Puzzling ponds

The duckweed majors in the pond in the front of the snap left and is suspected of weakening the marsh marigold in the middle. We are not convinced that it would be dying back in the summer if the duckweed was not sucking all the goodness out of the water. And quite thick enough that the likes of wasps and bees can now walk on water.

We clear the duckweed out from time to time and have tried various folk remedies, but it always comes back. I expect the only real answer is to empty the tub and start again, but that is not going to happen any time soon.

A very weak presence in the left hand pond, the one with the reeds. They seem quite strong enough to keep it at bay.

And a very weak presence in the right hand pond, the one with the lilies. Slightly puzzled why the duckweed has not covered the patches of open water there. Maybe a bit shady under the lily leaves, but there is a very shady pond on the common completely covered in duckweed, so that cannot be the whole story. Maybe there is something living in this pond - at around 15 inches a lot deeper than the other two - which eats the stuff. But not the remaining newts as they are carnivores.

PS: I recall reading that in the hot parts of the USA they farm duckweed in shallow lagoons. The fastest way of converting sunlight into carbohydrate known to man.