Minutum Blooms Research Articles

Nitrogen and phosphorus requirements of an Alexandrium minutum bloom in the Penzé Estuary, France

During an intense (up to 33 X 106 cells L−1) Alexandrium minutum bloom in the Penzé estuary (France), total NO3, NH4, and PO4 requirements of the bloom were, respectively, 184, 25, and 20 µmol L−1, with peak uptake rates of 43, 6, and 4.8 µmol L−1 d−1. The measured ambient concentrations of NH4 and PO4 were far short of this peak demand, whereas those of NO3 were far in excess, indicating that PO4 supply is important for sustaining the bloom. Comparison of the measured NO3 uptake rates with advective fluxes indicates that a reduction of NO3 concentrations in river waters to <200 µmol L−1 would be necessary to contain the bloom in the Penzé estuary. The role of NO3 was restricted to sustenance of the bloom, whereas warm conditions resulting in a water column stability seem to have triggered the bloom, and a self‐shading, probably coupled with a phosphorus limitation, caused its decline.

Relationship between vegetative cells and cyst production during Alexandrium minutum bloom in Arenys de Mar harbour (NW Mediterranean)

A recurrent Alexandrium minutum bloom in the Arenys de Mar harbour (Catalan coast, North Western Mediterranean) was monitored in order to establish the relationship between vegetative cells and cyst production. The bloom lasted from January 21 to February 24, 2002 and reached cell concentrations of up to 47 � 10 6 cell L � 1 . Two aspects related to the resting cysts deposition were studied: (i) production of resting cysts during the bloom period (by means of sediment traps) and (ii) distribution of resting cysts in the sediment after the bloom (May 2002). Cyst formation in Arenys clearly started in a period with high vegetative cell densities in the water column. Once production was initiated encystment fluxes remained constant for two weeks, and covering the periods of maintenance and decline of the bloom. High cyst fluxes (up to 6000 cysts cm � 2 day � 1 ) were quantified as a result of the high vegetative cell concentration. Moreover, encystment occurring in less than 1% of the total population indicates that mostof the cellsare not involved in resting cysts formation. A comparison of the resting cyst flux values obtained from the sediment traps and the resting cyst concentrations in surface sediment (628‐3270 cysts cm � 3 ) three months later, revealed that the number of cysts in the sediment decreased during that time. The studies of excystment showed a high germination percentage (91%) and germling viability (100%). These data, together with the resting cyst distribution in the sediment, are important in assessing the role of resting cysts in the bloom dynamics of A. minutum in confined waters.