Predicting the effects of reservoir impoundment on phytoplankton and shoreline vegetation communities using the space-time substitution method

The prediction of the influence of reservoir impoundment on water quality and phytoplankton community is the basis of ecological compensation or restoration. The aim of the current study was to predict the effects of reservoir impoundment on phytoplankton and shoreline vegetation communities using the space-time substitution method. The Huangjinxia Reservoir under construction on the Han River was selected as the research object. The space-time substitution method indicated that the average values of the total phosphorus (TP) and ammonia (NH4 þ-N) increased from 0.049 and 0.279 mg L 1 to 0.139 and 1.132 mg L , respectively, after reservoir impoundment. The percentage of diatom biomass exceeded 95% before the reservoir impoundment. However, it was gradually decreased to 75% after the reservoir impoundment. Meanwhile, the biomass of Chlorophyta, Cryptophyta and Pyrrophyta increased significantly, accounting for 32, 20 and 13% of the total biomass, respectively, after reservoir impoundment. Cynodon dactylon (65.3%), Polygonum hydropiper (51.7%) and Aster subulatus (50.3%) were the dominant shoreline vegetation before the reservoir impoundment, whereas after the reservoir impoundment, the dominant species shifted to Alternanthera philoxeroides (62.3%), Lobelia chinensis (55.7%) and C. dactylon (53.9%). Our results suggested that the percentage of bloom-forming phytoplankton would gradually increase after the reservoir impoundment. In addition, A. philoxeroides, C. dactylon and L. chinensis would be the plants suitable for living in the shoreline of reservoirs in this area.


INTRODUCTION
Reservoir construction is an important means to utilize water resources and hydro-energy efficiently. It alleviates the contradiction between the lack of water resources and the demand for a large amount of electric energy to a great extent. However, reservoir construction will also cause a series of ecological and environmental problems, such as changing the original ecosystem, causing water quality deterioration or the formation of water blooms (Joung et al. ; Xue et al. ). Thus, the above problems should be considered in the process of reservoir design and construction so as to formulate a series of ecological compensation or restoration measures. Particularly, the prediction of the influence of reservoir impoundment on water quality and phytoplankton community is the basis of ecological compensation or restoration.
Phytoplankton are the primary producers in rivers and reservoirs playing critical roles in maintaining the health and balance of aquatic ecosystems. Mhlanga et al. () monitored phytoplankton community in the Tugwi-Mukosi reservoir (Zimbabwe) 9 months after impoundment. Their results showed that Cyanophyta, dominated by Microcystis aeruginosa and Aulacoseira granulata, was the predominant phylum accounted for 50-70% in the total phytoplankton community. Wang et al. () found that reservoir impoundment affected silicon cycling thereby affecting the total amount of phytoplankton and the proportion of diatoms. Wu et al. () compared water quality and phytoplankton community composition between rivers and reservoirs before and after impoundment of the Three Gorges Reservoir. Their results showed that the proportion of diatoms decreased significantly but dinoflagellate increased gradually after impoundment. Bi et al. () analyzed the variation in phytoplankton community in the Xiangxi River before and after impoundment. Their results showed that phytoplankton diversity decreased after impoundment. These results showed that although the variations in phytoplankton community caused by reservoir impoundment in different reservoirs showed a similar pattern, there was still much uncertainty due to differences in natural geographic conditions and species of indigenous phytoplankton.
Generally, phytoplankton community variation was mainly induced by hydrodynamics changes after reservoir impoundment ( Jati et al. a, b). de Souza et al.
() showed that reservoir impoundment directly affected the hydrodynamics, mixing and underwater light field, thereby influencing the composition of phytoplankton functional groups. In addition, the influx of exogenous pollution after reservoir construction also directly affected the compo-

METHODOLOGY Study area and sampling sites
The Huangjinxia Reservoir under construction is located upstream of the Han River. As shown in Figure 1, a reservoir already in operation (Shiquan Reservoir) is located 40 km downstream of the Huangjinxia Reservoir. The average annual precipitation was 890 mm, and the perennially mean temperature was 14.6 C in this area. The space-time substitution method could thus be carried out by comparing the water quality, phytoplankton community and shoreline vegetation community. Three sampling sites were set up upstream of each reservoir.

Samples collection
Sampling was carried out in September 2020 when the Shiquan Reservoir had filled. At each site, three samples in a cross-section were sampled and mixed. Surface water samples were collected 0.5 meter below the surface with a plexiglass water sampler (3 L). The mixed water samples (1 L) were stored in plastic bottles in car refrigerator for water quality analysis. In addition, another 1-L mixed water sample was fixed with 15 mL Lugol's iodine solution for phytoplankton identification and counting. All samples were translated to the laboratory as quickly as possible.

Water quality analysis and phytoplankton counting
Water samples for water quality analysis were divided into two groups. One group was directly used for the total nitrogen (TN) and total phosphorus (TP) analysis. The other group was filtered through 0.45 μm cellulose membranes, and the filtrate was used for the analysis of the total dissolved nitrogen where S is the number of phytoplankton genera found at each sampling site; N is the total number of phytoplankton individuals found at each site; P i ¼ N i /N, with N i as the number of phytoplankton individuals in the ith genus; and N max is the number of phytoplankton individuals in the most abundant genus.
The relative dominance of plants is calculated using the following equation as follows: where R is the degree of relative dominance; N is the number of a scale's occupying; V 1 is the value of the scale fields; T is the total number of fields of weed community cluster; and V 2 is the value of the highest scale.

Water quality
The and 2(f)).

Phytoplankton community composition
A total of six phyla and 51 genera of phytoplankton were identified in the current study. There were distinct differences in the composition of phytoplankton before and after reservoir impoundment (Figure 3). The percentage of Bacillariophyte biomass exceeded 95% before the reservoir impoundment. However, it was gradually decreased to 75% after the reservoir impoundment. Meanwhile, the biomass of Chlorophyta, Cryptophyta and Pyrrophyta increased significantly, accounting for 32, 20 and 13% of the total biomass, respectively, after reservoir impoundment.   The accumulated phytoplankton biomass directly increased the measured TP concentration because of the high phosphorus content of phytoplankton.
In the current study, Bacillariophyta was dominated in the lotic natural river without reservoir. However, in the reservoir, Pyrrophyta and Cryptophyta were dominated.
Although both phyla were considered to be mainly auto- Our knowledge in this area was still very scarce and needed to be explored in future research.
The proportion of Chlorophyta in the reservoir area near the dam reached one-third of the total phytoplankton, but Bacillariophyta was dominated in the natural river without reservoir. Our investigation showed that TP concentration in the reservoir, which was slightly higher than the value in the upstream river, was not significant. In addition, the concentration of TN was reduced due to reservoir impoundment.
Although Compared with sites H1-H3, the proportion of Euglena, Cryptomonas and Peridiniopsis increased in sites S1-S3. Our results also showed that the upland plants were

CONCLUSION
Our results showed that Bacillariophyta was dominated in the lotic natural river without reservoir. However, in the reservoir, Pyrrophyta and Cryptophyta were dominated.
The proportion of Chlorophyta in the reservoir area near the dam reached one-third of the total phytoplankton, but Bacillariophyta was dominated in the natural river without reservoir. The succession from Bacillariophyta to Chlorophyta would be caused by the lower water flow after reservoir impoundment. A. philoxeroides (65.3%),