The y-axis of Fig. Generally, it increases productivity. In general, P and N separately or combined, are the most common limiting nutrients in terrestrial systems. The phosphate ion dissolves poorly, so P flowing from adjacent land tends to precipitate quickly and become unavailable. Adding P to freshwater systems usually lead to a great increase in algal biomass, and a consequent increase in eutrophication.
Fertile freshwater is far less desirable than fertile land, because eutrophication leads to anoxia.
In terrestrial systems, such as in the tundra experiments, productivity remained high for several years following fertilization. In aquatic systems, productivity tends to decline rapidly once P addition is stopped. One of the major sources of water pollution in this country was once phosphates in detergents, which were added as water softeners. The phosphate created algal blooms in lakes and rivers, which led to massive fish kills due to anoxia. The result was a dramatic improvement in water quality in a relatively short period of time.