That's an interesting idea, and it seems to support the hypothesis that the Redfield ratio (or any ratio ) is not the best way to think about it since higher nutrients at any fixed ratio may cause darkening relative to lower nutrients at the same ratio. I think it goes back to the idea that trying to keep both N and P in desirable absolute ranges (e,g., 2 to 10 ppm nitrate and 0.02 to 0.1 ppm phosphate, or whatever ranges folks prefer) is unlikely to ever be worse than targeting ratios of any kind independent of the absolute values.
Do you have any idea of the magnitude of the effect you are talking about?
For it to be significant, some fairly complicated things need to happen...
Darkening of algae in elevated nutrients is presumably from additional chlorophyll and the algae trying to take advantage of the available nutrients. Absolute concentrations of chlorophyll in the ocean is often taken as a good measure of the local productivity of photosynthesis. So that would be different here.
For there to be a darkening effect that reduces overall productivity, one would need that the chlorophyll in the upper layers of algae are absorbing light but not using it as effectively for growth as a lower down algae would. That could possibly be the case, if the algae is getting all the chemical energy it can handle and is wasting some excess. I've never looked to see if that sort of thing is known to happen with excess nutrients.
I know that organisms do have protective mechanisms when there is too much light which removes the excess energy to prevent damage. But I've not seen an indication that higher nutrients could trigger such effects
Researchers from MIT and the University of Verona have discovered how a key photoprotection protein allows moss and green algae to protect themselves from too much sun by dissipating the extra energy as heat. Learning more about how this protein works could allow scientists to alter it in a way...
news.mit.edu
"The researchers found that the protein, embedded in the membranes in the chloroplast, can switch between different states in response to changes in sunlight. When moss and green algae absorb more sunlight than they need, this protein releases the energy as heat, preventing it from building up and damaging the cells. The protein can act within seconds of a change in sun exposure, such as when the sun appears from behind a cloud."
The other thing needed is that not only are the lower down algae more efficient, but they would need to be enough more efficient to offset other losses of light (e.g., absorption, scattering, reflection back up) while passing through the first algae bodies before it gets to algae deeper down.
I personally would not have predicted this overall effect to happen (that is, decreased growth due to greening up of the algae) , but it is certainly a plausible effect.
