When there is too much phosphorus and you get blooms of those same algae that are harmful to humans, they are also unsuitable food for those zooplanktons. They can't feed on them and they're toxic.
In some cases it's because if there are not enough top predators those alewives will get so abundant that they mop up all the zooplankton, and again, there's no way for that phosphorus to get up the food chain, which is a situation we had in the 1960s and 1970s.