Yup, it sure is likely when ponds are heavily planted and liner ponds,
there's a definite limit to what plants can do. You are headed toward
the opposite of 'eutrophic' conditions, first obvious signs are water
lilies not forming buds and foliage yellowing.

Until you thin out the plants and supply effective fertility, the
plants will take advantage of the situation, the more invasive plants
will try to smother more delicate ones and mulch them...

the likes of mulch fluid, super phosphates, and typical tomato feed
should perk aquatics as much or as little as you like, however its
always a good idea to cut back the less interesting more invasive
plant varieties prior to winter, those which are likely to dump a lot
of dead foliage in the water

Regards, andy
http://www.members.aol.com/abdavisnc/swglist.html
(andys aquatic plant list for interesting swaps
http://community.webshots.com/user/adavisus
(photo albums of aquatic plants and descriptions)

So I ask...is it possible, contrary to popular belief, to overstock with
plants and filter nutrients out faster then the fish can produce them?