When they avoided using water during upwelling, the problem was solved.
Yes, because the upwelling is more acidic.
Those upwellings never killed oysters before. What changed? Oh, that's right, they got even more acidic.
Incidentally, ocean upwelling brings up mercury, cesium and every other heavy chemical pollutant from the depths (probably snail bait too)
Yes, and nobody thought to test for such things. You're the first person in the world to think of testing for chemical contaminants. Go inform the oyster farmers of your brilliant insight, as they never ran such tests.
Oh wait. They did. And they found nothing.