I would try imazapyr on the water shield. Very rarely have I seen regrowth in the same year after using imazapyr. I've had mixed results with navigate but some people swear by it. It doesn't seem to have as long lasting of a kill though. Your water Alkalinity will drastically impact how well your pond holds a bloom, low alkalinity ponds don't tend to hold a bloom for more than a couple of weeks. Water shield isn't going to provide nutrients for your fish, but it will suck up the fertilizer you intended for a plankton bloom.