Analysis on the Characteristics of Annual Changes and the De-tention Effect of Nitrogen and Phosphorus Nutrients in Plain Reservoirs

. In order to study the annual variation characteristics and the retention effect of nitrogen and phosphorus nutrients in plain reservoirs, taking Daheiting Reservoir in North China Plain as an example, the annual variation characteristics of total phosphorus (TP), total nitrogen (TN), ammonia nitrogen (NH 3 -N) and nitrate nitrogen (NO 3 -N) in Saheqiao and dam front water of Daheiting Reservoir from January 2018 to December 2018 were analyzed. On this basis, the retention rate of nitrogen and phosphorus and the actual retention rate of Daheiting Reservoir are calculated and analyzed. The results show that the annual average actual retention rate of Daheiting Reservoir in 2018 is only 65.70%, and other indicators are negative. The overall performance of water quality indicators in the year is that the retention effect in wet seasons is greater than that in dry seasons, and the retention effect is affected by the absorption and transformation of algae in the reservoir, the precipitation of particulate nutrients, the release of sediment and the operation and regulation of the reservoir, in which the operation and regulation of the reservoir plays a decisive function. The results show that Daheiting Reservoir has obvious interception effect on TP and still plays the role of "sink", while it basically maintains the balance of nitrogen.


Introduction
Reservoir is one of the main water sources of urban drinking water in China [1] . However, due to the interception effect of the dam, the hydraulic retention time is greatly extended, and the original hydrological characteristics of the river and the transformation and transport flux characteristics of nitrogen and phosphorus nutrients will have a series of changes [2,3] . Stenback and others [4] analyzed the monitoring results of the nitrate nitrogen in 26 years, and found that the reservoir has a great interception effect on the nitrate nitrogen in the water body. Beusen and others [5] found that about 13% of the particulate nutrients are retained in the reservoir every year after the reservoir is built in natural rivers. Stanley and others [6] found that the retention rate of nutrients in poor nutrient water can reach more than 70%. Ding Shuai and others [7] found that the retention effect of nutrients in the Three Gorges Reservoir is different, and the retention rate of phosphate is higher than that of inorganic nitrogen and silicate. For natural rivers, human activities seriously interfere with the material transportation of natural rivers [8][9][10] . For reservoirs, if a large amount of nitrogen and phosphorus nutrients are intercepted for a long time, the water body will gradually change from oligotrophic to eutrophic, and then cause water bloom and other water quality deterioration.
It is generally believed that the retention effect of the nitrogen and phosphorus nutrients in reservoirs is closely related to the reservoir scale, the operation mode and the biological composition [11] . Caraco and others [12] think that the retention effect of large reservoirs on nutrients is more obvious, and Garnier and others [13] hold this view. Behrendt and others [14] think that the retention effect of reservoirs on phosphorus is more obvious under the low specific runoff and the hydraulic load. Morri and others [15] think that when the reservoir is playing the role of flood control, the reservoir will retain more particulate nutrients, and the particulate phosphorus deposition is the main way of the phosphorus retention. Burford and others [16] found that the retention rate of the nitrogen and phosphorus nutrients in different periods of the year is not the same. When the upstream flow of the reservoir is less, the retention rate of nutrients is negative, and the reservoir sediment is stable. According to Kelly and others [17] , the phosphorus retention rate varied from 30% to 80% in different periods. The role of the biological composition mainly refers to the effect of phytoplankton on the retention of nitrogen and phosphorus. Phytoplankton can transform inorganic salts in the surface water into the particulate organic salts. D'Angelo and others [18] think that in artificial reservoirs, phosphorus retention is more controlled by the temperature and the flow rate. Humborg and others [19] think that reservoirs will lead to the increase of HRT and transparency, which will lead to the increase of phytoplankton biomass, and further increase the retention efficiency of the nitrogen and phosphorus in reservoirs. Therefore, for eutrophic reservoirs, the main causes of the reservoir eutrophication can be clarified by studying the retention effect of the nitrogen and phosphorus nutrients, which plays an important role in the guarantee of the water quality and the normal operation of reservoirs.

Overview of the study area
Daheiting Reservoir is located in Qianxi County, Tangshan City of Hebei Province, with a controlled drainage area of 3.51×104km 2 and a total storage capacity of about 337 million m 3 . Together with Panjiakou Reservoir, it is called "Panda Reservoir". Panda Reservoir was listed in the first batch of "national important drinking water source list" in 2006. It is one of the important drinking water sources in Luanhe River System and an important water source reservoir in North China Plain. It mainly supplies water to Tianjin and Tangshan. In the 1990s, large-scale cage fish culture began in Daheiting Reservoir, and a large number of surplus bait settled down and deposited at the bottom of the reservoir, causing sediment pollution. After the net cage fish culture was completely removed in 2017, the eutrophication of the water body did not improve, and large-scale water blooms often broke out during the early summer and the late autumn of the year.

Data sources and calculation methods
This study mainly collected the monthly monitoring data of Saheqiao point and the point in front of the dam in the upstream of Daheiting Reservoir in 2018 (as is shown in Figure 1). The flow data mainly came from the Luanhe Water Diversion Project Administration, and the water quality data were taken to the laboratory for detection and analysis after the team members collected water samples on site. The detection method followed the relevant provisions of "water and wastewater monitoring and analysis method". The detection indexes include the total nitrogen, the ammonia nitrogen, the nitrate nitrogen, the nitrite nitrogen, the total phosphorus, phosphate, iron, manganese and other water quality indexes. Excel 2016 is used for the data processing, and Origin2017 is selected for the data drawing.
In this study, the retention effects of nitrogen and phosphorus in the reservoir were calculated from two aspects: the retention rate of the nutrient concentration R C (%) and the actual retention efficiency RET (%). The calculation method is as follows: In this equation, C IN and C out represent the concentration of nutrients in and out of the reservoir (mg/L); M in and M out are the flux of nutrients in and out of the reservoir respectively. Q i is the runoff of each month, and C i is the concentration of nutrients in each month.

Variation characteristics of the annual inner diameter discharge
As a regulating reservoir, Daheiting Reservoir is mainly used for the joint operation with the upstream Panjiakou Reservoir to realize the flood control and the water supply functions. The storage capacity of Panjiakou Reservoir is about 10 times that of Daheiting Reservoir. Therefore, Daheiting Reservoir often has a large amount of the incoming water and the abandoned water before the flood season. According to the monitoring data of the inflow and the outflow of Daheiting Reservoir(as is shown in Figure 2), the total annual runoff of the inflow and outflow of Daheiting Reservoir are 1.925 billion cubic meters and 1.916 billion cubic meters respectively, and they are mainly concentrated in April to August. The runoff of inflow from April to August is 1.849 billion cubic meters, accounting for 96.07% of the total annual inflow, and that of the outflow from April to August is 1.802 billion cubic meters, accounting for about 30% of the total annual outflow 94.04% of the runoff.

Variation characteristics of the nitrogen and phosphorus concentration in a year
Nitrogen, phosphorus and other nutrients are the basic nutrients required by the biological life activities, and are also the important factors causing water eutrophication and bloom. Under the suitable environmental conditions, nitrogen and phosphorus nutrients are often released to the water body again, resulting in the abnormal reproduction of planktonic algae, eutrophication phenomenon, and affecting the normal operation of the reservoir.
The water quality data of Saheqiao and the monitoring point in front of the dam respectively represent the water quality of the incoming and the outgoing water bodies. As is shown in Figure 3, the incoming TP concentration of Daheiting Reservoir in 2018 varies from 0.03 to 0.3mg/L, and the outgoing TP concentration varies from 0.024 to 0.17mg/L. From January to August of the year, the outgoing TP concentration is higher than the incoming TP concentration only in April, and the incoming TP concentration is much higher than the incoming TP concentration in other months. The maximum concentration difference was 0.19mg/L, which appeared in January, and the TP concentration in September and De-cember was basically the same.
The TN concentration of Daheiting Reservoir changed from 3.23 to 6.01 mg/L, and the change of the TN concentration in the outlet is from 1.8 to 5.42 mg/L. During April to August and October to December, the TN concentration of Daheiting Reservoir was higher than that of Daheiting Reservoir, and only in April, the TN concentration of Daheiting Reservoir was higher than that of Daheiting Reservoir, and the maximum concentration difference was 1.54 mg/L, which occurred in April. The TN concentration of Daheiting Reservoir was lower than that of Daheiting Reservoir in other months, and the maximum concentration difference is 57 mg/L, which appeared in September.
The concentration of NH 3 -N in Daheiting Reservoir area varies from 0.036 to 0.443 mg/L, and the concentration of NH 3 -N varied from 0.029 to 0.452 mg/L. The concentrations of NH 3 -N in and out of the reservoir were basically the same during the year. The concentration of NH 3 -N was higher in the reservoir area from January to March and September.
The variation of NO 3 -N concentration in Daheiting Reservoir area is between 2.65 and 4.24mg/L, and that in the reservoir area is between 1.28 and 4.63mg/L. The variation of NO 3 -N in the reservoir area is consistent with the annual trend of TN, in which the concentration of NO 3 -N in the reservoir area is higher than that in the reservoir area from May to August and from October to December, and the maximum concentration difference occurs in July, about 1.6mg/L, and that in the other months is lower than that in the reservoir area. The maximum NO 3 -N concentration difference occurred in August, about 1.68mg/L.

Variation characteristics of nitrogen and phosphorus monthly transportation in a year
According to the inflow and outflow runoff and the nitrogen and phosphorus nutrient concentration of Daheit-ing Reservoir, the monthly transportation of nitrogen and phosphorus in the year can be obtained. As is shown in Figure 4, the input of TP in Daheiting Reservoir is about 265.14t/a, and the output is about 90.93t/a. The maximum inflow and outflow transportation occurs in August, with the inflow transportation of 95.94t and the outflow transportation of 24.99t. The inflow and outflow transportation of TN are respectively 7070.53t/a and 7284.61t/a. The maximum in and out of the warehouse transportation volume occurred in August, and the in warehouse transportation volume was about 2962.75t and 3982.98t. The transportation volume of NH 3 -N was 208.20t/a and 250.06t/a respectively, and the maximum transportation volume of NH 3 -N was 116.67 t and 169.02 t in August. The transportation volume of NO 3 -N was 5768.64t/a and 6121.84t/a respectively, and the maximum transportation volume of NH 3 -N was 2264.28t and 3402.43t in August.
On the whole, the maximum throughput of TP, TN, NH 3 -N and NO 3 -N in and out of Daheiting Reservoir occurred in August of this year, while in terms of the total annual throughput, the difference of TP, TN, NH 3 -N and NO 3 -N in and out of Daheiting Reservoir was 174.21t/a, -214.08t/a, -41.86t/a and -353.2t/a, respectively. Among them, TP was dominated by the phosphorus input, while TN, NH 3 -N and NO 3 -N were dominated by the output.

Retention rate of nitrogen and phosphorus concentration
The concentration retention rate (R C ) can clearly reflect the change of the nutrient concentration before and after the dam interception. From table 1, it can be seen that the annual average concentration retention rate of TP in Daheiting Reservoir area is about 45.32%, of which the retention rate is the largest in July, about 85.19%. The overall performance is that the concentration in the reservoir is greater than that in the reservoir, indicating that the TP in Daheiting Reservoir area is affected. The concentration of TP in the upstream water has a great influence, and a large amount of TP settles into the water or is absorbed and consumed by algae, and is finally intercepted in the reservoir area. The average annual retention rate of TN in the reservoir area is about 2.45%, and the overall performance is that the concentration of TN in the reservoir area is slightly higher than that in the reservoir area. The retention phenomenon of TN in the reservoir area mainly occurs from April to July, in which the retention rate of TN in July is the largest, about 44.27%, and the average annual retention rate of NH 3 -N in the reservoir area is about -5.88%, and the overall performance is that the concentration of TN in the reservoir area is slightly lower than that in the reservoir area. The retention phenomenon of NH 3 -N in the reservoir area mainly occurs in April and September, and the maximum retention rate of the year occurred in April, about 69.34%. The average annual retention rate of NO 3 -N in the reservoir area was about 1.42%, and the overall performance was that the concentration of NO 3 -N in the reservoir area was slightly higher than that in the reservoir area. The retention phenomenon of NO 3 -N in the reservoir area mainly occurred from May to July and from October to December, and the maximum retention rate occurred in July, about 55.56%.

Actual retention rate of nitrogen and phosphorus
According to the monthly runoff and the nutrient concentration of the reservoir, the monthly inflow and outflow flux of nutrients and the annual total flux can be calculated, and the actual retention rate and the annual actual retention rate of nutrients in each month can be further obtained (as is shown in Table 1).
The actual retention rate (RET) can indicate the true retention rate of nutrients after interception. On the whole, Daheiting Reservoir shows the obvious retention effect of TP, and the actual retention rate of TP ranges from -2628.6% to 100%, and the average annual retention rate is 65.70%, while for TN, NH 3 -N and NO 3 -N, it shows the opposite trend, in which the actual retention rate of TN ranges from -841% to 100%, and the average annual retention rate is -3.03%. The actual retention rate of NH 3 -N ranges from -734.2% to 100%, and the average annual retention rate is -734.2%. The actual retention rate is -20.11%, and the actual retention rate of NO 3 -N ranges from -802.3% to 100%, and the average annual actual retention rate is -6.12%. The actual retention rate of nitrogen and phosphorus in the reservoir during the wet seasons was significantly higher than that in the dry seasons.

Analysis of nitrogen and phosphorus retention factors
For large reservoirs, the existence of the thermal stratification structure is an important factor affecting the retention effect of nitrogen and phosphorus. Daheiting Reservoir has a long hydraulic retention time from May to September, resulting in stable thermal stratification in the vertical direction. On the one hand, the temperature and light conditions of the surface water are very suitable for the growth of algae, leading to the outbreak of the algal blooms [20] . Rapid propagation will consume a lot of nitrogen and phosphorus nutrients. The inorganic phosphorus and inorganic nitrogen carried by the upstream water are transformed into organic phosphorus and organic nitrogen, which are intercepted in the reservoir, resulting in high retention efficiency of nitrogen and phosphorus in this period. This study is consistent with the results of Wang Yaoyao [21] . On the other hand, during the thermal stratification of Daheiting Reservoir, there is obvious anoxic phenomenon in the bottom water [22] , which will lead to the diffusion and accumulation of a large number of nitrogen and phosphorus nutrients from the sediment interstitial water to the overlying water [23] . When the thermal stratification structure of the reservoir is destroyed, it will spread to the whole water body, resulting in the decrease or even negative of the actual detention efficiency of the reservoir. Figure 5 shows the relationship between the concentration of nitrogen and phosphorus in Daheiting Reservoir area and the concentration of nitrogen and phosphorus in the reservoir area in each month. The ratio k 0 is compared with the slope k=1, when k 0 is less than k, the concentration of nitrogen and phosphorus in the reservoir area is lower than the concentration of nitrogen and phosphorus in the reservoir area, and the sedimentation of nitrogen and phosphorus in the reservoir area is greater than the release. At this time, if there is more water coming from the upstream of the reservoir, nitrogen and phosphorus nutrients will be intercepted to the reservoir, and the actual retention rate is positive. When k 0 is greater than k, the concentration of nitrogen and phosphorus in the reservoir area is higher than that in the reservoir area, and the nitrogen and phosphorus deposition is less than the release. At this time, if there is large-scale inflow and discharge of the reservoir, the nitrogen and phosphorus nutrients will be carried out of the reservoir, and the actual retention rate is negative. In August 2018, the k 0 values of TN, NH 3 -N and NO 3 -N were greater than 1, and the sediment release was large. At this time, the inflow and outflow runoff of Daheiting Reservoir were more than 700 million cubic meters, and the actual retention efficiency of TN, NH 3 -N and NO 3 -N decreased to -34.4%, -44.9% and -50.3% respectively. The operation and dispatch of Daheiting Reservoir is also an important factor affecting the retention effect of nitrogen and phosphorus [24] . In terms of the year, the inflow and outflow of the reservoir can directly affect the input and output of nitrogen and phosphorus in the res- ervoir. According to the monthly runoff of Daheiting Reservoir in the year, it can be seen that during the period from January to February and from October to November, the operation condition of the reservoir is only inflow without discharge, so the nitrogen and phosphorus nutrients carried by the upstream water must be intercepted in the reservoir, and the actual retention efficiency of TP, TN, NH 3 -N and NO 3 -N is 100%. On the contrary, in December, the inflow runoff of the reservoir is basically 0, so the actual detention efficiency is far lower than 0, showing the negative detention. In addition, it can be seen from Figure 4 that during September 2018, the k 0 values of TP, TN, NH 3 -N and NO 3 -N were all greater than 1, and the reservoir sediments acted as the "source". However, the actual retention rates of nitrogen and phosphorus were positive due to the fact that the inflow in the upstream was more than the discharge in that month, and the reservoir as a whole still played the role of "sink". Therefore, for Daheiting Reservoir, the operation of the reservoir plays a decisive role in the retention of nitrogen and phosphorus.

Conclusion
(1) The concentration of TP in Daheiting Reservoir was significantly higher than that of water quality out of Daheiting Reservoir, mainly from January to March and from May to August, and the concentration of TN and NO 3 -N in Daheiting Reservoir was significantly higher than that of water quality out of Daheiting Reservoir from May to August and from October to December, while the concentration of NH 3 -N in Daheiting Reservoir was basically the same as that of the water quality out of Daheiting Reservoir.
(2) The annual actual retention rate of TP in Daheiting Reservoir in 2018 is 65.7%, which indicates that the reservoir has interception effect on TP, while the annual actual retention rates of TN, NH 3 -N and NO 3 -N are -3.03%, -20.11% and -6.12% respectively, which indicates that the reservoir has no obvious retention effect on the nitrogen nutrients.
(3) The retention effect of the nitrogen and phosphorus in Daheiting Reservoir varies greatly in the whole year. The retention effect of the nitrogen and phosphorus in wet seasons is larger. The main factors affecting the retention effect of nitrogen and phosphorus are algae propagation, absorption and transformation during thermal stratification, pollutant migration and diffusion in the bottom layer after the destruction of stratification structure and the operation regulation of the reservoir. Among them, the reservoir operation plays a decisive role.