Final answer:
The main source of phosphates is human activities such as the use of artificial fertilizers and animal husbandry runoff, leading to eutrophication. Natural sources include weathering of rocks and movement of ocean sediments to land.
Step-by-step explanation:
The main source of phosphates is often attributed to human activities, particularly the use of phosphorus in artificial fertilizers and runoff from animal husbandry which often leads to eutrophication in aquatic systems. Phosphates in nature originate from the weathering of rocks and are found as the phosphate ion (PO4³-). They enter the oceans and other water bodies through surface runoff, groundwater flow, and river flow. Over geologic time scales, phosphates are also moved to land from oceanic sediments, which are primarily formed from the bodies of ocean organisms and their excretions as well as volcanic ash and mineral dust.
In aquatic ecosystems, phosphate can lead to eutrophication, which is a process where increased phosphate levels lead to algal blooms followed by decreased oxygen levels and sometimes fish kills. Protective measures like no-till agriculture and the use of hedgerows can reduce phosphate runoff.