More information on phosphorus, nitrogen, etc.
Eutrophication is defined as: the process by which additional nitrogen and phosphorus enter a body of water from fertilizers, human and animal waste and cause photosynthetic organisms like algae to multiply exponentially. The problem is worsened by the decrease in wetlands, which serve as a natural filter, as well as other factors. Eutrophication is a major contributor to dead zone formation, and the resulting cyanobacterial growths just add to the dead zones.
Nitrogen and Phosphorous:
Phosphorus is an element that is essential for all life, from bacteria to plants to animals (it is in DNA). Phosphates are compounds that contain phosphorus, and are popular as fertilizers. They are very often used commercially on big farms, but they aren’t always used efficiently. Phosphates can be found in rock deposits in various parts of the world.
Nitrogen is also essential for life, and is also a component in DNA. Cyanobacteria and many plants are able to obtain it from the atmosphere and convert it into usable forms, and other organisms including animals must obtain it second hand from those sources.
When the ratio of nitrogen to phosphorus is less than 15:1, then toxic toxic varieties of cyanobacteria like Microcystis flourish. If the ratio of nitrogen to phosphorus is greater than 20:1, then non-toxic varieties of cyanobacteria will flourish instead. What this means is that phosphates are more limiting than nitrogen since many cyanobacteria can fix nitrogen, but phosphorus (in the form of phosphates) must come from outside sources.
Microcystis can survive even when Phosphorus is low because it has genes that increase uptake. But when Nitrogen is high, more microcystin is produced as a defense mechanism. Keeping Nitrogen low is difficult because other Cyanobacteria can fix it from the atmosphere. But if Phosphorus is low enough, those others will not survive to fix it. This will allow Microcystis to survive, but if it isn’t competing and doesn’t have access to excess Nitrogen, blooms will not be toxic.