A study on the phytoplankton community structure in the Diaohe River section of the Middle Route of the South-to- North Water Diversion Project in winter

As the first span and aqueduct of the Middle Route of the South-to-North Water Diversion Project, Diaohe Aqueduct has brought good engineering and social benefits since its completion. In order to understand the present water quality of the Diaohe River section of the route, four samplings were conducted at Diaohe Aqueduct in the winter of 2018. The main results of this study are as follows: 62 species (or variants) of phytoplankton belonging to 32 genera, 22 families and six phyla were identified during the study period, among which Bacillariophyta are the most dominant species. The identified phytoplankton fall into eight indicator grades: os; ps, αm; ps, αm, βm; αm; αm, βm; αm, βm, os; βm; and βm, os. A total of 17 functional groups are detected, among which TB is the absolute dominant functional group. Based on the composition characteristics of phytoplankton, the water quality trophic state of Diaohe Aqueduct is mesotrophic. However, other physical indexes show that the water quality of the Middle Route of the South-to-North Water Diversion Project is good. The conclusions drawn using phytoplankton alone as the evaluation index may not be comprehensive. In future research, how to build a comprehensive index system combining other water quality indexes for the Middle Route of the South-to-North Water Diversion Project needs yet to be improved and tested.


INTRODUCTION
As the largest inter-basin water diversion project in the world, the Middle Route of the South-to-North Water Diversion Project (MRP) is a major strategic infrastructure for alleviating water shortages in northern China, reasonably allocating water resources, ensuring sustainable economic and social development, and comprehensively building a well-off society in China. Since the MRP provides domestic and industrial water to northern China (e.g. Beijing), the water quality in the MRP is required to meet the Grade II Chinese Environmental Quality Standard for Surface Water (GB3838-2002;. However, the MRP is composed of a long canal and complex hydraulic structures, and transfers drinking water to Beijing, Tianjin and other cities in open channels, which has caused a certain threat to the MRP water quality (Tang et al. ; Zhu et al. ). Therefore, it is particularly important to monitor the water quality in the MRP.
Phytoplankton is the principal primary producer in aquatic ecosystems, it is highly sensitive and responds rapidly to environmental changes (Yuan et  Harbor, and divided the aquatic system into oligotrophic, mesotrophic and eutrophic water types. However, due to the short running time of the MPR, there are few studies on determining water quality by studying phytoplankton community. Therefore, it is necessary to monitor the water quality in the MRP by studying the phytoplankton community. Known as 'the first span of the massive water diversion project', Diaohe Aqueduct is the first aqueduct for water conveyance in the MRP, and it has brought good engineering and social benefits since its completion. As the beginning section of the Middle Route of the South-to-North Water Diversion Project, the water quality of the Diaohe River section has a direct impact on the condition of drinking water for residents in the following areas receiving the water supply. Therefore, this paper investigates the phytoplankton in the Diaohe River section of the Middle Route of the South-to-North Water Diversion Project, with the aim of providing an important basis for the ecological protection of the water of this main canal (Diaohe River section) in the Middle Route.

Study area
The beam-type Diaohe Aqueduct is located on the river From December 12, 2014, when the Middle Route of the South-to-North Water Diversion Project was officially put into use, to October 1, 2017, the total cumulative amount of water entering the canal has reached 10 billion m 3 . The aqueduct has yielded good engineering and social benefits.

Sampling time and sites
In this experiment, four samplings were conducted at the Middle Route (Diaohe section) of the South-to-North Water Diversion Project in November 2018 (on November 7, 14, 20, and 27, 2018, respectively). The monitored canal section (Diaohe section) is in two different channel shapes: straight and curved. The light intensity varies on different sides of it, and the curved section has concave and convex banks which lead to different flow velocities.
All these factors will affect the growth and distribution of algae. As a result, three sampling points were set up after comprehensively considering the above circumstances and the operability on the site. They are located on the left and right banks of the pontoon bridge of Diaohe Aqueduct (straight section) and the water quality sensitive position of Diaohe Aqueduct (curved section), as shown in Figure 1.

Environmental factor analyses
The pH, water temperature (WT), dissolved oxygen (DO), and conductivity, oxidation-reduction potential (ORP) were measured by YSI 6600EDS at 0.5 m below the water surface.
At the same, a deep water sampler was used to collect the mixed water sample for an additional index at 0.5 m below the water surface, and 500 ml of mixed water was extracted and put into a polyethylene plastic bottle, then the water samples were transported to the laboratory within 24 h, and stored in a refrigerator at 4 C before being analyzed.
Additional indexes included the total phosphorus, orthophosphate, total nitrogen, ammonia nitrogen, nitrate nitrogen, permanganate index, chemical oxygen demand and chlorophyll a. These indexes were measured according to Chinese standard methods (see Wei et al. () for details).

Algae sample collection and identification
Collection of samples for qualitative analysis: use the plankton net no. 25 (mesh diameter being 0.064 mm) to collect the sample. Specifically, drag the net slowly at a speed of 20-30 cm/s by making a '∞' shape repetitively for 3-5 min in the space between the water surface and 0.5 m deep below the water surface. Add Lugol's solution into the collected sample for fixation, label the sample bottle, and make various records (Jin & Tu ). To perform qualitative analysis of the phytoplankton, classify the plankton samples, make them into temporary specimens for microscope slides and then identify them. In this process, the phytoplankton need to be photographed and preserved, and identified by authori- ). The cell count on each slide is no less than 200, and the average of the counts of three slides is taken for each specimen. Since the specific gravity of phytoplankton is close to 1, the volume of phytoplankton can be directly converted to biomass (wet weight). That is, the biomass of phytoplankton is equal to the number of phytoplankton multiplied by their average volume, and the unit is mg/L. The biomass of unicellular organisms is mainly obtained by measuring their individual shape (Jin & Tu ).

Statistical analysis
The following equations of three diversity indices are used to calculate community metrics of phytoplankton. These are the Shannon-Wiener diversity index (Shannon & Weaver ) (shown in Equation (1)), Margalef richness index (Margalef ) (shown in Equation (2)), the Pielou evenness index (Pielou ) (shown in Equation (3)):

Water quality indexes
The water quality indexes in Diaohe Aqueduct during the study period are summarized in  Figure 2 shows the composition of phytoplankton and Table 2 lists the phytoplankton species during the study period).

Composition of phytoplankton pollution indicator species
As early as 1908, German scientists Kolkwitz and Marsson (Kolkwitz & Marsson ) used algae to monitor the environmental quality of water bodies, and they were the first to propose a sewage biological system indicating  Table 3.

Phytoplankton diversity
The and it also can be used as a main indicator to evaluate water quality. As shown in Table 4 (Table 4 shows

Water quality evaluation with the Margalef richness index
The Margalef richness index indicates the richness of species in a biological community. Generally speaking, the  number of species is high in a healthy environment, and the number of species is low in a polluted environment (Ma ).

The Margalef richness index of phytoplankton in
Diaohe Aqueduct ranges from 6.98 to 7.21. The range of changes is big, with the characteristics of index changes in  space being left bank> water quality sensitive point> right bank. The index at all the three monitoring points is greater than 6, so the water quality is relatively healthy and the pollution level is clean.

Classification of phytoplankton into functional groups
The composition of phytoplankton functional groups can be analyzed according to the physiological and growth   Fragilaria and Navicula, and the secondary functional groups are P and MP, which mainly include Diatoma,

Discussion
During the study period, although the β-p hypertrophic phy-