Phosphorus exists almost exclusively in nature as
phosphate; however, this is cycled between soluble and insoluble forms. This
conversion is pH-dependent, and if phosphate is only present in an insoluble
form, it will act as a limiting nutrient. This explains the sudden surge in the
growth of plants, algae and cyanobacteria when a source of soluble phosphate
(typically fertiliser or detergent) enters a watercourse. Unlike the elements
discussed above, phosphorus hardly exists in a gaseous form, so its main
‘reservoir’ is in the sea rather than the atmosphere.