Effect of pH on photo-oxidation of dissolved organic carbon by hydroxyl radicals in a coloured, softwater stream

Abstract

Previous work has shown that the photo-oxidation rate of dissolved organic carbon (DOC) exposed to solar radiation was significantly enhanced in acidic stream waters and that the average photo-oxidative loss of DOC in the UVA region was more than 60%. This study examined the effect of pH on photobleaching and the photo-oxidative loss of DOC and the degree of photo-oxidative loss by hydroxyl radicals ( ∑ OH) over the pH range 4-9 in a coloured, softwater stream. Binding iron with NaF lowered the photo-oxidation rate constant by about 50% at pH 4 and by 10-20% at pH 4.6 and 4.9. Hence, iron plays an important role in the photo-oxidative loss of DOC at pH 4 with declining importance as the pH increases. The importance of ∑ OH in photo-oxidative loss of stream DOC was evaluated by adding KI to quench ∑ OH. Aquatic SciencesAfter 69 hours of artificial irradiation without KI, DOC loss decreased as pH increased from pH 4 to 9 whereas the minimum loss of absorbance at 300 nm occurred at pH 7. KI significantly reduced loss of DOC at pH 4, 5 and 7 but not at pH 9 with the fraction of DOC lost by non-∑ OH mechanisms gradually increasing from 58% to 75% between pH 4 and 7, and 100% at pH 9. Since many northern freshwater lakes on the Precambrian Shield receive large fluxes of UV-labile humic matter from peatlands and have pH < 7.5, photo-oxidative loss of DOC by both non-∑ OH and ∑ OH mechanisms is important in C dynamics in northern freshwaters but with photo-oxidative loss dominated by non-∑ OH mechanisms likely operating in the UVA region.

Long‐term patterns of dissolved organic carbon in lakes across eastern Canada: Evidence of a pronounced climate effect

“…Of the large number of regional and global variables considered, only photosynthetically active radiation (PAR) and precipitation were found to be strongly related to the long-term pattern of DOC in their lakes. Although short-term studies (e.g., days) had previously documented that solar radiation (both UV and visible) degraded DOC (e.g., Gennings et al 2001;Molot et al 2005), this was the first study to provide evidence that a negative long-term effect was present. However, the generality of their result to other regions beyond the Dorset lakes is not clear and further testing was thought to be warranted.…”

“…Through photochemical processes, solar radiation, particularly UV, has been shown to cause reductions in DOC of ,20-60% over the course of days (11-70 d [Kö hler et al 2002;Molot et al 2005]). Hudson et al (2003) compared the long-term DOC pattern with regional and global-scale variables and determined that TSR had a negative correlation with the long-term DOC pattern in eight lakes in Dorset, central Ontario.…”

“…An increase in regional precipitation is expected to cause a greater loss of organic carbon from terrestrial systems (Clair et al 1994) and a reduction in precipitation is, thus, expected to cause a decline in the export of DOC from terrestrial to aquatic systems (Schindler et al 1997) because DOC mass export and runoff are tightly coupled (Dillon and Molot 2005). Increases in regional precipitation would, therefore, be expected to increase the export of DOC; however, an increase in precipitation may not always correspond with an increase in the concentration of DOC in lakes, in fact, heavy precipitation events may actually dilute lake DOC concentrations, particularly if such events occur when DOC is not readily available from terrestrial or wetland Table 4.…”

“…DOC originates mainly from terrestrial systems and from in-lake processes (Aitkenhead-Peterson et al 2003;Lennon 2004). The quality and quantity exported from terrestrial systems is affected by watershed properties, such as the proportion of wetlands (Dillon and Molot 1997), vegetation type (Sobek et al 2007), land use (Findlay et al 2001), runoff (Dillon and Molot 2005) and geology, morphology, and geography of the surrounding area (Moore 1998). Furthermore, within-lake characteristics also affect DOC.…”

“…Furthermore, within-lake characteristics also affect DOC. For example, lake acidity and Fe concentration affect the rate of DOC photodecomposition (Anesio and Graneli 2003;Molot et al 2005).…”