Urea uptake by phytoplankton at different fronts and associated stratified and mixed waters on the european shelf

Abstract

Urea uptake by phytoplankton was investigated in stratified, mixed and frontal waters at tidal mixing fronts, a plume front and a topographical front in the Irish Sea and German Bight. In general, fronts and the stratified and mixed waters associated with them have a marked effect on the distribution of rates of urea uptake. This varied at different types of fronts and at different times of the year. Often, maxima in urea uptake occurred on the fronts where there were peaks in chlorophyll a. Rates of urea uptake in mixed waters where inorganic nitrogen was high were generally low. Stratified offshore waters bordered by tidal mixing fronts were characterized by high rates of urea uptake in the euphotic zone above the thermocline where there was a notable seasonal variability. High rates occurred during the summer when stratification was strongest and nitrate concentrations were low. Urea and ammonium made up the majority of the total nitrogen available for phytoplankton growth in these euphotic stratified waters. The results suggest that other organisms such as protozooplankton and macrozooplankton which are also more numerous above the thermocline than either below or in the adjacent mixed waters provided regenerated nitrogen, in the form of both urea and ammonium, for continued phytoplankton growth. © 1986 Taylor & Francis Group, LLC.