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.