Is there any way a turtle has gotten into your pond and you haven't seen him. Or maybe the Koi have decided to try a different menu. Sound like something is biting them off. Glenda

What do the stems look like? Are they rotted at all? Are they torn, or cut straight across? Is there any disruption to the actual plant? It does sound like something is biting/pulling them off. If they are potted, they definitely need thinned every couple of years. And they should be fertilized every month during the growing season.
If they are hardy lilies, you shouldn't have to remove them in winter in your zone.
Maybe you could get a cheap night video camera and see what's doing this? I hope you figure it out. It must be very disconcerting to find all those torn lily pads.

Koi will do that too.

Thanks for the information on the koi. As the plants and stems look perfectly healthy, it seems like the koi are most likely culprit (I would be really impressed if a turtle made it into our backyard in the middle of LA). The question is how to keep the koi from continuing to eat at the stems? I will try to feed them more often, any other ideas?

You could probably fashion something yourself but they make these floating nets specifically to protect plants from koi.
Vanessa

Here is a link that might be useful: plant protector

My lillies are pretty weak and do the same thing this spring, but I just repotted them all. they're getting stronger and I think the problem will resolve itself. I plan to fertilize them tomorrow.
Have you ever repotted them?

Here is a link that might be useful: Matt's Koi Pond

You do NOT need to remove your waterlilies and put them inside for winter in your climate. Not even tropical waterlilies and since yours are pink, they are likely hardies.

You should divide and repot them every year though for best blooming though it sounds like they've done well without.

Koi love to root around in plant pots. They do also nibble on plants. Many cannot keep waterlilies in koi ponds because they destroy them.

I do have a ponding friend in LA who has had his neighbor return his turtle, found it in his pool. IF you have a neighbor with a turtle it could find your pond. Many don't give them a proper enclosure, thinking they won't leave but they do.