At midlatitudes in spring, a phytoplankton bloom is often preceded by . Multiple Choice vertical mixing exceeding the critical depth vertical mixing becoming shallower than the critical depth the critical depth being equal to the compensation depth the critical depth becoming shallower than the thermocline

Vertical mixing becoming shallower than the critical depth

Explanation:

Spring phytoplankton bloom occurs in all mid and high altitude regions. If the vertical mixing depth becomes less than the critical depth, this conditions favors the rapid growth of the phytoplanktons and a phytoplankton bloom is seen. Vertical mixing depth is the depth above which the water is thoroughly getting mixed either by wind or by convection. The conditions that favor the high growth rate of phytoplanktons in the spring are as follows -

the answer to you question is   jet streams