Nitrite profiles show the activity of nitrifying bacteria

Nitrite is an intermediate product of nitrification and will not accumulate to high concentrations. Still, it shows the horizons of high nitrification activity. Note the sharp peak just above the oxicline. Here, in the lower part of the halocline, the nitrifying bacteria accumulate. They need oxygen, hence nitrification ceases completely in the anoxic zone.

More on nitrification here...
More on denitrification here...