Chemical properties of the deep winter mixed layer in the Northeast Atlantic (40–47°N)

Abstract

We show the vertical distributions of temperature, salinity, nutrients, oxygen, CO2 fugacity (fCO2), 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 CO2 and a decreasing in temperature, salinity, oxygen, pH and chlorophyll-a. Taking into account these observations, we analyse the winter mixing effect on the CO2 uptake of the newly formed water masses. The mixed-layer CO2 values were very close to the expected values supposing that the new water masses were formed under conditions of CO2 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 CO2 levels of the newly formed water masses and it is an indicator of the extremely fast vertical velocities through the winter mixed layer.