Fluxes and Pools of Phosphorus in the Biosphere

Conceptual models of sub-global phosphorus cycles (after Jahnke, 1992) Conceptual ideas behind simulation of P cycling are related to the construction of models for freshwater terrestrial ecosystems and a generalized oceanic system and understanding the restrictions of its application. 

Unlike a temperate lake, stratification in the ocean does not completely break down in the winter and only a few nutrients come up due to exchange between deep and surface water layers. More important sources of P to the photic zone (surface oceanic layer) are the major upwelling regions. Under oceanic currents and wind influence the open oceanic waters exchange with waters from the continental margins accompanied by the corresponding input of nutrients. 

Once in the photic zone, P is readily incorporated into biogenic particles via the photo synthetic activity of plankton and begins to sink. The majority of these particles decompose in the surface waters or in shallow layers of sediments and the P is recycled directly back into the photic zone to be incorporated into biological particles. A small part of these particles escapes the surface layer and sinks into the deep ocean. Most of these particles eventually decompose and the cycle is repeated. Only a very small faction of these particles may incorporate into sediment without decomposition. 

The additional forms in which P is removed from marine waters are the burial of P adsorbed onto the CaCOs surface and the formation of apatite in continental margin sediments. In lesser degree, this is related to the adsorption of P onto iron oxyhydroxide phases and decomposition of fish debnis. 

Turnover of P in the aquatic ecosystems is expressed by significant values, because of its importance for living aquatic organisms (Box 6). 

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