Investigations on the Urea Uptake by Phytoplankton in the Baltic Sea

Urea uptake by the planktonic community was studied in the central Baltic Sea in July/August 1987, in March 1988 and in August 1988. Uptake was measured with ¹⁴C-urca incubations. Uptake rates were determined in the range of 0.8 … 5.7 nmol/dm³ · h urea in March and 25.6 … 48.0 nmol/dm³ · h urea in August. Respired CO₂ was found to represent 98 to 99 = of total uptake. Light stimulated urea uptake; parallel dark incubations showing usually 4.5 to 80.0 = of the uptake in the light. These results and size fractionation studies indicated that phytoplankton was dominating urea uptake. Urea proved to be an important nitrogen source for phytoplankton during the regenerated phase of the phytoplankton succession.