Does the age of metal-dissolved organic carbon complexes influence binding of metals to fish gills?

We investigated the effect of aging on the strength of dissolved organic carbon (DOC) complexation of metals, as indicated by the ability of DOC to keep copper and cadmium off trout gills. The addition of 5–20 mg C 1 −1 DOC reduced the toxicity of Cu and Cd to small rainbow trout ( Oncorhynchus mykiss, 1 g). Exposures were to 0.2 μM Cu and 0.03 μM Cd for 2 weeks, in synthetic soft water. The DOC was isolated from surface waters of a marsh through reverse osmosis. Copper was kept off the gills by DOC but Cd was not, in agreement with the relative binding strengths of the metals to DOC and to the gills themselves. Aging 0.5 μM Cu, 0.1 μM Cd, and 5 mg C 1 −1 DOC solutions for 3 weeks did not change the ability of DOC to keep Cu or Cd off trout gills: 3 week old and fresh metal-DOC solutions kept Cu off the gills equally well, whereas Cd was always only partially kept off the gills by DOC. Our results indicate that efforts to model metal-gill interactions in fish from waters of differing chemistries and DOC content do not need to consider the age of metal-DOC complexes, because metal-DOC complexes do not become appreciably stronger with time.