Phosphorus is an essential element for all life, forming DNA, RNA, and ATP, which are central to cellular energy transfer. It is naturally present in various geological formations and is released through processes like continental weathering. Runoff from land surfaces acts as a primary conduit, transporting phosphorus into marine environments.
Forms and Movement in Ocean Waters
Once phosphorus enters the ocean, it exists in several forms. Dissolved inorganic phosphorus, primarily as phosphate (PO₄³⁻), is the most readily available form for marine organisms. Phosphorus also occurs as dissolved organic phosphorus (DOP), a complex mixture of organic compounds. Additionally, particulate phosphorus, attached to sediments or organic matter, is transported into the ocean, particularly via riverine influx.
Ocean currents and mixing processes distribute these different forms throughout the water column. Upwelling brings phosphorus-rich deep waters to the surface, supporting biological activity, while downwelling moves phosphorus-rich surface waters to deeper layers. This constant movement ensures that phosphorus circulates and transforms within the marine environment.
Biological Integration and Cycling
Marine organisms actively incorporate dissolved phosphorus into their biological processes. Phytoplankton, the microscopic primary producers at the base of the marine food web, absorb dissolved inorganic phosphorus from the surrounding water for their growth. Phosphorus is a fundamental building block for their DNA, RNA, and ATP.
As phytoplankton are consumed by zooplankton and other marine life, phosphorus moves through the marine food web. When organisms excrete waste or die, decomposers, such as bacteria, break down the organic matter. This process, known as remineralization, releases phosphorus back into the water column in its inorganic form, making it available again for uptake by primary producers. This continuous cycle is fundamental to marine productivity.
Deposition and Long-Term Reservoirs
Eventually, phosphorus leaves the active water column and becomes stored in long-term reservoirs. Dead organisms, fecal pellets, and other particulate organic matter sink towards the seafloor. This sinking material carries phosphorus out of the surface waters and into deeper ocean layers.
Over time, these phosphorus-rich materials accumulate on the ocean floor and undergo sedimentation, becoming buried within marine sediments. This process locks phosphorus away for extended periods, forming a long-term geological reservoir. Authigenic minerals, such as phosphorite, can form directly within these sediments, persisting for millions of years.
Consequences of Phosphorus Accumulation
When excessive amounts of phosphorus enter oceans, due to human activities like agricultural runoff and wastewater discharge, environmental problems can arise. This oversupply of nutrients can lead to eutrophication, stimulating rapid growth of algae, known as algal blooms. These blooms can be harmful, blocking sunlight from reaching underwater plants and disrupting the marine ecosystem.
When these large algal blooms eventually die, their decomposition by bacteria consumes dissolved oxygen in the bottom waters. This oxygen depletion creates hypoxic or anoxic conditions, often referred to as “dead zones,” where marine life cannot survive. Such zones reduce fish populations and negatively impact overall marine biodiversity.