This experiment seems to indicate reduced algae growth at increased phos levels;
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3434001/
wouldnt this indicate that once a tipping point has been reached, you'll never get enough growth to reduce it ? Don't mind if I'm wrong on this by the way, but it's what it sais to me.
and therefore that after this tipping point is reached, if ever, you will need to reduce it artificially to promote screen growth once again!