The Islay front: Physical structure and phytoplankton distribution

A shelf sea front exhibiting strong salinity influence is described. Temperature plays only a secondary role in controlling the density although the frontal position conforms to the h/u³ model. Velocity measurements by current meters and radio drogues have defined the tidal flow and indicated residual currents of ∼20 cms^-1 parallel to the front. These flows were time dependent with flow reversal occurring in the lower part of the water column. The observed velocity shear was of the same order as, but not in precise agreement with, the geostrophic shear inferred from the density gradient. In vivo and extracted chlorophyll measurements showed that phytoplankton standing crop at the front was several times greater, and was healthier, than that in the mixed water inshore. Chlorophyll was vertically stratified at the front and in the density-layered water offshore. High standing crop at the front may be explained in terms of nutrient availability to algae in the euphotic zone. The complex physical and biological processes influencing frontal productivity are discussed.