Element biolimiting

The biolimiting elements (also called nutrients or micronutrients) tend to be present at low, and often undetectable, levels in the surface waters at mid- and low latitudes. Examples of nutrient depth profiles are provided in Figure 9.1 for a mid-latitude site in 221... 

The nutrient profiles are characterized by much higher concentrations in the deep-waters than in the surface. In some locations, such as shown in Figure 9.1, mid-water concentration maxima are present. The depth region over which concentrations exhibit the largest vertical gradients is usually defined by the thermocline. All biolimiting elements have similar depth profiles, having surfece-water depletions and deep-water enrichments. 

As exemplified by the silicate profile, all biolimiting elements do not behave identically. In the case of dissolved silicon and TDIC, their concentration maxima lie below the nitrate and phosphate maxima. This reflects the different mechanisms by which the elements are resolubilized. Nitrate and phosphate are regenerated from soft parts. This process seems to occur more readily than the dissolution of hard parts, which releases silicate causing the nitrate and phosphate concentration maxima to lie at shallower depths. Since TDIC is released in nearly equal amounts from soft parts as CO2 and the dissolution of calcareous hard parts as CO3, the resulting concentration maximmn lies below that of nitrate and phosphate. 

The vertical distribution of biolimiting elements is characterized by deep-water enrichments and surface-water depletions. As described above, this vertical segregation is caused by the remineralization of biogenic particles in the deep sea. Not all particulate matter that sinks into the deep zone is remineralized. Some survives to become buried in the sediments. How much of the biogenic particle flux escapes from surfece waters How much of this particle flux is remineralized in the deep zone How much is lost from the ocean by burial in the sediments What effect does this have on the concentrations of the biolimiting elements ... 

Box model for the biolimiting elements. Source From Broecker, W. S. (1974). Chemical Oceanography, Harcourt, Brace and Jovanovich Publishers, pp. 14-15. 

From the perspective of the surface box, the biolimiting elements are supplied via river runoff and from upweUing. The elements are removed via the sinking of biogenic particles and downwelling. Since this model considers only the transport of materials into and out of the ocean and between the two reservoirs, details as to what happens to the elements while they reside in the boxes are not needed other than that they are present in a steady state. In such a case, the input rate of a biolimiting element will equal its output rate. For the surface-water reservoir, the mass balance that describes this steady state is given by... 

The first three fluxes can be calculated by multiplying the biolimiting element concentration (C) by the annual water transport (p) that is,... 

If the ocean is in steady state with respect to a biolimiting element, then its flux into the ocean must equal its flux out that is. 

The overall oceanic recycling efficiency of a biolimiting element is given by the fraction of the river input that is buried in the sediments during one complete mixing cycle. This is calculated as... 

The residence time of a biolimiting element can be calculated ffom/x and the average mixing time of the ocean (lOOOy) as follows ... 

Table 9.1 contains a summary of recycling efficiencies for a variety of representative elements. The results fall into three groups, which have been termed biolimiting, biointermediate, and biounlimited. The biolimiting elements have 1 and, thus, are almost... 

Some biointermediate elements, such as calcium, have residence times longer than the biolimiting elements. In the case of calcium, this element is a major component of... 

In Chapter 11, we will discuss a fourth category of elements, one with vertical profiles nearly opposite to the biolimiting elements. These elements have surfece-water enrichments and bottom-water depletions. Most are trace metals that adsorb onto sinking particles enabling their transport to the sediments. These elements tend to have shorter residence times than the biolimiting elements because they lack the remineralization step. Still other elements have a foot in both camps in some locations, they exhibit biolimiting behavior and have profiles with surface-water depletion and bottom-water enrichments, and in other locations, the profiles appear to be controlled by particle adsorption. Iron is an example of such an element. 

Nutrients are carried back to the sea surface by the return flow of deep-water circulation. The degree of horizontal segregation exhibited by a biolimiting element is thus determined by the rates of water motion to and from the deep sea, the flux of biogenic particles, and the element s recycling efficiency (/and from the Broecker Box model). If a steady state exists, the deep-water concentration gradient must be the result of a balance between the rates of nutrient supply and removal via the physical return of water to the sea surface. 

As indicated in Table 10.1, this ratio is greater than 1 for all the biolimiting elements, demonstrating that their deepwater concentrations are greater in the Pacific than in the Atlantic Ocean since at most locations, is much less than Cj ep The... 

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