Diel variation in urea decomposing activity in the euphotic zone of brackish Lake Nakaumi

Diel variations in urea decomposing activity in the euphotic zone of brackish Lake Nakaumi were measured under fixed light intensity. The decomposition rate of urea was 17 to 44 μ mol urea m⁻³ h⁻¹ in the light and 10 to 27 μ mol urea m⁻³ h⁻¹ in the dark. Higher decomposition rates were obtained in the upper euphotic zone. A clear diel periodicity in the urea decomposition rate was observed, with high rates from 1200 to 1600 and low rates from 0000 to 0400. Chlorophyll a specific decomposing activity ranged from 12 to 21 μg urea C mg chl.a ⁻¹ h⁻¹ in the light and 7 to 13 μg urea C mg chl.a ⁻¹ h⁻¹ in the dark. In the light, high values were obtained from 1600 to 2000 and low values from 0400 to 0800. The diel change in specific decomposing activity exhibited a similar pattern to that of the photosynthetic assimilation number, following the diel change in photosynthetic activity. Received: March 10, 1999 / Accepted: October 22, 1999