Microdynamics and seasonal changes in manganese oxide epiprecipitation in Pinal Creek, Arizona

Abstract

In Pinal Creek, Arizona, Mn oxyhydroxides (MnOx) collect as thick precipitates on surface sediment, within the streambed, beneath algal mats, and on submerged and emergent plants and mosses. The proximate source of Mn is a thick, alluvial alkaline aquifer that was contaminated by past acid mine waste disposal practices associated with copper mines located upstream in the Globe–Miami area. Almost every organism in Pinal Creek is coated with MnOx. Some are actively precipitating manganese, and others are doing it passively. The variety and seasonality of epilithic biological processes resulting in Mn oxidation (epiprecipitation) was studied for more than a year by analyzing artificial substrates placed in surface water having different flows and different vegetation types and densities. Most epiprecipitation took place on the holdfasts of the green alga, Ulothrix sp., and the iron bacterium, Leptothrix discophora. Extensive patches of MnOx also coated extracellular polymeric substances of fungal hyphae and bacterial filaments. The dominant macroscopic precipitation was in the form of MnOx clumps on mosses, green algae, and cyanobacterial mats, consistent with precipitation by pH elevation during photosynthesis. Most oxidation occurred in the spring and summer, in agreement with thermal, biological, and chemical activity models. More biological oxidation occurred in swifter water, consistent with oxygen elevation models. The efficiency of this naturally occurring, diverse ecosystem suggests that remediation efforts to remove metal contaminants such as Mn should focus on creation of habitats that raise biodiversity.