MEPS 130:201-211 (1996)  -  doi:10.3354/meps130201

Urea uptake of the planktonic community was studied in a series of experiments on the SW coast of Finland, the Baltic Sea, in midsummer. Uptake was measured with ¹⁴C-urea incubations, where respired ¹⁴CO₂ was found to represent 98 to 99% of total uptake. Uptake kinetics were characterized by high affinity for urea (0.97 to 1.84 ug N l^-1), fast turnover rates (29 to 99% h^-1) and moderate maximum uptake velocities (0.52 to 1.31 ug N l^-1 h^-1). Light stimulated urea uptake; parallel dark incubations usually yielded uptakes 60 to 80% of those in the light. No surge uptake of urea was observed, as all kinetic parameters remained constant in a short-term time series experiment. Size-fractionation studies and light/dark incubations indicated that phytoplankton dominated urea uptake. The time course of urea depletion in an enrichment experiment followed the diel primary production pattern, and the depletion rate was of the same order as the ammonium depletion rate. Inorganic nitrogen additions (ammonium, nitrate) did not produce immediate inhibitions of urea uptake, but with a time lag of 1 night, ammonium inhibition was observed. Phosphate addition stimulated urea uptake. Evidence on stimulated bacterioplankton uptake of urea was found the next night after organic carbon source addition. Alternative nitrogen sources (NO₃, NH₄, urea) were utilized mainly according to their availability. Urea proved to be an important nitrogen source for phytoplankton during the regenerated phase of plankton succession.

Urea . Uptake kinetics . Phytoplankton . Bacterioplankton . Nitrogen sources . Preference . Inhibition . Baltic Sea