Daphnia eating wood?

I have an outdoor "pond" consisting of a 48 inch kid's wading pool. I've often used it as a source of live food for picky fish, or for raising fry by just throwing a gravid livebearer in there for a while. I've added different invertebrates over time, including daphnia and scuds, but I had mostly seen the pond produce mosquito larvae, bloodworms, and a few helgrammites, with never a sign of the creatures I had added.

A few days ago I added some cut wood to soak so I could add it to a tank after it got waterlogged. The wood was freshly cut, but had been down for maybe six weeks.

The next day there was a huge bloom of daphnia (probably ones that were already in there that I hadn't seen). Rather than free swimming as I'm used to seeing them, they were closely associated with the wood, and most of the big ones were stuck to it the way you see aphids sucking plant stems. They appear to be feeding on the wood, both on cut or broken interior surfaces and on the exterior bark. For three days I've netted off all I could, and they come back in spectacular numbers.

Has anyone ever observed this before? Do they eat cellulose, or sap, or what? There's no real sign of green water in the pond, but there are a lot of dead leaves which I imagine were providing bacteria as a food source previously.

Fresh cut wood is quickly colonized by various bacteria, fungi, and other "saprophytes"; the daphnia are probably feeding on these, and other microbes that feed on the saprophytes, rather than directly on the wood. You'll see the same effect if you toss an animal carcass in the water.

Look closer: you might have Ostracods in addition to Daphnia. Ostracods graze on surfaces more than Daphnia do, and are less tasty; many small fish won't eat them.

There are ostracods, copepods and cyclops present, but the daphnia are by far the most numerous.