Originally Posted by ShaneS

I proposed this question to one of my professors (Horticulture Phd, specializes in Floriculture) and he suggested allelopathy. There is very little research on the subject and not nearly enough for anyone to have a complete understanding, especially in aquatic environments. We know that a lot of large canopy trees will 'poisen' the ground under them with leaf litter and chemicals that are released as the leaves decompose. This frees more nutrients for their roots and keeps other trees from growing into their canopy and shading them.
Now there is proof that allelopathic chemicals exist, and it is a pretty sound theory, he is an intelligent guy. Algae can use No3, po4, and whatever so obviously in the case of EI (I am so far unfamiliar with PPS) the algae should be growing. In an empty take with lots of light and these chemicals (In ANY RATIO, the ratio is not important to algae and plants) we expect to see algae. Therefore something about the presence of healthy higher plants prevents excessive algal growth. I think of algae as a scavanger, it will take in nutrients down to the part per BILLION, and is more efficent than higher plants and living off low levels of nutreints (Although probably wont be prolific at low levels).
I dont think we can answer this question, but we can make some educated guesses. I'm still in college trying to figure things like this out... maybe one day i will have a better answer haha.