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April 2009 Williams and Denholm 81 ion complex AgC12" which is negatively charged (Pavlostathis et al. 1998). Each factor is discussed in more detail in the fol- lowing sections. 5.2 Dissolved Or4anic Carbon Many pubiished studies have identified Dissolved Organic Carbon ("DOC") as a significant factor in reducing the bioavailability of silver ion. For ex- ample, Karen et al. (1999) studied the effect of DOC (as humic acid) on the toxicity of silver ni- trate to rainbow trout (Oncoryhyncus mykiss), fathead minnows (Pimphales promelas) and wa- ter fleas (Daphnia magna). For at1 three species, increased concentrations of humic acid, measured as the percentage of car- bon, significantly increased the LC6o values in all treatments. In other words as the dissolved or- ganic carbon concentration increased, the silver ion became much less toxic. This cleady illus- trates the protective effect of DOC. In a study on the toxicity of silver to fathead min- nows and water fleas, reported by Erickson et al. (1998), the researchers were able to demon- strate that for the fathead minnows, Increasing the organic carbon to 2.5 mg C/L increased the 96-hr LC50 by 350%, and increasing organic car- bon to 10 mg C/L increased the 96-hr LC50 by 450°!0. As they noted, this is similar to other metals for which compiexation by organic matter also re- duces bioavailability. Furthermore, Erickson ef al. (1998) also found that the toxicity of silver ion to both organisms was much reduced when water from the St. Louis River was used in the test rather than normal laboratory water. The reduction in toxicity was some 60-fold for water fleas, although this is re- garded as a more sensitive organism than the fathead minnow. The researchers surmised that the higher organic carbon content in the St. Louis River water was responsible for the reduced tox- icity. The data are shown in Table 3 below. In a study of silver complexation in river waters in central New York, Whitlow and Rice (1985) had noted that the determined values for silver in the river waters were lower than calculated from a speciation model. They attributed the discrep- ancy to additional complexes formed with dis- solved organic carbon and or colloids that were not further identifieci. The importance of DOC complexes with silver, has also been emphasized by Hogstrand and Wood (1998) in their review of the bioavailability, physiology and toxicity of silver in fish. They nated that Janes and Playle (1995) have esti- mated a log K-- 9 for natural DOC collected from a marsh. The greater protectiveness of DOC compared with that due to hardness, is seen as particularly important for regions of soft-water which contain much organic carbon. Tabla 3; Acute toxicity of silver nitrate to juvenile fathead minnows and < 1-d-old Daphnia magna in laboratory water and St. Louis River water Tost Orpanism Test Water ? (m? A?? 95 ?G Conftdenco Lim{ts Pimpha/es prome/as Laboratory 10.4 8.6 - 12.5 River 106 97 - 114 ? Daphnia magna Laboratory 0.58 0.56 - 0.61 ? River ? -- -- ---- - 35 ------ 32 - 39 - --- , , LCso = median {ethal concentration - Scientific Papers -