PHYTOPLANKTON DYNAMICS DURING THE SPRING BLOOM IN THE SOUTH-EASTERN ENGLISH-CHANNEL

The two main phases of a phytoplankton spring bloom in the South-easte rn English Channel were studied during two 3-day cruises in March and May 1992. Physico-chemical parameters were measured, such as temperatu re, salinity, density, turbidity and nutrients, as well as biological parameters (in situ chlorophyll a fluorescence, photosynthetic pigment s and fatty acids). Photosynthetic pigments and fatty acids were used as taxonomic and physiological markers of phytoplankton populations. D ata suggest the existence of two 'biological provinces' north and sout h of the Bay of Somme. In the Northern province, the bloom starts earl ier, probably due to the shallower coastal water, and is characterized by high proportions of diatoms and, successively, of Prymnesiophytes (Phaeocystis sp.). The bloom maintains high biomass levels sustained b y inputs from the Somme River and probable nutrient regeneration. The Southern province, directly influenced by the Seine River, is characte rized by a deeper coastal water column and the presence of phytoflagel lates. Despite the higher supply of nutrients from the Seine River, th e bloom starts later and supports a lower phytoplankton biomass. The d ifferences between both areas are analysed on the basis of the hydrody namism of the area, and are interpreted as two different stages of the same process. To understand the spatio-temporal variations of phytopl ankton dynamics, interactions between biology and hydrodynamical chara cteristics of this area are discussed. (C) 1996 Academic Press Limited