How about letting chloroplast convert waste to body instead? That would simulate the fact that plant life need more than just sunlight to grow, like minerals and such. When body reaches 32000 they can't convert any more waste and it will start to build up instead, which would simulate over-fertilizing like some plants are sensitive too, and just assume it's like this for algae too. You get an algae bloom with lot of waste in the water, but there is a limit how much they can use of it.
..and a .fdwaste sysvar to convert waste directly to energy like some primitive bacteria.
..and change the "Don't decay nrg shots" feature to "Keep nrg & waste shots" or something like that.