Urea decomposing activity of fractionated brackish phytoplankton in Lake Nakaumi

The influence of brackish phytoplankton cell classes upon the response of urea decomposition was investigated in Lake Nakaumi. The urea decomposition rate was 5 to 350 μmol urea m⁻³ h⁻¹ in the light and 3 to 137 μmol urea m⁻³ h⁻¹ in the dark. The urea decomposition rates in the light were obviously higher than in the dark. An extremely high rate (350 μmol urea m⁻³ h⁻¹) was observed in Yonago Bay. The rate in the smaller fraction (<5 μm) exceeded that in the middle (5–25 μm) and larger fractions (>25 μm). The chlorophyll- and photosynthesis-specific rates for urea decomposition in the light were 0.5 to 3.9 μmol urea mg chl.a ⁻¹ h⁻¹ and 0.3 to 1.3 μmol urea mg photo.C⁻¹. The specific urea decomposing activities were higher in the smaller fraction than in the other two fractions. The present results suggest that in brackish waters urea decomposition occurred with coupling to the standing crop and photosynthetic activity of phytoplankton. Received: May 22, 1999 / Accepted: August 15, 1999