VintageReefer
10K Club member
It’s from shading. The only photosynthesis that occurs is where light hits, and the denser the algae becomes, the more shading to the inner parts. The outsides get the most light and grow the fastest. The outsides also have the most exposure to the water containing the nutrients that it consumes. This is not a limiting factor, until it reaches a certain density, as you mentioned.
