Something I hadn't considered, is if there something the water lettuce
exudes that kills algae. That's really the only major change in the pond's
ecosystem that has occurred in the last week or so.

In sal****er aquariums, both macroalgae and corals routinely engage in
chemical warfare. The corals especially, but even the macroalgae. It's
a possibility for fresh water plants, I suppose, although I am ignorant
of the subject.

C//