The Phosphorus Cycle
Within ecosystems phosphorus is a crucial component within the cells of living organisms. For example in the cell membranes within an organism's cell, phosphorus makes up components of the phospholipid, hydrophilic heads/ ends of the membrane (see Diagram A). Phosphorus also forms most of the chemical backbone in RNA and DNA strands (see Diagram B), and gives these strands their structural components. The cardinal reserves of phosphorus in the biosphere can be located in the Earth's crusts and with weather conditions, rocks disperse the phosphorus in the form of phosphate. Phosphate then seeps into the soil at which point, plants absorb the phosphate through their roots.
Diagram B

Diagram A

When animals consume plant matter, they incorporate some of the plant's phosphorus that they consumed into their own body's make-up. Then when organisms (plants, animals etc.) die or emit waste, the phosphorus is then available to decomposers, for example bacteria. Decomposers then break down tissues and emit phosphorus into the soil, so that plants can absorb it again.
Phosphorus is also introduced into water based eco-systems such as rivers and oceans via run-off, leaching or within the cells of the organisms that live in the eco-system. When phosphorus is in the ocean/ river it then enters aquatic food-chains. One tiny phosphorus atom can cycle through the water and the organisms that live in the water for an approximate average of 100,000 years until it will finally rest on the ocean floor in the form of sediment which will then proceed to make rocks. (5)

Phosphorus can possibly stay contained within sediment for over 100 million years and contrary to the carbon and nitrogen cycles, phosphorus does not filter through the Earth's atmosphere. The Phosphorus Cycle in turn is a 'sedimentary cycle'. Phosphorus within oceans and rivers sediment needs to be lifted by external geological components to get back to the earth's surface once again, at which point the cycle can continue (5). Human influences have altered the naturally occurring Phosphorus Cycle via occurrences such as mining and agricultural activities such as cropping and fertilising. Fertilizer contains copious amounts of phosphorus, which then gets taken in via the different environments. Also, sewage plants put extra matter containing phosphorus into rivers, oceans and lakes. This additional phosphorus can influence and increase the growth of particular organisms for example algae, and alter the balance of organisms within any particular eco-system.
Below is a diagram that summarises the Phosphorous Cycle and will help with understanding how Phosphorus Cycles through both water based Eco-systems and land based Eco-systems and also how humans now contribute and have changes the cycle.

Diagram C: The Phosphorus Cycle

An issues in detail: An issue related to phosphorous within an agricultural context is that excessive amounts of phosphorus in fertilisers can cause a rapid increase in the productivity and growth of organisms such as algae and create a bloom and ultimately cause eutrophication (4). A bloom is when an organism, for example algae, would become so plentiful and dense that it would cut off the sunlight available to aquatic life and could alter an entire ecosystem's functioning. Without sufficient light aquatic life cannot photosynthesize, which means that aquatic life cannot produce the food they need to survive and ultimately will die off. An increased growth of algae can also cut off the oxygen supply to the water. Living organisms within the water need oxygen to survive so if the oxygen supply is limited or cut off then organisms such as fish will die off (4). To summarise, when copious amounts of phosphorus are present in an aquatic environment this will cause phosphorous pollution, and when phosphorous pollution occurs it speeds up the rate of a process called eutrophication which is basically a lake or aquatic environment's death due to diminished availability of sunlight and oxygen. Below is an image of a lake in the Dianchi Lake in China which is over-run with algae due to excessive uses of fertilisers in the area that ran into the lake when it rained and caused eutrophication. (4)

Eutrohpication of the Dianchi Lake
Eutrohpication of the Dianchi Lake

In relation to the health sector blooms of blue-green algae will create neurotoxins which affect the nervous system and hepatotoxins which affect the liver within humans, and can create a huge problem for public health along with harming aquatic environments. In relation to economics increased algae growth caused by phosphorus pollution raises water rehabilitation costs, decreases fishing and boating activities, and devalues tourism and property prices. Phosphorus pollution is an increasingly difficult problem to fix but can be done through water conservation and the removal of phosphorus rich fertilisers from an area. (6)