Assessment of phytoplankton community structure and water quality in the Hongmen Reservoir

To ﬁ nd effective measures to control the water quality of the Hongmen Reservoir, it is necessary to better understand its phytoplankton composition, abundance and spatial and temporal distribution. Samples were collected at three sampling sites in January (dry season), May (wet season) and September (normal season) in 2019. Trophic level and stability status were assessed on the basis of the Shannon diversity index ( H ), species richness ( S ) and evenness ( J ) index. The different relationships between phytoplankton and the concentrations of several physicochemical parameters and the main soluble nutrients were evaluated by statistical tests. The results showed that there were 75 species belonging to seven groups of phytoplankton, including Chlorophyta (44 species), Bacillariophyta (12 species), Cyanophyta (9 species) and others (10 species). The phytoplankton community composition belongs to the Chlorophyta – Bacillariophyta – Cyanobacteria type structure; and Microcystis , Anabaena azotica Ley , Aphanizomenon , Melosira granulata were the main contributors to the dissimilarities in the temporal distributions of their communities. The phytoplankton density ranged from 4.42 × 10 6 to 8.99 × 10 6 particles/L, with an average of 6.45 × 10 6 particles/L, and the biomass was 4.42 × 10 6 ∼ 8.99 × 10 6 particles/L, with an average of 6.45 × 10 6 particles/L. The variation ranges of the Shannon – Wiener index ( H 0 ), Margalef index ( D ) and Pielou evenness index ( J ) were 2.05 ∼ 2.85, 4.12 ∼ 6.60 and 0.61 – 0.78, respectively. This research shows that the water in the Hongmen Reservoir is clean and that the pollution level is light. The correlation analysis shows that total phosphorus and dissolved oxygen are the main factors affecting phytoplankton community structure in the Hongmen Reservoir.


INTRODUCTION
Fuzhou city, Jiangxi Province, is a comprehensive water conservancy project, mainly for power generation, flood control, irrigation, shipping, aquaculture, etc. In the past ten years, a few studies have found that the nutrient levels of reservoirs have increased, resulting in cyanophyta becoming the dominant population in the high-temperature season (Zhou et al. ). Rapid economic development and human activities in and near the Hongmen Reservoir have changed estuarine dynamics, increasing the pressure on ecosystem management (Gong et al. ). However, the relationship between phytoplankton and environmental factors in the Hongmen Reservoir has not yet been reported.
Here, we use monitoring data to explore the general patterns in anthropogenic forces on the environmental drivers of the phytoplankton community. The main objectives of this work are to investigate: (1) the qualitative and quantitative analysis of phytoplankton, (2) the temporal and spatial organization of phytoplankton, (3) the influence of some abiotic parameters and (4) the assessment of trophic status. The results obtained will be helpful to assess and predict the growth and development of phytoplankton blooms and would be a reference for further strengthening the protection of water resources and water environment management.

METHODOLOGY Sampling point location and sampling time
The Hongmen Reservoir is located in the lower reaches of the Litan River, which belongs to the Fuhe River system. The main tributaries of the Hongmen Reservoir include the Zifu River, Longan River and Longhu River.
The Hongmen Reservoir has a typical subtropical monsoon humid climate with four distinct seasons, abundant rainfall and sufficient sunshine. The river basin above the dam site of the Hongmen Reservoir covers an area of 2,376 km 2 , with an average annual runoff of 2.458 billion m 3 and a surface area of 69.58 km 2 . The total length of the reservoir is 36.5 km, with a maximum width of 1.5 km and an average width of 0.3 km. The average water depth of the reservoir is approximately 17.5 m. The total storage capacity of the reservoir is 1.214 billion m 3 , and the average water change cycle is 178 days. According to the morphological characteristics of the Hongmen Reservoir and the inlet location of the main tributaries, four sampling points were set up in the main channel inlet and reservoir core (Figure 1), including the Cifu water inlet (S1), reservoir core (S3) and dam front (S4).

Sample collection and processing
Samples were taken at a station located in the deeper portion of the reservoir in December 2018, May 2019 and September 2019. Water samples were collected at the surface, middle and bottom of the water and then mixed for determination using a Niskin bottle. Before the collection of samples, an oxygentemperature meter (YSI 6200) was used to measure the pH, dissolved oxygen (DO), water temperature, electrical conductivity (EC) and salinity in situ. The water transparencies of the lakes were determined with the aid of a 20-cm Secchi disk. The samples were stored in a cold, dark environment for laboratory analyses of ammonium (NH 4 þ -N), nitrate (NO 3 À -N), nitrite (NO 2 À -N), total nitrogen (TN), permanganate index (COD Mn ), biochemical oxygen demand (BOD 5 ), total phosphorus (TP) and chlorophyll a (Chl a) following the water and wastewater monitoring and analysis method (4th).
For the phytoplankton analyses, the one-liter water samples were preserved using 1% Lugol's iodine solution for identification and counting of phytoplankton species.
Algal analysis involved sample concentration through several settling steps from an original volume of 1 L to 50 mL is determined according to the occurrence frequency of species and the density of individuals and the calculation formula is as follows: where Y is the dominance degree, f i is the occurrence frequency were selected to evaluate species diversity (Belokda et al.

).
The formula is as follows:

Eutrophication status of reservoir
As shown in Figure  The nutrient index has obvious seasonal characteristics.

Phytoplankton community composition
In this survey, a total of 75 species of phytoplankton from 7 families were identified (only 1 species of the genus was identified), among which 44 species were chlorophyta, 12 species were Bacillariophyta, 9 species were cyanobacteria, 4 species were Euglenophyta, 3 species were Pyrrophyta, 2 species were Cryptophyta and 1 species was Chrysophyta.
The proportion of chlorophyta in the Hongmen Reservoir is 58.7%, followed by Bacillariophyta at 16.0% and cyanobacteria at 12.0%, and the proportion of the three reaches 86.7% (Table 1), indicating that the reservoir shows typical characteristics of chlorophyta-Bacillariophyta-cyanobacteria. Figure 3 shows that the species composition of phytoplankton varies greatly in different periods, with the most abundant and diverse species in the dry period and the fewest species in the wet period.

Phytoplankton density and biomass
During the survey period, the average phytoplankton density in the Hongmen Reservoir was 6.6 × 10 6 cells/L.  According to the relevant density classification standard, the Hongmen Reservoir was at the level of mesotrophication, which was consistent with the analysis results of the total nutrient index. As shown in Figure 4, the change trend of phytoplankton density in different periods is (7.55 ± 1.81) × 10 6 (in normal season) > (7.37 ± 0.72) × 10 6 (in wet season) > (4.91 ± 0.36) × 10 6 (in dry season), among which there is a significant difference (P < 0.05) in the density of phytoplankton between dry season and wet season. The temperature in the dry season is significantly lower than the temperature in the wet season and normal season, which is not conducive to the growth of phytoplankton,   Table 3, indicating that phytoplankton density is closely related to water environmental factors in the Hongmen Reservoir.  The characteristics of the phytoplankton community are   The average density of phytoplankton in the reservoir is 6.96 × 10 6 particles/L, which is higher than the previous study results of 2.83 × 10 6 particles/L. The change trend in phytoplankton density at different periods, in the normal season > in the wet season > in the dry season, may be associated with the temperature of the reservoir. The average water temperature of the reservoir in the normal and wet seasons was 18.4 and 27.1 C, respectively (P < 0.05), which was significantly higher than the average temperature in the dry season (6.2 C, P < 0.05). The prolonged illumination time is conducive to the growth of algae. In the dry season, the water temperature is low, which is the leading factor in low phytoplankton density. The density of cyanobacteria in phytoplankton is the highest in the normal season, followed by cryptophyta, which is related to the increased water temperature, relatively less precipitation, and the lower water level in the reservoir area due to irrigation. Although the temperature is higher in the wet season, a large amount of exogenous water enters the reservoir, diluting the algae in the water.