Water velocity has to be considered in the mix. Phosphorous, in the low concentrations we're discussing, could certainly be limited (or not) by water motion. Mass transfer through a boundary layer is dependent upon thinning of said layer (high water motion) or thickening (low water motion) is a distinct possibility. If water velocity is so low that P mass transfer doesn't come into play, Liebig's Law of the Minimum applies (that states photosynthesis is limited by the least available nutrient.) I'm really going to have to give this some thought.