Yes.It is stressed time and again on here about problems of having too much light. Is it possible to not have enough?
Your plants die.And what are the consequences?
Yes, basically it is cause and effect, the Duckweed and Pennywort have intercepted the PAR, and this has meant that the lower leaves of the Water Wisteria have not reached light compensation point (LCP) and have been discarded. At this time of year I give the floaters quite a trim as the tanks will receive less ambient light during the winter. In the summer I'm happy with 2/3 cover, in the winter I keep it to below 1/2.This corresponds with my pennywort and duckweed going mental and taking over the surface. It looks wonderful but I wondered if it was cutting out too much light.
The other way I get around this is that once the tank has matured I don't have any stem plants, other than Ceratopteris, Ceratophyllum and Cabomba caroliniana, and I just use these as sub-surface floaters.When I start the tanks I make sure I have a high plant mass right from the start. The permanent planting consists of "easy" Cryptocoryne, Aponogeton, Echinodorus spp. with Anubias, Ferns and mosses. These all have slow growth rates, persistent "leaves" and low LCP values
If I were you I would check CO2 an flow/distribution first.
Cheers,