Nitrogen source for uptake by Gyrodinium cf. aureolum in a tidal front

Measurements of ammonium and nitrate uptake by a Gyrodinium bloom in the Ushant frontal region showed that ammonium is the preferred form and 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 µ m. Comparison of the conditions before and after the bloom formation, and a mass balance of N, shows that the G. aureolum bloom in the frontal region can be maintained almost exclusively by in situ remineralization of remnants of previous diatom blooms.