Study on Spatial and Temporal Distribution of Nutrient Salts in Typical Seaward Rivers in Hainan Island

. Hainan Island, China's second largest island, is located in the tropics and its rivers are significantly affected by precipitation. With the construction of free trade port with Chinese characteristics, Hainan Island's industry, agriculture, tourism and urbanization have developed in an all-round way. With the rapid development of economy, water environment ecology has been destroyed. Based on the two seasons of drought and rain, this paper took Ningyuan River and Wanquan River as the research object to study the spatial and temporal distribution of nitrogen and phosphorus nutrients in the rivers entering the sea in Hainan, and found that the concentration of nutrients in Ningyuan River was higher than that in Wanquan River. The total nitrogen pollution in dry season was higher than that in rainy season, and that in tributaries was higher than that in main stream. The maximum nitrogen content in Ningyuan River reached 2.99 mg/L and 2.37 mg/L in the dry season and rainy season, and human activities were the main reason for the increase of nutrient concentration.


Introduction
In the past few decades, the intensification of global climate change, the significant increase in the duration of extreme heavy rainfall, the rapid development of urbanization and the increase of human activities have led to a large amount of nitrogen (N), phosphorus (P) and other nutrient elements entering rivers and lakes and transporting them to coastal waters, posing a serious threat to water quality and aquatic ecosystems [1]. Inorganic or organic nutrients, especially nitrogen and phosphorus, are enriched in a specific water environment, leading to the change of excess nutrition in the water area, which is called eutrophication [2]. Eutrophication has become an important problem for global surface water bodies [3].
Rivers are an important link between the sea and the land and a vital link in the circulation of matter in nature. A large amount of non-point source pollution flows into rivers along with surface runoff, resulting in the decline of their water quality [4]. Studies have shown that the global river nitrogen output is estimated to be about 60 Tg/y, which has increased by more than two times compared with before the Industrial Revolution [5]. Zheng's study shows that 57% of nitrogen and 67% of phosphorus in Chinese water bodies come from agricultural production [6]. With the development of urbanization, the emission of pollutants from household and agricultural sources increases day by day, and a large amount of nitrogen and phosphorus nutrients enter lakes and bays along with rivers, causing the concentration of nitrogen and phosphorus to increase continuously and the proportion of nutrients to be unbalanced [7]. Especially in estuarine area [8], with the increase of nutrient levels and structural changes, severe eutrophication and bloom outbreak have been caused, and a vicious cycle has been formed between cyanobacteria bloom formation, eutrophication and climate warming, greatly increasing the difficulty of aquatic environment restoration [9]. The freshwater discharge of China's three major rivers (Yellow River, Yangtze River and Pearl River) accounts for 73% of the total discharge of China's rivers into the sea, and the seawater with the highest degree of eutrophication is mainly distributed in these three largest estuaries [10]. According to the study of Li , the input of dissolved total nitrogen and dissolved total phosphorus in seaborne rivers needs to be reduced by 78-93% before 2050 to avoid coastal eutrophication [11]. Therefore, the detection and research of river nutrient salts is the key to control water eutrophication and protect water ecosystem.
Hainan Island is the second largest island in China, where industry, agriculture and tourism have developed rapidly [12]. Due to the tropical climate of high temperature and high humidity, chemical fertilizers are heavily used in agricultural production. The over-input nutrients cannot be effectively used by crops and are lost to the environment, resulting in a large amount of farmland nutrient loss. In addition, heavy rainfall in the rainy season leads to soil and water loss, which makes Hainan Island face serious risk of non-point source pollution and eutrophication risk. Therefore, it is very important to study the spatial and temporal distribution of nutrients in the seaward rivers of Hainan Island to maintain the stability of river water quality.
In this study, Wanquan River and Ningyuan River, two typical rivers in Hainan Island, were selected as research objects, and 12 representative sampling points were set respectively. Since Hainan Island has a typical tropical monsoon Marine climate, two samples were collected in the dry season and the rainy season to obtain the spatial and temporal distribution characteristics of nitrogen and phosphorus nutrients, and to evaluate the pollution status.

Study area
Hainan Island is located in 18°10 '~20°10' north latitude and 108°37 '~111°03' east longitude, with a tropical monsoon Marine climate (Figure 1). The four seasons are not distinct, the annual temperature difference is small, the annual average temperature is above 23 ℃, the precipitation is abundant, the dry season and the rainy season are obvious, every year from May to October is the rainy season, the total rainfall is about 1500 mm, accounting for 70% to 90% of the annual rainfall.
Wanquan River is located in the central and eastern part of Hainan Island. This basin is the most abundant area of precipitation in Hainan Island, with annual precipitation of more than 2100 mm, and general rainfall of about 1900 mm in the rainy season. The total length of the main stream is 157 kilometers, which is the third longest river in Hainan Province. With a total length of 83.5 kilometers, the main stream of Ningyuan River is the fourth longest river in Hainan Province and also the longest river in southern Hainan Island. The annual rainfall is about 1400 mm, and the Ningyuan River basin is one of the regions with the least rainfall in Hainan. The Wanquan River basin is dominated by towns and the Ningyuan River is dominated by farms. N-4, N-7, and W-5 are tributaries of rivers.

sampling strategy
In this study, two samples were collected in July 2022 and February 2023, respectively, reflecting the river water quality in the rainy and dry seasons in Hainan Island.
For each sample, surface water (0-50 cm below the surface) was taken by using a 3 L plexiglass water sampler. After the water sample was washed, the samples were collected. The electrical conductivity, temperature, dissolved oxygen, pH value and turbidity were measured by portable instruments. After the filtration, the glass fiber filtration membrane with 0.45μm (Φ47mm) was put into the sealed bag with tweezers and stored away from light immediately at -20℃ in the vehicle refrigerator. Mixed water samples were collected three times, and a total of 500 mL water samples were taken. Use an eyedropper to add 2~3 mL of 1 mol/L sulfuric acid for easy preservation. All water sample bottles were put into the storage box and put into the ice pack. The storage time was 14 days. The sampling time lasted for about a week. Before sampling, all water samples were washed with detergent, then with tap water, and then washed three times with distilled water.

Ecological assessment
The single pollution index method was used in this study. The single factor pollution index selected the single index with the worst water quality to determine the water quality pollution situation of the subordinate waters. P i =C i /S i (1) P i is the single pollution index, C i is the measured concentration, S i is the Class Ⅲ water standard of China surface water environmental quality standard(TN≤1.0mg/L 、 TP≤0.2mg/L 、 NH3-N≤1.0mg/L). When P i ≤1, it means that the water is not polluted. P i > 1 indicates water pollution. N-1 N-2 N-3 N-4 N-5 N-6 N-7 N-8 N-9 N-10N-11N As shown in Figure 2, the water temperature is 24.0℃~30.4℃ in dry season and 29.5℃~35.2℃ in rainy season, the difference is obvious. For salinity, values were almost zero of most points and near the estuary would increase under the influence of seawater backflow, and it was more affected by seawater in the dry season. The pH value fluctuates little in the rainy season (7.52~9.26) and the dry season (7.79~8.93), showing weak alkalinity. Turbidity in dry season is usually higher than that in rainy season, and turbidity in tributaries is obviously higher. The dissolved oxygen was 5.25±0.85 mg/L in rainy season, which was significantly lower than 6.19±0.84 mg/L in dry season. Affected by temperature, the higher the temperature was, the lower the dissolved oxygen would be.

Spatiotemporal distribution of phosphorus
As shown in Figure 3, the TP ranges from 0.004 mg/L to 0.210 mg/L in the rainy season with an average of 0.066±0.039 mg/L, and from 0.028 mg/L to 0.254 mg/L in the dry season with an average of 0.076±0.043 mg/L. TP in the dry season was higher than that in the rainy season, and the highest value appeared in the tributary N-7. From the main stream, TP rises from upstream to downstream. According to the single pollution index, only N-7, a tributary, was polluted by TP (P TP =1.05 in the rainy season and 1.27 in the dry season), due to the influence of residents.

Spatiotemporal distribution of nitrogen
As shown in Figure 4, the TN ranges from 0.330 mg/L to 2.371 mg/L in the rainy season with an average of 0.918±0.453 mg/L, and from 0.392 mg/L to 2.990 mg/L in the dry season with an average of 1.114±0.619 mg/L. TN in the dry season was higher than that in the rainy season. From the main stream, TP rises from upstream to downstream. In the rainy season, a large amount of fertilizer enters the tributary under the action of rain, and the highest value appears in N-4, while in the dry season, the highest value appears in N-7, which is seriously affected by domestic sewage. In tributaries and estuaries, TN concentrations increase significantly. According to the single pollution index, In the dry season, the average TN concentration of the river was polluted, and N-7 was moderately polluted (P TN =2.99). During the rainy season, N-4 was moderately polluted (P TN =2.37), while N-7, N-11, W-2, W-6 and W-9 were mildly polluted(1<P TN <2).

Conclusions
Our investigation elucidated the temporal and spatial variations of nutrient salts in typical seaporic rivers in Hainan Island. From upstream to downstream, the nutrient concentration of nitrogen and phosphorus increased, and the nutrient concentration in dry season was higher than that in rainy season. The total phosphorus hardly exceeded the standard, while the total nitrogen in tributaries and docks was affected by rural non-point source pollution and domestic wastewater discharge, which resulted in pollution, especially in the dry season. Therefore, controlling external inputs such as rural non-point source pollution and domestic wastewater discharge is particularly important for maintaining the stability of river water quality in Hainan Island.