Intense bloom off coast of Devon and Cornwall, July 2000
SeaWiFS and Ship Measurements of a Red Tide Dinoflagellate
SeaWiFS images from mid -July in the western English Channel showed very high concentrations of chlorophyll a over an area of approximately 4,500 square-kilometres. In order to validate the SeaWiFS image and to sample the bloom, data were obtained from the Plymouth Marine Laboratory research vessel Sepia on 25 July. Microscope estimation showed that the bloom was comprised of the species Gyrodinium aureolum, a red- tide dinoflagellate, with cell concentrations over 10,000 cells per millilitre. During the cruise, data were taken on phytoplankton pigments (HPLC) and optics, using a rig with Satlantic upwelling radiance, downwelling irradiance at 7 wavelengths, and a fast repetition rate fluorometer to gauge phytoplankton photosynthesis characteristics. Samples were taken for analysis by Analytical Flow Cytometry, for phytoplankton primary production (using 14C incubations), and to characterise viruses and bacteria. The data are currently being analysed.
A SeaWiFS false-colour composite created from the wavebands at 555, 510 and 443 nm, showed the bloom as red-coloured, which corresponds to absorption at 443 and 510 nm. At sea, the bloom looks dark greenish- brown. G. aureolum is believed to be present in the English Channel in most years, at the base of the mixed layer, but this year the concentrations at the surface have been the highest observed with SeaWiFS. Blooms have previously been measured by CZCS and ship campaigns (e.g. Holligan et al 1983). G. aureolum, recently found to be conspecific with Gymnodinium mikimotoi (Hansen et al., 2000) is believed to be harmful to fish, irritating the gills. This aspect is of particular interest since PML are involved in a European Space Agency project to investigate the use of near-real time ocean colour data with a harmful bloom monitoring system along the Norwegian coast.
Also found within the bloom was a second plankton species, Noctiluca scintillans, which grazes on the G. aureolum. Noctiluca are visible to the naked eye (~1 mm in diameter), and in calm conditions float near to the surface. By virtue of the high numbers they appear as red patches, or streaks caused by the wind. Noctiluca are bioluminescent; that is, they glow in the dark, which is particularly noticeable if you sail through a patch at night.
The Plymouth Marine Laboratory Remote Sensing Group are funded by the UK Natural Environment Research Council (NERC) to process SeaWiFS data for the UK marine science community. PML also processes AVHRR data, and recently received a grant to develop MODIS data processing applications. The images are received and archived at the NERC receiving station at the University of Dundee.
References:
Hansen, G., Baugbjerg, N., and Henriksen, P., (2000): Comparative study of Gymnodinium mikimotoi and Gymnodinium aureolum, comb. nov (Gyrodinium aureolum) based on morphology, pigment composition, and molecular data, J. Phycology, 36, 394-410.
Holligan PM, Viollier M, Harbour DS, Camus P, Champagne-Phillippe M (1983): Satellite and ship studies of coccolithophore production along a continental shelf edge. Nature, 304, 339-342.
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