Marine Phosphorus Cycling

Fig. 14-6 Profiles of potential temperature and phosphate at 21= 29'N, 122= 15'W in the Pacific Ocean and a schematic representation of the oceanic processes controlling the P distribution. The dominant processes shown are: (1) upwelling of nutrient-rich waters, (2) biological productivity and the sinking of biogenic particles, (3) regeneration of P by the decomposition of organic matter within the water column and surface sediments, (4) decomposition of particles below the main thermocline, (5) slow exchange between surface and deep waters, and (6) incorporation of P into the bottom sediments.

Fig. 14-6 Profiles of potential temperature and phosphate at 21= 29'N, 122= 15'W in the Pacific Ocean and a schematic representation of the oceanic processes controlling the P distribution. The dominant processes shown are: (1) upwelling of nutrient-rich waters, (2) biological productivity and the sinking of biogenic particles, (3) regeneration of P by the decomposition of organic matter within the water column and surface sediments, (4) decomposition of particles below the main thermocline, (5) slow exchange between surface and deep waters, and (6) incorporation of P into the bottom sediments.

temperate lake in summer. Also included in the figure are the major processes responsible for controlling this shape. In general, dissolved P is nearly undetectable in the euphotic zone (generally the upper 20-100 m) and increases to maximum concentrations of 1-3 fiM at approximately 1000 m. The distribution can best be envisioned as the balance between the incorporation of P into organisms with the eventual sinking of some fraction of this P from the surface waters and the constant slow rate of return of P to the surface layer by physical processes. The majority of ocean deep water is formed in the North Atlantic and slowly spreads sequentially to the South Atlantic, Indian, and finally to the Pacific Oceans. Because of the continuous rain of particulate P into the deep waters from the surface layers, the deepwater PO4 concentration increases progressively from the North Atlantic to the Pacific.

Unlike the temperate lake, stratification in the ocean does not completely break down in the winter. Except for specific high-latitude regions of the ocean, processes other than deep convec-tive overturn are responsible for returning the P stored in the deep waters to the surface. A relatively slow exchange occurs between water layers everywhere in the ocean and this supplies some P to the surface ocean (process 5 in Fig. 14-6) from the intermediate layers below. More important sources of P to the photic zone are the major upwelling regions generally located adjacent to the western, sub-tropical continental margins (1) and in equatorial divergence zones. In the western margins, the prevailing winds tend to transport surface water offshore. This water is replaced by nutrient-rich water from below. At these locations P input (along with other required nutrients) is not limited by slow diffusive transport processes but is enhanced many-fold by the upward advection of water. For this reason, upwelling areas are capable of supporting extremely high rates of biological primary production and abundant populations of higher organisms. Thus, the major fisheries of the world are concentrated in upwelling regions such as off Peru. A significant amount of P is also returned to the surface ocean in cold, high-latitude regions where decreased stratification results in greater vertical mixing than in the temperate and equatorial regions.

Once in the photic zone, P is readily incorporated into biogenic particles (2) via the photo-synthetic activities of plants and some fraction of the biogenic materials subsequently sinks. Increasingly, improved technologies permit the dynamics of this system to be followed by studying 32P and 33P distributions. The majority of the particles decompose in the surface layer or in shallow sediments and the P is recycled directly back into the photic zone (3) to be reincorporated into biological particles. A small portion of the particles produced in the surface layers, however, does escape the surface layers and sinks into the deep ocean. Most of these particles eventually decompose (4) and the cycle is repeated. A very small fraction of these particles, however, escapes decomposition and is incorporated into the sediment (6). P appears to be buried in sediments primarily as organic P, apatitic P (including both authigenic apatite and fish debris), P associated with other mineral phases (primarily CaC03 and FeOOH) and P loosely sorbed on to other solid phases (Rutten-berg, 1993; Follmi, 1996; Anschutz et al, 1998).

14.3 The Global Phosphorus Cycle

The main reservoirs and exchange pathways of the global P cycle are schematically presented in Fig. 14-7. This representation is primarily taken from Lerman et al. (1975) and modified to include atmospheric transfers. The mass of P in each reservoir and rates of exchange are taken from Mackenzie et al. (1993) and Follmi (1996).

In choosing these reservoirs to describe the P cycle, compromises were made to maintain a general focus and global scale and yet avoid being too general and hence lose information about important transfers and reservoirs. The following is a brief discussion of the rationale behind the choice of the reservoir definitions and their estimates. For the purpose of discussion, the reservoirs have been numbered as presented in Lerman et al. (1975) with the addition of the atmosphere (reservoir 8). The total P content of each reservoir and comments concerning the estimate are provided in Table 14-3.

land biota

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