Study of Formation and Disappearance of Thermal Stratification and Hypoxic Water, and Benthic Fluxes of Phosphorus from Sediment in Lake Kitaura in Summer Using Vertical Continuous Measurements

The formation and disappearance of thermal stratification and hypoxic water were investigated by vertical and continuous observations of the water temperature, dissolved oxygen concentration, and oxidation–reduction potential from an automatic profiler sensor in Lake Kitaura in summer. Furthermore, the benthic flux of the dissolved inorganic phosphorus (PO4-P) from sediment in Lake Kitaura was estimated from the results of water quality observations at short intervals using an automatic water sampler. The observed results showed that the conditions for forming thermal stratification were a temperature of 25 ℃ or higher and a wind velocity of less than 6.0 m s-1. Thermal stratification disappeared when the wind velocity exceeded 6.0 m s-1 for 2 h or more, internal Froude number (Fri) increased from 0.3 to 1.0, and Wedderburn number (We) decreased from 0.5 to less than 0.1. Hypoxic water was formed 1–2 days after the formation of thermal stratification. The estimated benthic flux of PO4-P from sediment in the field in summer may be 99.1 mg m-2 d-1 or higher.