I am trying to overwinter some Duckweed, Water Hyacinths and Water Lettuce. One of my experiments has a solid dark green fibrous film covering the water surface. It is solid/plastic like and prevents water from evaporating. The film has shrink-wrapped around the water lettuce and the duckweed seems to be trapped underneath the green film. The water is clear.

Any ideas what this is or if it will be harmful to reintroduce to my pond in the spring?

I am including two pictures. In the second picture, I removed a few of the water lettuce and you can see the green film is attached. The clump of duckweed on the right is upside down to show you the underneath side of the green film.

Sure looks like FA (filamentous algae) which is present in all ponds to some degree or another.

Go here to the vegetation forums of PB...

http://forums.pondboss.com/ubbthreads.php?ubb=postlist&Board=14&page=1

and search on "FA" for a lot of reading.

FA can be a horrible problem, but it's most always the conditions of the pond/lake not the fact that it's present. Shallow waters, high nutrients, and ample sunlight reaching the bottom are all conditions that promote the slime.

I wonder if your plants are dying back creating some nutrients for the slime to live off of, some partial water changes may help reduce it's growth.

Welcome to forums!