Vertical and horizontal controls of a haptophyte thin layer in the Bay of Biscay, France.

Investigations into the phytoplankton of the Bay of Biscay in July 2006 revealed an extensive bloom of an unidentified haptophyte species.High-resolution sampling techniques identified the region in the vicinity of the mouth of the Loire estuary as an area of high (10^610^7 cells l^1) cell densities of the haptophyte in a thin sub-surface layer.This organism dominated the phytoplankton population regionally. Measurements of the vertical distribution of horizontal shear showed that the population, located at ca.20m depth, was confined to a layer lying between two density discontinuities with high shear.Physicalmeasurements indicated that the population was being advected south eastwards,along the coast.Seaward spreading of the population was also noted during the survey.Horizontaldiffusion could bederived from the changes in the offshore gradient in the populations distribution with time, with estimatesof Kx, thehorizontaldiffusion coefficient, of the order of 100 m2 s1. The magnitude of horizontal dispersion is compared with that of horizontal advective flux, vertical dispersion and growth.