Phytoplankton Dynamics During the Spring Bloom in the South-eastern English Channel

The two main phases of a phytoplankton spring bloom in the South-eastern English Channel were studied during two 3-day cruises in March and May 1992. Physico-chemical parameters were measured, such as temperature, salinity, density, turbidity and nutrients, as well as biological parameters (in situchlorophyllafluorescence, photosynthetic pigments and fatty acids). Photo-synthetic pigments and fatty acids were used as taxonomic and physiological markers of phytoplankton populations. Data suggest the existence of two ‘ biological provinces ’ north and south of the Bay of Somme. In the Northern province, the bloom starts earlier, probably due to the shallower coastal water, and is characterized by high proportions of diatoms and, successively, of Prymnesiophytes (Phaeocystissp.). The bloom maintains high biomass levels sustained by inputs from the Somme River and probable nutrient regeneration. The Southern province, directly influenced by the Seine River, is characterized by a deeper coastal water column and the presence of phytoflagellates. Despite the higher supply of nutrients from the Seine River, the bloom starts later and supports a lower phytoplankton biomass. The differences between both areas are analysed on the basis of the hydrodynamism of the area, and are interpreted as two different stages of the same process. To understand the spatio-temporal variations of phytoplankton dynamics, interactions between biology and hydrodynamical characteristics of this area are discussed.