It is speculated
that is why aquatic plants produce red color to protect from UV(have you ever seen any specific studies on submersed aquatic plants that state this in the literature????....I mean, since we are considering it "prudent to focus peoples attention on light as the one essential factor they often obfuscate, instead of distracting them with the exceptions and peripheral issues"...........submersed aquatic plants live in relatively low light locations.
I've found the reddest colored Ludwigia for example at nearly 20ft depth in springs, whereas I've found the same species green near the surface. Same water, same location by a few feet.
The spring had hardly any NO3, the shore had lots from soil to draw N from.
Note, this is for only one species. The Fe in the soil was 4000X higher than that of the river(so Fe limitation, adding more Fe is questionable as to increasing red color), the plant had attached to hard gravel substrate and loose rock, no nutrients where extracted.
It's also been speculated that the red chemical is namely an anti herbivory agent.
In observations of aquatic plants, I'd more suspect the latter.
If you assume UV protection, this does not address why the colors are so red in deeper waters.
Overall, being specific is the most useful approach many aquarist generalize a lot but without addressing the other issues as to why it works or not.
Are there alternative hypothesis that explain things?
Can such hypothesis be falsified? I've given one such example and a reference that suggested the high light or UV protection is not a model at least for one or more plants.
Those that are red color variable are so due to low N in their environments.
There is research on subsmersed plants that suggest low N rather than light is the main variable" see Spencer (UC Davis-same place, same building and same lab as me). Nice guy and a good scientist.
Spencer, D. F. & G. G. Ksander, 1990. Influence of temperature, light, and nutrient limitation on anthocyanin content of Potamogeton gramineus. L aquat. Bot. 38: 357–367.
Maybe we are wrong........
The Chl mask the red color in this specific case.......... if you reduce the N you will increase the red color in many aquatic species. We can also see this.
Ascribing cause to red color is still one based on stress, not health.
We generally do not have enough light o troast the Chl, so we are left with NO3.
The amount of light used is near natural light, about 3 more than than even a 1000 w MH over a 20 gal at 12".
The lower ranges of light suggest otherwise.
The plant responds namely from preserving the Chl is does have and is able to use it at full efficiency in intense light rather than damaging the photosystem machinery. So in this case, with very intense light, more than nay aquarist adds, it does appear that red color can bleach the Chl out and maintain plant color that way, but I do not see it in our tanks generally.
So this is a case that suggest otherwise about N limitation.
Dave Spencer has another study that suggest NO3 limitatuion is causing the red color increase. I'll see if I can hunt it down.
New growing tips also tend to be redder due to new growth lacking as much Chl as older tips. as the leaves develop, their chl increases, at faster growth rates, the catch up time is greater. So you see more reddish tips etc with more light as more light = faster growth rates in general.