The classic Sverdrup model of the initiation of a phytoplankton bloom has been criticized because

A) Phytoplankton have been found to mainly bloom in the summer
B) When the phytoplankton bloom begins, the water column is surprisingly cold and vertically mixed
C) Measures of the mixing depth show that is actually gets deeper as the spring progress.
D) Nutrients have been found to be more limiting in winter surface waters than previously supposed.