Phosphorus is very important in the marine environment. It is a necessary component of the chemicals of life, ATI' (Adenosine TriPhosphate) and DNA (DeoxyriboNucleic Acid), and is also part of bones, teeth, and shells. Phosphorus enters the marine environment from the land. The breakdown of rock and sediments releases inorganic phosphorus to wash from land to sea. Phosphorus released from sewage and fertilizer also eventually finds the sea. Phosphorus occurs in three forms in marine waters: Dissolved Inorganic Phosphorus (DIP) as orthophosphate and phosphate, Dissolved Organic Phosphorus (DOP) as part of dissolved organic compounds, and as Particulate Organic Phosphorus (POP) in organic particles, Including components of detritus. Phosphorus is not a part of the atmosphere and there is no exchange toward equilibrium with atmospheric gas. However, dissolved inorganic phosphate (DIP) readily leaves seawater incorporated in tiny droplets of salt spray along with other organic and inorganic compounds. DIP coats air bubbles in seawater, travels with the bubbles to the surface, and leaves the water with the transport of salt spray. This mechanism for phosphate loss from seawater is more of a factor in marine aquariums where spray particles are
lost from the system than in the natural environment. In fact, these "aerosol particles" composed of phosphate and organic compounds form particles that are an important food source to planktonic organisms. DIP is converted to POP though air/water interaction at the sea surface.
readily take up dissolved organic phosphate (DOP) and dissolved inorganic phosphate (DIP), and in the natural environment, the amount of phosphate present is often the limiting factor in the bloom of microalgaes. Phosphate levels are higher in coastal areas near rivers and near oceanic upwell ings of water from the deeps. It is in these areas that phytoplankton blooms are most common. Phosphorus moves from algae as DOP and POP when the algaes decompose and also into the tissues of animals when consumed. A great deal of phosphorus becomes part of the detritus in the form of particu late organic detritus (POP). Were it not for the legions of bac teria and animals that feed on detritus and transform POP into DIP in the process, most marine phosphorus would be lost in the sediments. The phosphorus cycle in seawater is illustrated in Figure 10.
In the natural environment, especially over coral reefs, the amount of DOP and DIP is very low, less than 30 parts per billion in most coral reef environments. Phosphorus tends to accumulate in marine aquarium systems, however, through excretion from plants and animals, decay of plant and animal tissues, and animal and bacterial processing of detritus. Excess phosphorus, over I to 2 parts per million (mg/l), vigorously encourages plant growth, and interferes with calcification in coral growth; and unless there is a luxuriant growth of macro algaes, growth of undesirable blue-green algaes and encrusting red algaes are stimulated. Aside from algal growth, phosphorus accumulation can be controlled by water exchange, protein foam skimming, and removal of excess detritus. Reduction of the animal loading of the aquarium and/or increasing the total water volume of the system are other methods that will improve the balance of plant and animal life and reduce nutrient accumulation.
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