Dissolved Phosphate

The circulation of the ocean is driven by inputs of dense water, fluxes of heat and fresh water at the surface, and by the winds. Embedded in this general circulation are dissolved and particulate inorganic and organic nutrient forms of nitrogen, phosphorus, silicon, and trace metals such as iron, copper, zinc, etc. Nutrients are chemicals that marine organisms require for growth that must be taken in from the water column. Their concentration result from physical and biogeochemical processes that balance sources and sinks. Photosynthetic plants use nutrients in the water column to synthesize chlorophyll, the compound used in photosynthesis to turn light energy into chemical energy, releasing oxygen and taking up carbon dioxide. Nutrients needed in relatively large quantities are referred to as macronutrients, and those in small amounts as micronutrients. When marine organisms die and decompose, their organic tissue breaks down releasing nutrients and carbon into the water while consuming dissolved oxygen in the process. With notable exceptions, the concentration or abundance of nutrients in the ocean is relatively low at the surface layer (< 100 m) because of nutrient uptake and high at depth because of oxidation of soft organic matter and dissolution of hard parts. The nutrient in the shortest supply relative to the others will be exhausted first and will thus limit biological growth (e.g., limiting nutrient). This biological uptake of nutrients, when available, by marine photosynthetic plants takes place primarily in the near-surface water column in what is commonly called the photic zone, the layer where sunlight penetrates allowing photosynthesis to take place. All organisms in the ocean contain more or less constant proportions of nutrients, oxygen, and carbon. Nutrient concentrations are generally higher in the Southern Ocean south of about 50°S and in the N.E. Pacific basin.

Nutrients are chemicals that marine organisms require for growth that must be taken in from the water column. Their concentration result from physical and biogeochemical processes that balance sources and sinks.  Phosphorus is present in all marine living cells as a necessary part of proteins, enzymes and metabolic processes involved in the synthesis and transfer of energy including photosynthesis. Phosphorus in seawater is available primarily as dissolved inorganic phosphate ions (ortho-phosphate), and lesser amounts of dissolved and particulate organic phosphorus. Phytoplankton and marine plants generally satisfy their phosphorus requirements by the uptake of available phosphate. With notable exceptions such as in the Antarctic, the concentration or abundance of nutrients in the ocean is relatively low at the surface layer (< 100 m) because of nutrient uptake and high at depth because of oxidation of soft organic matter.   For comparison, notice the difference between phosphate abundance at the surface and that at 500 m depth in the Atlantic Ocean. Surface waters are generally more depleted of nutrients than those at deeper levels. When deep water circulates to the surface, a process called upwelling, a spike in oceanic productivity usually occurs, associated with plankton blooms. For example, the west coast of the U.S. is a high upwelling region particularly during summer, and thus contains high marine production levels.  Excessive runoff of phosphate from the land, mostly due to agricultural fertilizers, and carried out to sea by rivers or wind deposition can create an overabundance of the nutrient, a process called eutrophication. As plankton bloom and subsequently die off, dead zones may occur as the bacteria decomposing organic matter consume available dissolved oxygen.

The phosphate maps represent the large-scale horizontal distribution of this nutrient at selected standard depth levels of the world ocean on a one-degree latitude-longitude grid in the World Ocean Atlas (Garcia et al., 2009). These maps were computed by objective analysis of all scientifically quality-controlled historical phosphate data in the World Ocean Database (Boyer et al., 2009). The online version of World Ocean Atlas 2009 Figures contains a collection

Boyer, T. P., J. I. Antonov , O. K. Baranova,H. E. Garcia, D. R. Johnson, R. A. Locarnini, A. V. Mishonov, T. D. O’Brien, D. Seidov, I. V. Smolyar, M. M. Zweng, 2009. World Ocean Database 2009. S. Levitus, Ed., NOAA Atlas NESDIS 66, U.S. Gov. Printing Office, Wash., D.C., 219 pp., DVDs. Available at NODC World Ocean Database 2009.