Final answer:
Phosphate concentrations in seawater are influenced by rock weathering, volcanic activity, and human activities like fertilization and detergents, which can lead to eutrophication. This cycle is a slow process, with phosphate having a long oceanic residence time.
Step-by-step explanation:
The concentration of phosphate (nutrient) in seawater is primarily influenced by several key processes. Phosphate enters the ocean through weathering of rocks, volcanic activity, and human activities such as the use of artificial fertilizers and runoff from animal husbandry. Once in the ocean, phosphate becomes part of the marine food webs where it can eventually settle to the ocean floor as sediment. The movement of phosphate is slow, with a single ion having an oceanic residence time between 20,000 and 100,000 years. Additionally, human-induced effects such as the use of phosphorus in detergents can lead to eutrophication, which in turn affects phosphate concentrations through increased growth of algae and subsequent depletion of oxygen when these algae die.