This study aimed to investigate seasonal changes in water quality and eutrophication indices in small-scale water bodies, focusing on Osongji and Deokjin Pond, located in Jeonju-si, Korea. Water quality was examined seven times between November 2022 and September 2023 at both locations. In the field, pH, Eh, electrical conductivity (EC), dissolved oxygen (DO), and water temperature were measured, while alkalinity, chemical oxygen demand (COD), chlorophyll-a, T-P, T-N, and anions (PO₄³--P, NO₃--N, and SO₄²-) were measured in the laboratory. COD and T-N values increased in the summer due to precipitation, and T-P, a key factor influencing algal growth, showed similar trends to chlorophyll-a. Nutrients carried by summer precipitation also increased PO₄³--P, NO₃--N, and SO₄²- levels. For eutrophication assessment, the TSI_KO, a modified version of Carlson’s Trophic State Index (TSI) adapted for Korea, indicated hypereutrophic conditions in Osongji during heavy rainfall in June and July 2023, reflecting typical seasonal variations of algal growth. In contrast, Deokjin Pond showed no significant changes, attributable to its artificial nature. This suggests that small water bodies in natural settings are more susceptible to seasonal fluctuations and multiple factors should be considered in water quality management.

Keywords: Eutrophication, Water quality, Algae, Seasonal change, Surface water