Abstract
Seasonal stratification in freshwater lakes suppresses vertical exchange, often leading to strong biogeochemical gradients in bottom waters. This study investigates the dynamics of near‐inertial internal waves and their role in driving turbulence in Lake Inawashiro, a medium‐sized, seasonally stratified lake in Japan. Microstructure profiler observations in June 2023 revealed elevated turbulent kinetic energy dissipation rates ( ε ), reaching approximately 3 × 10 −7 , 2 × 10 −8 , and 4 × 10 −9 W kg −1 in the epilimnion, metalimnion and hypolimnion, respectively. Near‐surface ε profiles followed Monin–Obukhov similarity scaling, consistent with contemporaneous wind speeds of 2–4 m s −1 , indicating a dominant influence of wind‐driven mixing. Complementary acoustic Doppler current profiler measurements from June 2020 captured strong near‐inertial oscillations (~ 17.9 h period) across the lake basin. Cross‐power spectral density analysis revealed high coherence and smooth phase progression among the three mooring sites. Plane wave fitting indicated southwestward propagation (~ 235°) of near‐inertial waves, with a horizontal wavelength of ~ 5.4 km and a phase speed of ~ 0.07 m s −1 (≈ 6 km d −1 ). The inferred spatial scale and propagation direction are likely shaped by the interplay between wind‐driven Ekman transport and shoreline constraints. These findings demonstrate that Earth's rotation can exert a dynamically significant influence even in medium‐sized lakes (~ 10–15 km), where the basin scale approaches the local Rossby radius. The observed enhancement of ε below the thermocline suggests that near‐inertial waves can effectively transmit energy into deeper layers, contributing to vertical mixing and the redistribution of nutrients and biogeochemical tracers during the early summer stratification period.