How do fish harvests and seabird guano return phosphorus from the ocean to land?

Phosphorus is a vital element that often moves unidirectionally from land to the ocean, where it accumulates in sediments. Because much of the phosphorus on Earth is lost to ocean deeps, the return of this nutrient to terrestrial ecosystems is essential for maintaining soil fertility.12

Fish harvests and seabird guano serve as critical mechanisms for this return:

• Fish Harvests: When humans harvest fish from the ocean, the phosphorus assimilated by those organisms through the marine food chain is physically transported back to land.12
• Seabird Guano: Seabirds that feed on marine life accumulate phosphorus in their bodies. When they deposit their excrement—known as guano—on coastal islands, they return significant amounts of phosphorus to the land. This substance is highly valued as a fertilizer because it is rich in phosphoric acid, nitrogen, and potash.13

These processes help mitigate the steady loss of phosphorus to the sea, though human intervention through the application of fertilizers remains necessary to sustain agricultural productivity.1