Phosphorus (P) is a key nutrient for the production of Biomass by photosynthesis. (Lawn fertilizer is mainly Phosphate and Nitrate.) So high levels of P are associated with excessive proliferation of aquatic weeds, algae, and cyanobacteria.
P accumulates from inflows that include runoff from agricultural land and discharges from wastewater treatment works. In addition, organic matter such as leaves fall into the lake and decompose in the sediments. Aquatic weeds, algae, and cyanobacteria tend to bloom in summer and then die off and sink to the bottom where they decompose and return P and other nutrients to the sedimentary stockpile.
Limnologists focus on managing the total “in-lake” phosphorus load, and this includes all P dissolved in the water as well as that stored in the sediments. So effective management of P loading must address both P in the water column and P in the sediment.
Popular strategies aimed at limiting further additional P inflows into eutrophic water bodies are therefore akin to closing door after the horse has bolted.