The development of a unispecies bloom of phytoplankton in the Rade de Villefranche (N.W. Mediterranean, near Nice) during June 1982 coincided with the disappearance of salps from the area. A phytoplankton haul was taken on 24 June and examined in detail. The sample consisted of virtually a unispecies diatom population; Rhizosolenium spp., together with a few small medusae and crustaceans. The diatom cells were unusual in that they appeared to be associated with large amounts of extracellular mucus.

The appearance of this virtually monospecies bloom provided a rare opportunity to examine in detail the biochemical composition of a natural field diatom species (see discussion in Ballantine, Lavis & Morris, 1979 and Morris, 1984). After removal of zooplankton the phytoplankton were collected on a 30 μm clear nylon filter. A detailed biochemical analysis was carried out as previously described (Morris, McCartney & Robinson, 1983; Morris, 1984).

The Rhizosolenium spp. sample was composed of almost equal amounts of lipid (23 mg), insoluble carbohydrate (19 mg) and protein (24 mg) with much smaller amounts of soluble carbohydrate (009 mg). The lipid was composed mainly of polar lipids (70% total lipids), neutral glycerides (20% total lipids) and sterols (8% total lipids). The detailed fatty acid and sterol composition of these major lipid fractions are shown in Tables 1 and 2.