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If you were going to go the Fluridone route, I'd apply it now. Fluridone will kill all the plants in the pond, and it really, really works well when applied correctly. It takes 60-90 days to work, so it'd be in the pond when the plants are growing. I hope Kelly will chime in here, he's the plant guru. My take on it is that the plants would die before they completely covered the pond, and that there would be less decaying organic material, which would use less Oxygen.

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