Continues to be a fantastic thread.
I wonder if the phytoplankton release algal exudates like macro algae do. In that case your tank would have shifted strongly towards a higher ratio of organic/inorganic nutrients.
Most of the current hypotheses in the hobby right now are pointing in the opposite direction as low organic/inorganic ratio of nutrients in the water being desirable. (Triton N-DOC etc...)
Some people have mentioned dead phyto products, and it's not really comparable, because it has zero potential to uptake nutrients, and is guaranteed to raise nutrients in the tank as it decays.
On the dinos issue, I've seen more go poorly - more cases of phytoplankton addition causing an intensification of a dino outbreak, and fewer instances of people saying it made it better. This is kind of counterintuitive because one would think biodiversity wards off dinos. But Dino's, especially the toxic types are mixotrophic and use mucus mats to trap food. Phyto and copepods can become trapped, die and directly fed the dino bloom. (Personally observed this in a test tank). In low toxin dino types, it may be more likely to go the other way.