Denitrification is conversion of nitrate to gaseous nitrogen, a net loss of available nitrogen in the system. Algae blooms, taking up a lot of nutrients. Area of Gulf of Mexico is dead zone, where no nutrients are left, so nothing is growing. In water algae take up phosphate, fish eat algae, excretion and decomposition returns phosphate to water. Some phosphate moved from water to land by sea birds eating fish, birds defecate on land Phosphate may be kept in ocean sediments for millions of years.
Humans have fertilizers, animal and sewage runoff get lots of phosphate into waters.
Soaps have phosphates. Clearcutting and erosion increases loss of phosphate from land, stays in oceans. Hydrological cycle is movement of water among land, ocean, atmosphere. Water leaves atmosphere as precipitation over land or ocean Evaporates from ocean surface, lakes, streams, etc. Water also seeps through soil to groundwater, which is in layers of rock called aquifers.