Seasonal Dynamics and Environmental Drivers of Phytoplankton Composition in a Tropical Dam Over 5 Years in Chiang Mai, Thailand

This 5-year study (2015 - 2019) investigates the dynamics of phytoplankton and their correlation with environmental factors including water volume, physicochemical and nutrient availability in Mae Kuang Dam, Northern Thailand. The study identified 177 taxa across 6 groups, with Charophyta being predominant (53.3 %) in both wet and dry seasons, specifically in the genus Staurastrum. Taxonomic richness was higher in the wet season, with green algae being the most abundant. In addition, Cyanobacteria, Bacillariophyceae and Chrysophyceae showed increased densities. Annual variations peaked in 2015, decreased in 2017 and exhibited a slight increase thereafter. Cyanobacteria experienced blooms in the wet season, accompanied by significant fluctuations in Charophyta, Cyanophyta, Dinophyta and Euglenophyta. Notably, Cyanophyta exhibited a 20-fold increase from 2015 to 2019. Redundancy Analysis (RDA) highlighted the influence of soluble reactive phosphorus (SRP), conductivity, air temperature, pH, total dissolved solids (TDS) and biochemical oxygen demand (BOD) on phytoplankton during the dry season. Lower water volumes and concentrations of water bodies may intensify the impact of certain factors on phytoplankton communities during this period. In the wet season, pH, conductivity, TDS, ammonia, air temperature and BOD played significant roles, as increased rainfall and nutrient runoff occurred. These results indicate that the combined effects of these environmental variables and hydrological events, including water volume and water static, could influence the dynamics of phytoplankton and water quality. This finding helps us understand the shifts in the aquatic ecosystem under the environmental factors in Thailand. Therefore, further research is needed, emphasizing the necessity for well-planned dam management to maintain both water quality and ecological stability. HIGHLIGHTS Biannual monitoring was conducted over 5 years to study the dynamics of phytoplankton in a tropical dam. Green algae, including Staurastrum and Cosmarium, dominated throughout the sampling period Green algae showed a decreasing trend in cell numbers, while Cyanophyta exhibited an increasing trend The low water volumes of water bodies may favor green algae, while the highest volume favors high nutrient levels, resulting in more cell numbers than in the dry season. Therefore, nutrient variability, temperature and water volume may favor the dominating growth of green algae and Cyanobacteria GRAPHICAL ABSTRACT