Zorfox: I think the relationship between algae and macrophytes is very interesting in the way they are antagonistic to one another but they mutually manage the nutrient loads. Do we really understand the way plants combat algae?
I think we are just beginning to understand this concept. In fact, as far science is concerned is relatively new.
If so, what is the main mechanism?
Quite simply Allelopathy
plays a large role in this. It is a fascinating subject of particular interest to hobbyist. Apparently macrophytes (plants) can excrete allelopathic substances which reduce algae blooms. I had no idea plants were capable of this until I started with planted tanks. I kept hearing that the plants will out compete algae for nutrients, CO2 and light. Well this isn't the African savanna where one antelope has to feed a lion pride. We are providing excess of all necessities to the plants. So I started researching it and realized they are not just competing but killing each other off lol.
Why is a high density of healthy plants a deterrent to algal growth?
The more plants the more allelopathic chemicals. I do wonder if water changes affect this defense mechanism. I suppose I have a lot more research to do.