A comparison is attempted between a semi enclosed bay (Rade de Brest), the vicinity of the Ushant tidal thermal front and the Celtic Sea shelf-break, with respect to phytoplankton and zooplankton overall abundance, using dissolved oxygen as a measure of in situ photosynthetic activity. A simple conceptual model, of a type termed agricultural, appears to be applicable to both the Rade de Brest and the shelf-break ecosystems, while the case of the Ushant front seems more complex. Different time constants in basic physical processes are tentatively put forward to account for the differences in ecosystem functional structures.