BIOGEOCHEMICAL DYNAMICS AND DIEL CYCLES IN STEPHENS LAKE: A MODEL OF AN UNPOLLUTED FRESHWATER SYSTEM

Stephens Lake (SL) is a pond in Franklin, Tennessee formed by damming the headwaters of East Fork Creek (EFC), a rural headwater stream in middle TN. A depth survey conducted in the study area revealed that depths were generally less than 1.6 meters, with the shallowest regions located where the stream enters and the deepest areas near the original channel. In-situ measurements were taken during two periods: October 24-26, 2022, and March 27-April 3, 2023. The study monitored various water quality parameters, including functional dissolved organic matter (fDOM), optical dissolved oxygen (ODO), pH, total algae (TAL), and temperature. In 2023, fDOM, ODO, pH, TAL, and temperature exhibited distinct diel signals. In contrast, the 2022 data showed strong diel signals for ODO, pH, specific conductivity (SpC), and temperature, while fDOM displayed a weak signal. The observed diel signals varied in their time lags relative to light intensity and were less pronounced compared to those recorded simultaneously in EFC, located 2,240 meters downstream of the dam. Correlational analyses indicated that, in 2022, pH had a negative correlation with both fDOM and SpC and a positive correlation with ODO. In 2023, fDOM and TAL demonstrated a negative correlation with temperature, whereas ODO and pH showed a positive correlation with temperature. Additionally, fDOM, TAL, and turbidity were positively correlated.

In 2023, water samples were collected every four hours over a 96-hour period and analyzed for 25 chemical species. All samples were identified as Ca-HCO₃ type water. Selenium (Se) and manganese (Mn) exhibited strong positive correlations with dissolved organic carbon (DOC), indicating potential complexation. Among the analyzed species, none exceeded regulatory limits, with the exception of Mn, which exceeded the limits in 7 out of the 24 samples. Based on phosphorus (P) concentrations, the pond is oligotrophic, consistent with ODO ranging from 6.7–13.2 mg/L in 2022 and 8.3–11.7 mg/L in 2023. Furthermore, the average total dissolved solids (TDS) concentration increased from 107 mg/kg in the upstream SL to 198 mg/kg in the downstream EFC, consistent with the progressive dissolution of minerals. The findings offer valuable insights into the natural biogeochemical cycles of an unpolluted freshwater lake and have implications for environmental monitoring and management.