NITROGEN-SOURCE FOR UPTAKE BY GYRODINIUM CF AUREOLUM IN A TIDAL FRONT

Measurements of ammonium and nitrate uptake by a Gyrodinium bloom in t he Ushant frontal region showed that ammonium is the preferred form an d its assimilation provides 90% of the N required. Uptake (3.75 mmol m -2 h-1) and regeneration (3.41 mmol m-2 h-1) of ammonium were tightly coupled. Regenerative flux of ammonium was through a microbial loop of bacteria, nanoflagellates, and ciliates in the size fraction < 30 mum . Comparison of the conditions before and after the bloom formation, a nd a mass balance of N, shows that the G. aureolum bloom in the fronta l region can be maintained almost exclusively by in situ remineralizat ion of remnants of previous diatom blooms.