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

The physical factors associated with the development, advection and disappearance 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 lifetime) in conjunction with thermal and visible satellite imagery [Advanced Very High Resolution Radiometer (AVHRR)]. The physical processes that appeared important in patch evolution and structure were differential stratification in an area of weak tidal currents, initial zero resultant wind conditions (allowing local bloom development), later strengthening NE winds (driving a coastal warm surface current), entrainment of the bloom water into the anticyclonic tidal circulation around the Isles of Scilly, and finally bloom dispersal by mixing and flow divergence. Sea-truth results simultaneous with the satellite images on 25 and 26 June were examined in relation to the bloom intensity and vertical structure (E.huxleyi cells, detached coccoliths and empty coccospheres). Bloom conditions were characterized by sections of temperature, chlorophyll a, inorganic nutrients, particulate organic and inorganic carbon, and carbon to nitrogen atom ratio. The phytoplankton composition of non-coccolithophore species presented significant variations inside and outside the bloom. Measurements of beam attenuation coefficient (660 nm), diffuse attenuation coefficient [photosynthetically active radiation (PAR)] and reflectance spectra (415-660 nm) were used to analyse the extent to which the bloom changed the local optical properties. Optical measurements at sea were correlated with coccolith concentration in order to quantify concentration estimates derived from remote sensing reflectance measurements.