Phosphorus dynamics in streams is influenced by physical, chemical, and biological processes (Figure 11.4). The principal biological processes are autotrophic and heterotrophic uptake of dissolved inorganic P and its assimilation into cellular constituents, transfer of organic P through the food chain, and its eventual release and mineralization by excretion and the decomposition of egested material. Algae and microbes in biofilms likely obtain the majority of their P from the water column, but rooted macrophytes and benthic algae can remove P from the sediments as well. Phosphorus may be excreted or released following cell lysis directly as dissolved inorganic P, or released as dissolved organic P, which subsequently is mineralized to orthophosphate by bacterial activity. The decomposition of organic matter including feces, dead organisms, and leaf litter also releases P into the water column and sediment pore water (Mainstone and Parr 2002). In addition to these biological processes, P availability is influenced by physical-chemical transformations. Sorption of orthophosphate onto charged clays and charged organic particles occurs at relatively high P concentrations, while desorption is favored by low concentrations. Sorption-desorption reactions thus act as a buffer on dissolved P concentrations. In addition, under aerobic conditions, dissolved inorganic and organic P both may complex with metal oxides and hydroxides (such as Fe(OH)3) to form insoluble precipitates. This phosphate is released under anaerobic conditions, and since the extent of the anaerobic zone tends to vary seasonally with organic matter loading, the availability of dissolved phosphate varies accordingly.
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