Threshold amounts of nutrients and the relationship with chlorophyll a during eutrophication phenomenon in small-scale artificial reservoirs

This study aimed at determining the relationship of chlorophyll a (Chl a) and the nutrients in two reservoirs of different sizes, namely Bueng Si Than and Bueng Thung Sang, which are located in Khon Kaen Province in Thailand. The evaluation of the trophic state index and the estimated threshold amounts of nutrients was consequently considered. Water samples were collected during both the dry and rainy seasons with twenty-seven sampling points in each reservoir. Parameters indicating water quality and nutrients were based on the APHA method. The results showed a significant difference in the average physicochemical properties and nutritional parameters of the water in a smaller proportion, with Bueng Si Than having more than Bueng Thung Sang (p < 0.05). Nutrient factors (TP, TKN) showed the impact on the occurrence of eutrophication during both the dry season and the rainy season. Average ratios of nutrients and Chl a together with the trophic state index revealed the occurrence of hypertrophic conditions in both reservoirs. The high value of Chl a:TP signaled that the algal growth in these reservoirs could be controlled by TP. The Pearson correlation analysis indicated a strong positive relationship of Chl a and TP in Bueng Si Than. Moreover, the obtained results from the simulation were achieved using MATLAB R2022a, and were performed with the fourth order polynomial equation, which indicated the boundary values of 28.25–29.42 μg/L and 4.15-4.19 mg/L for the threshold amounts of TP and TKN prior to Eutrophic occurrence, ascertain further eutrophication management in small-scale artificial reservoirs.