In this study, the water physicochemical parameters and phytoplankton community composition of the Ubol Ratana Dam were investigated to assess their relationships with water contaminants. The levels of nutrients, particularly nitrogen and phosphorus, were examined because they can lead to eutrophication and changes in the phytoplankton community. This study also highlights the challenges associated with assessing the diversity and community composition of phytoplankton and the consequences of temporal and seasonal variations in physical and chemical parameters. Overall, the results of this study revealed a relationship between water physicochemical properties, seasonal changes, site location, and phytoplankton and cyanobacteria biodiversity in the Ubol Ratana Dam from 2022–2023.
We identified 84 species of phytoplankton belonging to 8 phyla: Bacillariophyta, Chlorophyta, Cyanophyta, Euglenophyta, Pyrrhophyta, Chrysophyta, Cryptophyta, and Xanthophyta. In a previous study in 2018 by Muangsringam et al., six of these phyla were reported in the same reservoir: Bacillariophyta, Chlorophyta, Cyanophyta, Pyrrhophyta, Cryptophyta, and Euglenophyta. We identified all the phytoplankton at the species level, and M. aeruginosa was found to be the dominant species in the reservoir, consistent with a previous study by Muangsringam et al. (2019). Cyanobacteria frequently accounted for over 80% of the total phytoplankton. Several potentially toxic genera, such as Microcystis, Cylindrospermopsis, Planktotrix, Planktolyngbya, and Anabaena, were dominant throughout our study period (Barros et al. 2019; Crevecoeur et al. 2023; Mengchouy and Meksumpun 2022; Muangsringam et al. 2019; Passos et al. 2022; Tito et al. 2022). Our findings suggest that phytoplankton respond favorably to a variety of nutrients depending on location and season. The abundance of cyanobacteria (Cyanophyta) and the density of phytoplankton varied across different geographical locations, and distinct spatial and temporal variations were observed across sampling sites despite their high relative abundance.
Biological indicators are often used to assess the pollution status of aquatic ecosystems (Gogoi et al. 2019). Both the evenness distribution and Shannon‒Wiener index exhibited similar trends, showing significant increases during the winter and summer seasons and reaching their lowest points during the rainy season. The findings of this study align with those of Sun et al. (2023) and Arumugham et al. (2023). The seasonal changes in diversity indicate that the alterations occurred due to shifts in the evenness and Shannon‒Wiener indices rather than due to changes in species richness (Stirling and Wilsey, 2001; SureshKumar and Thomas, 2019). The Shannon‒Wiener diversity index ranged from 1 to 2, indicating moderate diversity (Parakkandi et al. 2021; Wihm, 1975). Seasonal hydrological changes may have also impacted these indices. The irrigation systems result in low riverine inflow and stable water availability in the winter and summer periods and generate changes in phytoplankton diversity, with significant increases during the winter and summer seasons, and the lowest levels occurred during the rainy season. The findings of the present study are consistent with those of Sun et al. (2023) and Arumugham et al. (2023). The seasonal changes in diversity indicate that the alterations occurred due to shifts in the evenness and Shannon‒Wiener indices rather than due to changes in species richness (Stirling and Wilsey, 2001; SureshKumar and Thomas, 2019). The Shannon‒Wiener diversity index ranged from 1 to 2, indicating moderate diversity (Parakkandi et al. 2021; Wihm, 1975). Seasonal hydrological changes may also affect these indices. The irrigation systems result in low riverine inflow and stable water availability in the winter and summer periods, resulting in changes in diversity.
The ecology of phytoplankton in aquatic environments is affected by seasonal changes, leading to fluctuations in species diversity, evenness, the Shannon‒Wiener index, and cell density (Sun et al. 2023; SureshKumar et al. 2019). Temperature is a significant factor that regulates the growth of phytoplankton (Somdee et al. 2013; Wang et al. 2023). During the study, it was observed that species richness decreased in the summer. The results showed an overall increase in the total phytoplankton population in the summer months, with a specific increase in the proportion of Cyanophyta. High temperatures led to a decrease in the proportion of other phyla, with Chlorophyta, Euglenophyta, Pyrrophytophyta, and Bacillariophyta showing decreases in relative abundances. This result is consistent with the findings of Sun et al. (2023), who noted that Euglenophyta and Bacillariophyta are susceptible to increasing temperatures. These genera may be unable to grow at high temperatures due to their physiological incapacity to tolerate high temperatures, while cyanobacteria flourish and reproduce rapidly under such conditions. Increasing water temperatures induce competitive exclusion, resulting in the dominance of cyanobacteria and the disappearance of some taxa (Briddon et al. 2022; Sun et al. 2023). Cyanobacteria can impact phytoplankton communities by influencing planktonic species when the water temperature reaches a certain level. The impact of seasonal variation on cell density seemed to be greatest in the summer and lowest during the rainy season.
It was challenging to determine the impact of temperature on phytoplankton abundance due to the influence of other factors, such as low water levels in summer, which led to increased nutrient content and phytoplankton biomass. However, the density of phytoplankton and cyanobacteria increased the most during the summer at all sites. This was evident from the positive correlation between water temperature and dominant phytoplankton, such as cyanobacteria. This finding is consistent with that of Kong et al. (2019). However, it remains unclear whether phytoplankton prefer high temperatures (Wang et al. 2022). It is possible that high temperatures do not lead to cyanobacterial blooms but rather intensify these phenomena by increasing thermal stratification and depth shifts through gas vesicles (Crevecoeur et al. 2023; Tito et al. 2022). Seasonal temperature significantly influences the regulation of phytoplankton reproduction, growth, and behavior. The growth of eutrophic phytoplankton is significantly affected by physicochemical factors, which cause fluctuations in phytoplankton biomass. These parameters change because of changes in weather-related processes. Phytoplankton can thrive well under a broad pH range of 6.5–10 (Ballah et al. 2019; SureshKumar and Thomas 2019). Cyanobacteria, particularly M. aeruginosa, can utilize HCO3− and CO2 for photosynthesis in alkaline water, which increases their density (Wei et al. 2022). According to our findings, the pH during the rainy season was approximately 8, and the CCA pointed to pH as a major variable. The observed alkaline pH may be explained by water input from runoff from nearby agricultural land, which includes both organic and inorganic components that lead to the breakdown of organic waste (Gogoi et al. 2019). However, the overall quantity of phytoplankton was lower than that in the other seasons. We can speculate the causes of the low phytoplankton abundance observed. In this study, alkaline pH was linked to the growth of M. aeruginosa, but this value was not optimal (Wei et al. 2022). Moreover, sudden changes in hydrology, such as increased inflow and outflow of water from rainstorms, make these areas unsuitable for phytoplankton growth. Moreover, increased water storage contributes to nutrient dilution (Gogoi et al. 2019; Parakkandi et al. 2021). This causes changes in the physicochemical characteristics of water bodies.
Increased levels of nutrients, particularly nitrogen and phosphorus, are essential for the growth, survival, and proliferation of phytoplankton, as they are necessary for phytoplankton metabolic processes (Arumugham et al. 2023; Barros et al. 2019; Briddon et al. 2022; Lv et al. 2011; SureshKumar and Thomas 2019). Higher levels of nitrate and ammonium were detected near the Non-Sang municipality community and fish farm locations throughout all seasons, which was caused by the use of organic and chemical fertilizers, excess feed, domestic wastewater, and anthropogenic activities, which contribute to the nutrient load in the inflowing water and in the water surrounding the reservoir (Akter et al. 2022; de Lima Pinheiro et al. 2023). Additionally, the changes observed with increased nitrogen content suggested that nitrates and ammonium strongly influence the growth of other phytoplankton, such as Euglenophytes. Taxa such as Euglena sp., which thrive in nutrient-rich water, frequently serve as bioindicators of hypereutrophic water pollution (Parakkandi et al. 2021). Surprisingly, although the stagnant water at the water outflow and water in the center of the dam had the highest total phytoplankton and cyanobacteria abundances, both nitrate and ammonium levels remained low in these areas. However, several studies have shown that cyanobacteria, particularly the common species M. aeruginosa, typically thrive in environments rich in nitrate and ammonium, which promote biomass growth (Sun et al. 2023). In contrast, a rapid utilization of nutrients may lead to decreases in nitrate and ammonium levels, generating a strong negative correlation. This finding is similar to that of Pitchaikani and Lipton (2016). Fluctuations in the levels of limiting nutrients such as ammonium and nitrate often regulate the growth of phytoplankton (Pitchaikani and Lipton 2016). The concentration of nitrogen sources decreases as nitrogen is rapidly consumed (SureshKumar and Thomas 2019). It is also possible that the availability of nitrogen from nitrogen-fixing cyanobacteria is sufficient to support the growth of nonheterocystous phytoplankton (Lv et al. 2011). For example, the nitrogen-fixing cyanobacterium C. raciborskii was present consistently throughout the seasons, along with M. aeruginosa (Crevecoeur et al. 2023). In nitrogen-limited environments, Microcystis is more abundant than other cyanobacterial species due to its effective assimilation of regenerated ammonium (Flanzenbaum et al. (2022).
The concentration of phosphorus in the water bodies was low during the rainy season and increased during the summer. This finding was supported by a strong positive correlation between orthophosphate content and an increase in dominant phytoplankton and total biomass. Cyanobacteria rely more on phosphate as a nutrient than do other phytoplankton (Lv et al. (2011). However, less dissolved phosphate may be utilized during the rainy season due to lower plankton density, which can be attributed to various factors. The reservoir water only drains in the winter season, and water evaporates in the summer, giving rise to greater nutrient concentrations. Several factors contribute to phosphorus accumulation, particularly in summer, including fertilizer inputs, decomposition of dead plants and animals, aquaculture, and residential activities such as detergent use and the discharge of domestic effluent (Ajayan et al. 2017; Kundu et al. 2015). The nutrient supply for phytoplankton comes from both the water inflow and the nutrients generated within the reservoir from the degradation of organic matter by bacteria (Crevecoeur et al. 2023). Our results in this regard are inconclusive. The nutrient levels in the central dam area with high phytoplankton biomass in the summer were not significantly different from those at the other stations. However, it is possible that there was enough consumption of available phosphorus. Cyanobacteria can regulate phosphate absorption, consume it in subsequent life stages, and store in the form of polyphosphate because of their high growth rate and prolonged life span (Sanz-Luque et al. 2020; SureshKumar and Thomas 2019). They exhibit phosphorus saturation when cultivated in phosphate-rich environments (Barros et al. 2019; Dolman et al. 2012). However, the significant occurrence of Microcystis blooms may be linked to high phosphorus concentrations since phosphorus is a limiting nutrient under low nitrogen‒phosphorus ratios (Barros et al. 2019; Jargal and An 2023; Lv et al. 2011). Our findings indicate that phosphate is a significant factor affecting the increase in phytoplankton biomass in this reservoir. We believe that rather than phosphate content, nitrate and ammonium contents play key roles in influencing the biomass of phytoplankton and cyanobacteria.
The CCA showed that the influencing factors varied with seasonal fluctuations. Potential factors are indicated by the length of the arrows in the CCA (Sharma et al., 2015). Nitrate and ammonium levels significantly influence the composition of the phytoplankton community throughout the year, suggesting that these factors are not impacted by seasonal changes and indicating that nitrogen levels restrict phytoplankton growth in the reservoir. We observed that total phytoplankton, particularly cyanobacteria, more efficiently utilized nitrate, ammonium, and orthophosphate when the temperature was optimal. Cyanobacteria appear to adapt effectively to environmental changes. The abundance of phytoplankton can be influenced not only by individual factors but also by interactions among multiple crucial factors. CCA revealed a broad pattern in the physicochemical variables in the rainy season, which might be related to sudden hydrological changes that lead to changes in the concentrations of nutrients in the water bodies. Based on the above discussion, the water management measures taken in reservoirs during each season have a significant impact on water physicochemical changes and nutrient dynamics. These factors ultimately control the composition of phytoplankton (Kumar et al., 2020; Malik and Rathi, 2022; Muangsringam et al., 2019). Seasonal variations in irrigation systems play an important role in the formation of unique phytoplankton communities. During the summer, when temperatures rise and the daily period of sunshine increases, a small amount of water flows through two gates at the confluence of the dam's outflow and center. This generates low agitation, low turbidity, stagnation, increased nutrients, and increased light availability at the epilimnion of the reservoir. These factors can positively impact and enhance the growth, photosynthesis, reproduction, and buoyancy of phytoplankton (Crevecoeur et al. 2023; Jargal and An 2023; Mânoca and de Lima Isaac 2023). A limitation of this study is that the physicochemical and biological parameters of the riverine water inflow that may influence phytoplankton population changes during seasonal variation were not investigated.
Although phytoplankton blooms are currently not a serious issue and the amount of microcystin does not exceed the standard values of the World Health Organization (WHO), which recommends a maximum level of 1 µg/L of microcystin per liter of water in this reservoir, it is important to monitor the water body as the concentration of nutrients such as nitrogen and phosphorus continues to increase. This can lead to a change in the phytoplankton community composition and to cyanobacterial blooms. These significant findings support the previous prediction that cyanobacterial proliferation occurred in the central dam zone during the summer.