The remaining part of phosphorus becomes distributed, absorbed, or precipitated in the form of orthophosphates of calcium, iron or aluminium. Because of such reactions, little if any, leaching of P occurs at least in very sandy and organic soils. Much of the applied P accumulates in the surface soil and remains there in insoluble form unless the sediment containing it happens to be carried away by runoff.
Very low concentrations of soluble P are often sufficient to lead to eutrophication of surface waters. However, virtually nothing is known about the behaviour of P in sediments, whether mineral or organic, especially with reference to the role of colloids in bringing about equilibrium conditions. In freshwater habitats, phosphorus often limits algal production (Schindler, 1977), but this occurs rarely in the sea. According to Ryther and Dunstan (1971), coastal waters contain about twice as much phosphate as can be used by the algae. Marine phytoplankton can concentrate phosphorus by a factor of as much as 1O7 over its concentration in the sea, and prolific algal growth can deplete the phosphorus to very low levels indeed. The cycling of phosphorus in most lakes is quite rapid, with a turnover occurring in a few minutes in summer or in a few days in winter. In the ocean, the phosphate turnover occurs in a matter of a few hours in summer or a few days in winter (Round, 1981).
Some of this rapid turnover is through bacteria, not algae (Harrison et al., 1977). In the upper layers of most lakes, light is not the factor that limits plant growth. In these layers in most natural lakes in temperate regions, phosphate is the growth limiting nutrient; nitrogen seems to be the chief limiting nutrient in most tropical lakes. Algae efficiently absorb inorganic phosphate. When they die, most of the absorbed phosphate is recycled back into the ambient water.
Two kinds of phosphate cycle operate in lakes, viz., (1) the metabolic or biological cycle, represented. Whereas the biological cycling is quite fast, the geochemical cycling can be very slow indeed. About twenty million tonnes of P are estimated to leach off from land into the oceans per year. The major pathway for returning P to land is the uplifting of marine sediments which, however, is a geologically intermittent process. A more continuous process is the deposition of guano by marine birds.
The guano birds eat ocean fish and by depositing their excreta on the land produce high concentrations of phosphates. Thousands of tons of phosphates are also returned to land by marine fishing, but still the fact remains that the rate of loss of P to the oceans is much greater than its rate of return to the land.