Biogeochemistry of wetlands in watersheds affected by aerial deposition of metals, and linkages to aquatic ecosystem recovery.

Abstract

Wetlands are prevalent in the boreal shield, and are efficient at retaining metals and thus modifying the chemistry of downstream receiving waters. The Sudbury region of Ontario, Canada has suffered over a century of elevated metal and sulphur deposition from mining and smelting activity, and wetlands in the area have provided an important ecosystem service by adsorbing large concentrations of metals over decades of emissions. However, metals can be released from peat in some circumstances such as during dry periods that result in a drop in water table height. With emissions declines in the region, there is the potential that wetlands have remained contaminated and are now acting as a source of metals at annual scales. Stream metal concentrations and dissolved organic matter (DOM) exports were evaluated in six Sudbury wetland-draining streams to address the question: Are wetlands in mining-impacted watersheds releasing potentially toxic metals to receiving waters? Water table declines occured in response to summer droughts, and some metals (Al, Co, Cu, Fe, Mn, Ni, and Zn) were released during rewetting events. Other metals that have a high affinity for DOM (Cu, Al, Fe) were also released from some wetlands in the summer in response to mineralization and DOM release. Concentrations in streams exceeded provincial guidelines and fluxes of some metals (e.g. Ni, Co) exceeded atmospheric deposition inputs to lakes by as much as 12 times. Changes in DOM concentration and composition occurred concurrently with the mobilization of metals after drought, with large decreases in aromaticity (SAC340) coupled with decreased DOC concentration. Biotic ligand modelling indicated that these drought-related DOM changes had a large effect on Cu toxicity, and while predicted Ni toxicity increased because of drought-related increases in Ni concentrations, predicted Cu toxicity increased primarily because of drought-related DOM changes. It is likely that these metal stores will take a very long time to deplete, iv and this offers important insight into the recovery trajectory of aquatic communities in watersheds affected by smelting. The results can be used to advance restoration efforts in the Sudbury region and other similar smelter-impacted areas affected by aerial depositon of metals.