EVOLUTION AND STRUCTURE OF A SHELF COCCOLITHOPHORE BLOOM IN THE WESTERN ENGLISH-CHANNEL

The physical factors associated with the development, advection and di sappearance of a shelf bloom of the coccolithophore Emiliania huxleyi were investigated in the Western English Channel in June 1992 from an early reflective stage to a mature and dissipative phase (3-4 weeks li fetime) in conjunction with thermal and visible satellite imagery [Adv anced Very High Resolution Radiometer (AVHRR)]. The physical processes that appeared important in patch evolution and structure were differe ntial stratification in an area of weak tidal currents, initial zero r esultant wind conditions (allowing local bloom development), later str engthening NE winds (driving a coastal warm surface current), entrainm ent of the bloom water into the anticyclonic tidal circulation around the Isles of Scilly, and finally bloom dispersal by mixing and flow di vergence. Sea-truth results simultaneous with the satellite images on 25 and 26 June were examined in relation to the bloom intensity and ve rtical structure (E.huxleyi cells, detached coccoliths and empty cocco spheres). Bloom conditions were characterized by sections of temperatu re, chlorophyll a, inorganic nutrients, particulate organic and inorga nic carbon, and carbon to nitrogen atom ratio. The phytoplankton compo sition of non-coccolithophore species presented significant variations inside and outside the bloom. Measurements of beam attenuation coeffi cient (660 nm), diffuse attenuation coefficient [photosynthetically ac tive radiation (PAR)] and reflectance spectra (415-660 nm) were used t o analyse the extent to which the bloom changed the local optical prop erties. Optical measurements at sea were correlated with coccolith con centration in order to quantify concentration estimates derived from r emote sensing reflectance measurements.