Looks like a raccoon visit to me.
Raccoons will mess with plants while looking for
snails, insect larva and crawdads (something they will find in
natural ponds and they don't know yours is not
natural). Raccoons will eat just about anything they
can get their paws which is why they've adapted so
wonderfully to suburban life.

Stuff that will work to deter them ~

- rent a humane trap from animal control and follow
their directives.

- put in an electric shock 'Fido Fence'.

- set up a motion activated sprinkler.

- netting the pond.

All these methods will work, depending on the set
up of your pond and your particular predator. Sometimes
you have to try a few, unfortunately, and that can add up.

Around here our two visiting raccoons ran smack into
two indignant labradors and spent the balance of their
visit up in a tree.

k :-)