Phosphorus is the limiting factor in the metabolic pathways. The lakes at BWCA are of this type because of their glacial past Iowa lakes are Eutrophic because they have more gradual edges than the scraped out deep glacial bwca lakes. In an eutrophic lake there are distinct regions where diverse flora occurs ie. BWCA lakes don't have many nutrients washing in because of the soil quality and nutrient sink quality of the trees. Forest fires release these necessary ingredients for new life.
Nutrients enter the lake from runoff water and things falling in.
Several detrimental factors can affect a lake. Acid rain or deposition can be caused by a pollution source from far away. In Iowa, there is little problem associated with acid rain effects because there is a large amount of limestone in the area. Mercury is the other toxin that affects the BWCA lakes. The toxin bioaccumulates progressively as it moves to higher levels of the food chain.