This is to a large amount what I would have said, with the addition of "photoinhibition". With hardcore photoinhibition, there is too much light for the amount of nutrients. A reduced period won't help, because then it either off, or photoinhibited, neither of which grow. The answer is to dim the light, which normally you never want to do because dim light causes celluose growth to slow and exudates to increase, neither of which removes nutrients.

Another component is organics. Some are consumed by algal growth, especially cladophora, which grows a lot in a SURF2. So this unseen and un-measurable component alters things too, and also feeds into the photosynthesis process.

Thus, the easy solution is to temporarily reduce the light.