A phytoplankton bloom was observed in late winter 2000, on the continental shelf offshore of southern Brittany, in northwestern Bay of Biscay. This bloom appeared initially along the 120-m isobath, in stratified and clear waters, at the interface between the oceanic water and the plumes of southern Brittany rivers (mainly the Loire and Vilaine). The development of the bloom was triggered by favourable meteorological conditions, characterised by solar irradiance reaching the maximum level expected for that period of the year.
Outside of the bloom area, the phytoplankton photosynthesis was irradiance limited: inshore, because of the stronger attenuation of the light; offshore, because of the weak stratification. The hydrological conditions at the onset of the bloom were observed in the field, during the oceanographic cruise MODYCOT. However, without SeaWiFS, the only observations related to this major event in the primary production would have been those of the coastal phytoplankton network (REPHY (REseau PHYtoplankton)).
Observed initially offshore by SeaWiFS, as early as March 5, 2001, the phytoplankton bloom extended onshore and was observed at REPHY stations by mid-March. On, March 15, fishermen reported that they were handicapped in trawling, the presence of mucilage in their nets carried clogging by very heavy slime. This mucilage has been shown to be produced by the diatom Coscinodiscus and, especially, C. tvailesii. These observations were made as the main bloom was declining.
A simple calculation, based upon the SeaWiFS chlorophyll concentration maps observed from March 5-16, shows that phosphate was probably totally depleted offshore, by March 16. The former bloom area appeared very poor in chlorophyll on the SeaWiFS image of April 7, which was the first image available after 3 weeks of an overcast sky. A second cruise undertaken at the end of April confirmed the phosphate depletion and the low chlorophyll concentration in that area.