I am nowhere near a professional on the subject but I don't think a clay base will be any problem for growing plant life, I have my dam built out of crappy clay coal rocky mix, the top of it from the deepest pit in the pond, 20 plus ft, and stripped all the topsoil out of shallow areas that I could in the coves, the plant life is as vibrant there as in any place around the pond.
Clay is a misunderstood product, it will actually grow pretty decent crops as far as productivity goes, the molecules are just so fine and tight that it will not absorb or hold moisture that crops need to do the best in. Most of your pond weeds and plants are mostly hydroponic anyways, very seldom do they have a big root system, other then cattails and maybe some lilies, the rest can pretty much be raked out with minimal effort, mostly just growing off the water that is available to them with just enough roots to keep them from floating around. jmo
