Bill, when you have more than 30-50% of the entire surface area covered in a subtropical or tropical shallow lake with macrophytes, adding PO4 = more weeds, the lakes in Florida, 7800 are 4 hectarces or larger, are gin clear regardless of the PO4 content in the water column.
You will get algae where the % of surface area is less, or where wind resuspenision or they have removed the plants etc occur.
They have found no correlation between PO4 in lakes with macrophytes and algae.
A good paper that refutes some of the older seminal work done by Philips 1978, was done at the old lab I worked at in UF.
The number of lakes in the study far exceeds any other and addresses specifically the type of system we are interested in natural ecosystem, lots of plants, warm temps(no freezing), high % of coverage, shallow, fish, snails etc, and a wide range of nutrient levels from oligotrophic to hypereutrohpic.
http://fishweb.ifas.ufl.edu/Faculty Pubs/CanfieldPubs/Aquatics2004LR.pdf
Take a long look at figure 1. You tell me if that figure has any high R^2 valve, eg, is PO4 correlated well with algae/plants etc?
A similar figure appeared after adding the fraction of plants that Philips, 1978 left out(actually he just did not consider the fraction of PO4 locked up in the plants, but added the algae PO4 from the water column, that great skewed the results!).
Removing such bias and parameters/methods that skewed things really makes is far less predictive.
So we search for other parameters that are predictive, such as NH4.
I have a non CO2 low light tank as well, if you think I've not added excess PO4, and I mean a lot, you would be mistaken, I've added 2-3ppm there, same thing with NO3, Fe etc.
Never was able to induce algae if the system was well planted and doing well to start with.
If the plants are hurting, or things are not that stable, then you can get algae.
Try moving all the plants all around and see.
That alone can easily do it, things happen slower in non CO2 planted tanks, the rate is slower for algae and plants.
That's why I chose to use high light a long time ago to get a better understanding about algae. Took less time and was easier to see the relationships.
If you do a large water change after moving the plants around, you do not get an algae bloom.
As the article states, plants define the nutrients and the system, not the nutrients defining the plants.
That realization has really been promoted in that group and they are very very sharp.
So for low light tanks, high light tanks, CO2, excel, non CO2(but not marine planted tanks, the max range seems about 0.5ppm for PO4) etc, the excess PO4 is going to induce algae.
If algae is already there, it may or may not depending on other issues.
Still, if algae is there, then there are not enough plants, too much disturbance, poor conditions for the plants.
This is the more technical paper:
http://fishweb.ifas.ufl.edu/Faculty Pubs/CanfieldPubs/macrophyte.pdf
Regards,
Tom Barr