Maybe I'll start by being a little bit technical, but then I'll bring it to a higher level.
Basically, as scientists, we look at dissolved and particulate phosphorus. These are technical terms; it depends on what passes through one of our laboratory filters, a very fine filter, a very fine mesh.
If it passes through this fine mesh, it's called “dissolved”. That means very small molecules. Because they're small, the algae can take them up, and that fuels their growth.
The other forms of phosphorus are the particulate ones, which don't pass through this very fine filter. They tend to be particles that are sediment associated. They're ones that are associated with soils or particles like that. This type of phosphorus isn't immediately available to fuel the growth of the algae. If you think of the fertilizers you use on your plants or your lawn, you're using pellets, usually, that have the soluble phosphorus bound onto them and can come off quickly and feed the grass and the plants.