Factors Influencing the Bioavailability of Fe and P Bound to DOM

Given that the Fe and P bound to the DOM in humic lakes are probably not directly available to organisms, the DOM-Fe-P complex would have to undergo some form of transformation to yield bioavailable Fe and P species. In terms of biological uptake, in general it can be said that Fe is preferred in the dissolved free ionic form and P as orthophosphate (P04). Several known processes result in the transformation of Fe and P from the DOM-Fe-P complex into biologically available species. Two of the mechanisms are physical-chemical processes (1) UV-mediated photoreduction and (2) DOM-mediated chemical reduction or dark reduction. Other mechanisms are 

Of all the mechanisms that should enhance the bioavailability of complexed Fe and P, the photochemical reduction of Fe and the subsequent release of P from the DOM-Fe-P complex have received the most attention 

FIGURE 6 The REDOX cycling of Fe bound to DOM via (A) photochemical and (B) dark and (C) biological reduction. Oxidation of Fe is considered immediate, given that the reactions are taking place in oxygenated water and in the presence of H202. Associated P04 release is depicted however, where this release has not been observed, but is suspected, it is denoted with a question mark. 

In a comparison between clear and colored lakes, Francko and Heath (1979) found that P04 was released with UVR exposure in a colored lake, but that UVR had no effect on P04 release in the clear system. Upon further investigation, the release of P04 was indeed linked to the reduction of Fe from the complex (Francko and Heath, 1982). Corner and Heath (1990) examined the effects of added Fe and UVR on the release of P04 in a temperate acid bog lake over the course of the growing season. They found that the amount of P04 released increased with added Fe and UVR exposure. 

Fe is made more bioavailable at depth and at night. Whether P is released via this dark reduction remains unknown, but this could also be an important mechanism of P release, particularly at depth. 

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