Biological Oceanography

The halocline acts as a strong mixing barrier, preventing full ventilation to the
surface layer from the water beneath that contains abundant nutrients (40 μM nitrate
at 300 m). In the central Gulf of Alaska, gradual fall (autumn) and winter erosion of
the seasonal thermocline raises maximum surface-layer nitrate to only ∼17 μM in
March. By April or May, reduced winds and solar heating establish a seasonal
thermocline at about 35 m, after which surface nitrate drops by about 8 to 10 μM from
March to August, but it does not drop below ∼6 μM (Fig. 4.8d), a level unlikely ever
to limit phytoplankton growth. Phosphate and silicate are not reduced to limiting
levels, either. Another important part of this is that limitation of vertical mixing by the
halocline keeps the microplanktonic food-web in place right round the year. It is never
disrupted by winter mixing to depths as great as occur in the subarctic North Atlantic,