Challenges Part 3: Phosphorus
In plants, phosphorus is a growth stimulant that allows the passage of other nutrients through the plant membranes. Phosphorus is an abundant nutrient in Florida, mostly contained within sedimentary rock and in Florida’s natural soil. However, if it is released in excess, it can become a pollutant. In The Everglades “excess” is only tiny amounts. The Everglades ecosystem evolved as a low-phosphorus environment, The lack of phosphorus was a limiting factor, determining which plants would be able to compete successfully to shape The Everglades into an array of sawgrass, wet prairies and open-water sloughs dotted with tree islands.
Over the last decades there has been a constant addition of phosphates to the soil by humans, for example through fertilizers used in the Everglades Agricultural Area, which are released via numerous canals into the southern parts of the Greater Everglades Ecosystem. In many areas phosphorus reaches concentrations well above the natural ones.
There is now excessive growth of cattails, and other vegetation that was not found, or only sparsely present in the Everglades. High levels of phosphorus cause cattails to crowd out native Everglades plants favored by wildlife.  (SFWMD) Also, the increasing phosphorus concentrations in surface waters increase the growth of organisms such as algae and duckweed, which prefer a higher level of nutrients. This changes the habitats of aquatic and non-aquatic wildlife. 
Phosphorus from agricultural runoff has impaired water quality in large portions of the Everglades and has been particularly problematic in Lake Okeechobee . The Caloosahatchee and St. Lucie estuaries, including parts of the Indian River Lagoon, have been greatly altered by high and extremely variable freshwater discharges that bring nitrogen, phosphorus, and contaminants into the estuaries and alter the salinities that control the abundance of estuarine organisms   .
Due to legacy phosphorus storage in the Lake Okeechobee watershed, the lake itself, and the Everglades Agricultural Area, current phosphorus loadings into the system could persist for decades. Attaining water quality goals throughout the system is likely to be very costly and take several decades of continued commitment to a system wide, integrated planning and design effort that simultaneously addresses source controls, storage, and treatment over a range of timescales. However, the current acreage of stormwater treatment areas (STAs), as managed, is not sufficient to treat existing water flows and phosphorus loads into the Everglades Protection Area.