Recycling in the Water Column

Association of P with major components of the particle flux was investigated to identify and quantify the processes controlling P removal and recycling in the water column. The approach involved coupling fluxes of particle components, derived from the components model, with the P concentration of the component obtained from chemical analysis of pure particle fractions. The isolation of clean samples of individual particle types was achieved through the combination of size fractionation and sampling over a range of times and depths. 

When considering the availability of nutrients, it is also necessary to examine the significance of nutrient re-use within the waterbody. These internal sources amount not to an additional load, but a multiplier on the recyclability of the same load. This nutrient recycling and the internal stores from which they are recycled are often misunderstood, but there is a dearth of good published data about how these recycling mechanisms operate. Microbial decomposition in the water column is one of several internal loops recognized in recent years, but these are not closed and the flux of nutrients recycled through them is delayed rather than retained. 

Most lakes affected by eutrophication will also have significant amounts of phosphorus in their sediments, which can be recycled into the water column (Section 4). The control of this source can be achieved by treating the sediments with iron salts or calcite to bind the phosphorus more tightly into the sediments. These methods have been used to some effect, but consideration has to be given to the quality of the materials used and whether or not the lake can become de-oxygenated in the summer. In the latter case this can be overcome by artificial de-stratification. 

The crude liquid chlorobenzenes stream leaving the second reactor is washed with water and caustic soda solution to remove all dissolved hydrogen chloride. The product recovery system consists of two distillation columns in series. In the first column (the benzene column ) unreacted benzene is recovered as top product and recycled. In the second column (the chlorobenzene column ) the mono- and dichlorobenzenes are separated. The recovered benzene from the first column is mixed with the raw benzene feed and this combined stream is fed to a distillation column (the drying column ) where water is removed as overhead. The benzene stream from the bottom of the drying column is fed to the reaction system. 

The amount of P supplied by resuspension was relatively small compared with water-column standing pools and major flux vectors. Thus, resuspension of bottom sediments may not be a major mode of phosphorus resupply. The pool of resuspendable P is finite. The deposition-resuspension cycle will not increase the amount of P in this pool unless P is added from another source (e.g., by diffusion of P from lower sediment levels). However, the diffusive flux would be relatively small. The resuspendable particulate P can be recycled during spring mixing by repeated deposition and resuspension, but this cycle does not increase the amount of P in the resuspendable pool. Eadie et al. (24) reported a resuspended P flux (sediment-trap-based) of3200 mg of P/m2, 66 times our estimate here. However, this large P flux would require the resuspension of over 2.0 cm of surface sediment and much higher suspended Al levels than were measured in the water column. 

Commercial production of ethanolamines (EOA) is by reaction of ethylene oxide with aqueous ammonia. The ethylene oxide reacts exothermically with 20% to 30% aqueous ammonia at 60 to 150°C and 30 to 150 bar in a tubular reactor to form the three possible ethanolamines (mono-ethanolamine - MEA, di-ethanolamine - DEA and tri-ethanolamine - TEA) with high selectivity. The product stream is then cooled before entering the first distillation column where any excess ammonia is removed overhead and recycled. In the second column, ammonia and water are removed and the EOA s are separated in a series of vacuum distillation columns. 

Recycling of N and P occurs in the water column and at interfaces between the water and substrata such as profundal sediments. In Lake Calado, regeneration of ammonium and phosphorus is dominated by planktonic heterotrophs less than 53 pm in size (Table 14.7, Lenz et al. 1986, Fisher et al. 1988a, Fisher et al. 1988b, Morrissey and Fisher 1988). Sediment-water exchange is smaller than planktonic processes, but is substantial... 

Figure 17.3 Si, P, and N budgets for the Amazon shelf. Recycling of P and N in the water column is necessary to sustain the primary production on the shelf. Little Si, P, or N are buried in the seabed. Approximately 40% of the bioavailable dissolved N supplied to the shelf, however, is deposited as organic matter and then converted to ammonium/nitrate and finally to molecular nitrogen, making it unavailable to oceanic plankton.

For the Amazon shelf, recycling of N and P in the water column is essential for sustaining primary production (providing -60% of the total nutrient uptake). In contrast, silicate supply to the shelf from rivers, upwelling, and surface mixing is sufficient to sustain all of the siliceous productivity on the shelf. 

Welsh, D. T. (2003). It s a dirty job but someone has to do it The role of marine benthic macrofauna in organic matter turnover and nutrient recycling to the water column. Chem. Ecol. 19, 321—342. 

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