Community Characteristics of Planktonic Microalgae in Aquaculture Ponds of Different Modes in Ningxia in Summer

Ningxia Hui Autonomous Region is located in Northwest China. Most of the culture ponds in Shizuishan City and Yinchuan City, in the northern part of Ningxia, are sulfate-type saline-alkali ponds. The central area of Ningxia is located on the Hetao Plain, where the salinity of water bodies is relatively low. Owing to the high pH value of saline-alkali water bodies, which indicates high alkalinity, water quality control has become the main challenge of saline-alkali aquaculture. The community structure of planktonic microalgae is closely related to the water quality of aquaculture ponds and the healthy growth of aquaculture organisms. The planktonic microalgae community structure of culture ponds in summer in Ningxia is rarely reported. To study the community structure characteristics of planktonic microalgae in different culture pond modes in Ningxia in summer, we collected planktonic microalgae samples in July 2021 from Pingluo County in Shizuishan City, Helan County in Yinchuan City, and Shapotou District in Zhongwei City in the Ningxia Hui Autonomous Region. We then analyzed the planktonic microalgae community composition and its correlation with water quality factors. The samples were collected from three typical aquaculture pond modes: the shed-pond relay mode (PT), soil pond mode (TC), and rice-fishing breeding mode (DY). We qualitatively identified and quantitatively counted planktonic microalgae under a light microscope and performed diversity analysis using the Shannon diversity index. SPSS software was used to carry out bivariate correlation analysis between the total number of algae, total biomass, algal biomass of each phylum, and the physical and chemical factors of water quality in aquaculture ponds. A total of 5 phyla and 27 genera of planktonic microalgae were detected, including 7 genera of Cyanophyta, 13 genera of Chlorophyta, 5 genera of Bacillariophyta, 1 genus of Euglenophyta, and 1 genus of Dinophyta. The total number of planktonic microalgae in the 9 ponds ranged from 1.52×104 to 2.39×108 ind./L, the biomass ranged from 0.16 to 97.78 mg/L, the Shannon-Wiener index as 0.03 to 3.31, and the biomass diversity index was 0.29 to 3.58. There were significant differences in the community structure of planktonic microalgae in different kinds of culture modes. The community structure of planktonic microalgae in PT culture ponds had no obvious common characteristics. Cyanophyta, Chlorophyta, and Bacillariophyta, such as Anabaenopsis sp., Anabaena sp., Oscillatoria sp., Pediastrum sp., Oocystis sp., and Cyclotella sp., were predominant. A single genus of planktonic microalgae dominated in TC. The three ponds were dominated by Pediastrum sp., Chlorella sp., and Microcystis sp. The diversity of microalgae in DY culture ponds was relatively rich. Chlorophyta such as Chlorella sp., Scenedesmus sp., Pediastrum sp, Oocystis sp., and Cladophora sp. and Bacillariophyta such as Cyclotella sp. and Nitzschia sp. were the dominant algae. Although the community structure of planktonic microalgae in most ponds was dominated by Chlorophyta and Bacillariophyta, some ponds in PT and TC still had Cyanophyta overgrowth which became the single dominant algae. Harmful Cyanophyta proliferate in large numbers in aquaculture ponds and easily form algal blooms, which cause significant adverse effects on the water quality and environment and endanger the life and health of cultured organisms. The ponds with Chlorophyta and Bacillariophyta as the dominant species had a relatively stable water quality, were rich in microalgae species, had excellent water color, and experienced less disease and better growth of cultured organisms. The characteristics of high summer temperatures, long sunshine hours, sulfate-type saline-alkali water in Ningxia, and the eutrophication of the culture ponds due to factors such as food and cultured biological excrement affect the structure of planktonic microalgae communities in local culture ponds. The biomass of Cyanophyta had a significant positive correlation with the concentration levels of NO3–-N, NO2–-N, and chemical oxygen demand (COD) (P < 0.01). The total nitrogen and COD contents of the ponds in the DY mode were lower than those in the PT and TC modes, meaning that the water environment was not conducive to the proliferation of harmful Cyanophyta and that the planktonic microalgae biodiversity index was relatively high in the DY mode. In addition, a large number of studies have shown that the adaptability of Cyanophyta to strong light and optimal growth temperature is generally higher than that of other algae. The results indicated that in Ningxia, due to the long hours of light in summer, cyanobacteria blooms dominated by Cyanophyta such as Microcystis and Anabaenopsis formed easily in aquaculture ponds with high water carbon and nitrogen nutrients. Therefore, this finding indicates that the hours of light should be used as one of the critical control points when regulating pond water quality to prevent and control the occurrence of adverse conditions such as reduced production and efficiency due to the outbreak of harmful algal blooms.