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 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; Tang et al. 2011). 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. 2016; Zhu et al. 2019). 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 al. 2018), and the changes of its community structure have a direct or indirect impact on other aquatic organisms, and can even affect the stability of the aquatic ecosystem. Learning phytoplankton structure could obtain a comprehensive and real understanding of the status of an aquatic ecosystem (Suikkanen et al. 2007). Many researchers determine the water quality of a study area by studying phytoplankton communities and environmental variables, for example, Gharib et al. (2011) determined the water quality of Matru Beach in the Mediterranean Sea in southeastern Egypt by studying environmental variables and the abundance and community structure of phytoplankton during 2009–2010. Zaghloul et al. (2020) studied the species composition and abundance of phytoplankton weekly from 16 June 2011 to 30 May 2012 in the Eastern 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.

The beam-type Diaohe Aqueduct is located on the river 500 m away from Yaoying Village, Jiulong Town, Dengzhou City, Henan Province, 2.5 km south from Dengzhou–Zhechuan Highway. The main structures of the aqueduct include 115.1 m of surplus escape, 40 m of inlet transition, 26 m of inlet controlling sluice, 20 m of inlet connection, 350 m of aqueduct, 20 m of outlet connection, 15 m of outlet maintenance chamber, 70 m of outlet transition, and 3.9 m of outlet open channel, with the total length being 660 m. The span arrangement of the aqueduct body is 8*40 m + 1*30 m, the upper structure of which is a pre-stressed box-type simply supported beam arranged in double lines and double troughs. The net width of a single trough is 13.0 m, with a maximum cross-sectional area of 83.98 m², a designed water depth of 5.75 m, and an increased water depth of 6.46 m. The trough is 15 m wide at the top, 15.1 m at the bottom, the inner wall spacing between the two troughs is 5.0 m, and a sidewalk slab is laid between them. The full width of the double-line aqueduct is 33.0 m wide at the top and 33.1 m wide at the bottom. Diaohe Aqueduct is the first water conveyance aqueduct on the main canal of the Middle Route of the South-to-North Water Diversion Project, known as ‘the first span of the massive water diversion project’. 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 m³. The aqueduct has yielded good engineering and social benefits.

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.

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. (2002) for details).

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 1990). 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 authoritative books (Shi 2004; Zhou & Chen 2004; Wang et al. 2008).

Collection of samples for quantitative analysis: use the plexiglass water sampler to collect the sample by holding it still at about 0.5 m below the water surface, collect 1 L of water, put it into the sampling bottle that has been soaked with nitric acid and cleaned beforehand, and then immediately add 15 ml of Lugo's solution with a mass fraction of 15‰ into the collected sample for fixation. After taking the sample back to the laboratory, shake it thoroughly and transfer it into a 1,000 ml separatory funnel. After leaving the separatory funnel still for 24 h, aspirate the supernatant from it by siphoning (using a siphon) until 250 ml of the sample is left, and remove the remaining liquid to a small container. Leave the sample still for 24 h again, and then pipette the supernatant until 30 ml of the sample is left. After shaking the remaining liquid thoroughly, pipette 0.1 ml of the shaken sample into a phytoplankton counting box (0.1 ml) and count the phytoplankton under a microscope with a 10 × 40 magnification lens (Liu et al. 2019). 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 1990).

The water quality indexes in Diaohe Aqueduct during the study period are summarized in Table 1. During the study period, the water temperature was maintained at about 14 °C, and pH was maintained at about 8.5. Chlorophyll a is an index that characterizes the content of algae; comparing the content of chlorophyll a at different times in the same place, it is found that as time goes by, the content of chlorophyll a shows a downward trend, while the content of nutrients and organic matter both show a downward trend, and the water temperature does not change. It is inferred that the phytoplankton had begun to enter a dormant period. Comparing with the Chinese Environmental Quality Standard for Surface Water (G3838-2002), it is found that the content of all water quality indexes meet the Grade II Chinese Environmental Quality Standard for Surface Water, that means the water quality in the Diaohe Aqueduct during the study period is clean.

In the samples collected at Diaohe Aqueduct in November 2018, Chlorophyta, Cyanophyta, Bacillariophyta, Pyrrophyta, Euglenophyta and Cryptophyta are identified. There are 62 species (or variants) of phytoplankton under 32 genera, 22 families and six phyla, including 30 species (or variants) of Bacillariophyta under 12 genera, accounting for 48.39% of the total phytoplankton; 16 species of Chlorophyta under 11 genera, accounting for 25.81%; nine species of Cyanophyta under four genera, accounting for 14.52%; two species of Pyrrophyta under two genera, accounting for 3.23%; one species of Euglenophyta under one genus, accounting for 1.61%; and four species of Cryptophyta under two genera, accounting for 6.45% (Figure 2 shows the composition of phytoplankton and Table 2 lists the phytoplankton species during the study period).

As early as 1908, German scientists Kolkwitz and Marsson (Kolkwitz & Marsson 1908) used algae to monitor the environmental quality of water bodies, and they were the first to propose a sewage biological system indicating organic pollution in rivers. It pointed out different types of ‘indicator organisms’ for different pollution zones, and these were low pollution zone, β medium pollution zone, α medium pollution zone and multi-contamination zone. Through the changes of ‘indicating organisms’ in different water bodies, the water quality of the water body can be judged. Since the concept of ‘indicating organisms’ was proposed, it has been used by many scholars and has been continually revised and improved.

In the samples collected at Diaohe Aqueduct in November 2018, 62 species (or variants) of phytoplankton under 32 genera, 22 families and six phyla are identified, and they fall into eight indicator grades: os (oligotrophic); ps (p hypertrophic), αm (α medium eutrophic); ps, αm, βm (β mesotrophic); αm; αm, βm; αm, βm, os; βm; and βm, ps. Among them, there are one species of os phytoplankton under one genus, accounting for 5.56% of the total indicator species; two species of αm phytoplankton under two genera, accounting for 11.11%; four species of βm phytoplankton under four genera, accounting for 22.22%; four genera of βm, ps phytoplankton under three genera, accounting for 22.22%; and two species of α-β-o mesotrophic phytoplankton under two genera, accounting for 11.11%. The phytoplankton pollution indicator species and their quantity in Diaohe Aqueduct are shown in Figure 3.

In this experiment, the identified pollution indicator species fall into six phyla, namely Chlorophyta, Cyanophyta, Bacillariophyta, Pyrrophyta, Euglenophyta and Cryptophyta. The Bacillariophyta include Cyclotella meneghiniana Kütz., Gomphonema sp., Melosira granulata (Ehr.) Ralfs, and Synedra ulna (Nitzsch) Ehr.; the Pyrrophyta include Ceratium sp.; the Cyanophyta include Microcystis robusta (Clark) Nygaard, and Aphanizomenon flos-aquae (L.) Ralfs; the Euglenophyta include Euglena viridis Ehr.; the Chlorophyta include Phacotus lenticularis (Ehr.) Stein, Eudorina sp., Oocystis sp., Pediastrum simplex Meyen, Planktosphaëria gelatinosa G. M. Smith, Radiococcus nimbatus (Wild.) Schm., Pediastrum duplex Meyen, and Staurastrum manfeldtii Delp.; and the Cryptophyta include Cryptomonas erosa Ehr., as shown in Table 3.

The Shannon–Wiener diversity index (H′) is a single-factor index for representing the relationship between the number and species of biomes with multiple kinds of organisms. The biodiversity in communities reflects the complexity of biomes or habitats. This index is widely used to monitor changes of marine benthos (Noman et al. 2019), and it also can be used as a main indicator to evaluate water quality. As shown in Table 4 (Table 4 shows water quality and ecological evaluation standards), the Shannon–Wiener diversity index (⁠⁠), the Margalef richness index (d) and the Pielou evenness index (J) were applied to evaluate the water quality and ecological environment of Diaohe Aqueduct (Figure 4 shows the changes in the biodiversity index for each sampling points during the study period).

The Shannon–Wiener diversity index of phytoplankton in Diaohe Aqueduct ranges from 1.42 to 1.66. The biggest value 1.66 was recorded at the curved section, followed by 1.56 at the monitoring point on the left bank, and the smallest value 1.42 on the right bank. The range of changes is relatively small, and the pollution level is rated medium (Miao et al. 2019).

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 2012).

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.

The Pielou evenness index gives a good representation of the distribution uniformity of the individuals of phytoplankton species. The value range of J is 0–1. When this value is big, it indicates that the distribution of individuals among the species is uniform; when this value is small, it suggests uneven distribution of individuals among the species (Ma 2012).

The Pielou evenness index of phytoplankton in Diaohe Aqueduct ranges from 0.42 to 0.51, and the changes are stable, so the pollution level is rated as medium-light pollution. The value at the monitoring point on the right bank is small, while that on the left bank is large.

The spatial variation of the Pielou evenness index of phytoplankton in Diaohe Aqueduct is characterized as water quality sensitive point > left bank > right bank. The water quality at the monitoring points on the left and right banks is worse than that at the water quality sensitive point. Their pollution level is medium pollution, with the water quality on the left bank better than that on the right bank and rated as light pollution. Generally speaking, the distribution of individuals among the phytoplankton species in Diaohe Aqueduct is uniform.

The composition of phytoplankton functional groups can be analyzed according to the physiological and growth characteristics of phytoplankton and the mechanism of their environmental adaptability as proposed by Reynolds et al. (2002) and Padisák et al. (2009). The identified phytoplankton were classified into different groups according to their functions. As shown in Table 5, the phytoplankton in Diaohe Aqueduct can be divided into 17 functional groups, namely: B, C, D, F, G, H1, J, LO, M, MP, N, P, T, TB, W1, WO, and X2.

If the dominance of a certain functional group is ≥0.02, its status in the water is dominant and we call it the dominant functional group (Wang et al. 2018). The monitoring results on the left bank of Diaohe River register the most species of Bacillariophyta, in which Fragilaria, Cymbella, Navicula and Diatoma vulgare Bory (in the order of dominance from large to small, the same below) are absolutely dominant. On the right bank of Diaohe River, Fragilaria, Navicula and Cymbella are absolutely dominant, and at the water quality sensitive point, Navicula, Fragilaria, Cymbella and Oscillatoria are absolutely dominant.

In the monitoring period, the degree of dominance of the TB functional group in all the four sampling investigations is ≥0.02, which makes it the absolute dominant functional group in the Diaohe section of the middle water diversion route. The TB functional group includes Fragilaria and Navicula, and the secondary functional groups are P and MP, which mainly include Diatoma, Cymbella and Oscillatoria. All the three functional groups are between mesotrophic and eutrophic. In this study, since the TB, P, and MP functional groups are the dominant functional groups in Diaohe Aqueduct in winter, it is inferred that the nutritional state of Diaohe Aqueduct may be mesotrophic.

During the study period, although the β-p hypertrophic phytoplankton indicator and β mesotrophic phytoplankton indicator dominated in the Diaohe Aqueduct, considering the phytoplankton diversity index (including the Shannon–Wiener index, the Magalef richness index and the Pielou evenness index) and the study of phytoplankton functional groups, it can be inferred that the water quality of the Diaohe section was mesotrophic. The Middle Route of the South-to-North Water Diversion Project diverts water from Danjiangkou Reservoir to Beijing, Tianjin and other cities. As Wan et al. (2020) reports, the water quality of Danjiangkou Reservoir in 2018 was in a mesotrophic state, and the Diaohe Aqueduct is only a section of the MRP, so it is speculated that the mesotrophic state of water quality in the Diaohe Aqueduct during the study period may be related to the water quality of the Danjiangkou Reservoir. However, the conclusions drawn using phytoplankton alone as the evaluation index may not be comprehensive, and needs to be further combined with other indexes for comprehensive consideration. So in the future the water quality index and the phytoplankton need to be considered comprehensively.

By analyzing the water quality monitoring data, it is found that the water quality of the Diaohe Aqueduct is relatively clean, which meets the Grade II Chinese Environmental Quality Standard for Surface Water (G3838-2002). The phytoplankton population in the Diaohe section of the Middle Route in winter consists of 62 species of phytoplankton under 32 genera, 22 families and six phyla, and the main dominant species or genera are Cyclotella, Navicula, Fragilaria, Diatoma and Cymbella under Bacillariophyta and Oscillatoria under Cyanophyta. Considering the pollution indicator species, dominant functional groups and diversity index of phytoplankton in the Diaohe Aqueduct, it is inferred that the water quality of the Diaohe section was mesotrophic in the winter of 2018. Although the conclusions analyzed by phytoplankton community may not be comprehensive, it is recommended that the local government should pay more attention to changes in the phytoplankton community while carrying out long-term water quality monitoring.

This paper was supported by the Major Science and Technology Program for Water Pollution Control and Treatment (2017ZX07108-001) and Special Project on Water Quality Guarantee and Environmental Impact of the Middle Route of South-to-North Water Transfer Project (JZ0205A222018).

All relevant data are included in the paper or its Supplementary Information.