How much of the carbon stays in the animal eating the bloom? I would think while it's tricky to calculate assuming them being eaten is bad might not be true. Of course we might end up making farting whales and adding loads of methane to the atmosphere knowing our luck...

Whenever we try playing god it seems we generally mess things up a few times so we'd need to monitor the actual outcomes in reduced CO2 in the atmosphere over several decades. It seems extremely high risk to assume there won't be unintended consequences...