different rivers tended to convergence to smaller in downstream reaches, especially in the wet season. The results demonstrated that karst features had a strong constraint on the spatial scale similarity of riverine nitrogen and phosphorus.
In addition, based on the spatial and temporal variations of nitrogen and phosphorus in river waters influenced by hydrological conditions, human activities and karst features, corresponding scientific suggestions have been proposed to control nitrogen and phosphorus pollution of the water system in Huixian Karst Wetland basin.
Clearly, our understanding of the links between hydrological conditions, human activities, karst features, and spatio-temporal dynamics of riverine nitrogen and phosphorus in Huixian basin is relatively restricted. For example, resource constraints dictate that this study has just focused on TN and TP in river waters at different catchment scales that are relatively easy to measure. However, nitrogen and phosphorus transfer in the surface water is just one aspect of nutrient cycling in the water system. A fuller understanding of nutrients transfer at different vertical depths is required, as river waters, soil, and groundwater are all the mediums in which nutrients flow, especially in a karst-affected basin where surface-water and groundwater are closely linked. Thus, linking the study of water cycling with the analysis of nutrient transfer processes will be an important aspect of nitrogen and phosphorus spatio-temporal variations research in Huixian karst wetland basin for further research.