We show the vertical distributions of temperature, salinity, nutrients, oxygen, CO 2 fugacity (fCO 2 ), pH and chlorophyll-a at the end of the winter 1984 in the northeast sector of the Atlantic Ocean between 40°N and 47°N. The intense vertical mixing due to winter cooling resulting in homogeneous water column layers as deep as 300 m at 47°N and, getting shallower further south to 100 m at 40°N. This northward increasing of the winter mixed layer was accompanied with a northward increasing trend in nutrients and CO 2 and a decreasing in temperature, salinity, oxygen, pH and chlorophyll-a. Taking into account these observations, we analyse the winter mixing effect on the CO 2 uptake of the newly formed water masses. The mixed-layer CO 2 values were very close to the expected values supposing that the new water masses were formed under conditions of CO 2 atmospheric equilibrium and were subsequently subjected to convective mixing with aged waters from below the main pycnocline.We also show the occurrence of a significant phytoplankton biomass through the entire winter mixed layer. This phenomenon, previously coined as ‘phyto-convection’ [Backhaus, J.O., Wehde, H., Hegseth, E.N., Kämpf, J., 1999. ‘Phyto-convection’: the role of oceanic convection in primary production. Mar. Ecol., Prog. Ser. 189, 77–92] affects the nutrient and CO 2 levels of the newly formed water masses and it is an indicator of the extremely fast vertical velocities through the winter mixed layer.