Pb and Fe flow through the mire-lake complex of Skogaryd catchment – a system under anthropogenic influence

Abstract

Sphagnum-dominated peatlands store not only vast amounts of carbon (C) but also metals derived from bedrock and anthropogenic sources. Some metals are involved in peat C stability, and some are toxic to living organisms. However, the pools of such metals and their export in relation to the export of C in the form of dissolved organic C (DOC) in mire-lake complexes have received little attention. We characterized a hemiboreal mire in Sweden previously exposed to heavy metal pollution in terms of physiochemistry, age, total C pool, lead (Pb) and iron (Fe) content. We investigated export of Fe and Pb in relation to DOC in the mire-lake complex. We found strong and linear correlations of Fe and DOC export in streams and the export occurred in similar ratios from both the mire and the downstream lake. The export of Fe and DOC seemed to be controlled by hydrological connectivity, while the production of DOC and mobilisation of Fe and Pb most likely was controlled by drought conditions. We found the Pb content exceeded threshold values for toxicity in the top layer of the peat and in stream water leaving the mire. Stream water concentrations of Pb were, as for Fe and DOC, highest after drought periods. Lead isotope analysis revealed that Pb leaving the mire accumulated through anthropogenic contamination (most likely from gasoline), while Pb leaving the lake appeared to contain natural sources as well. In addition, the lake appeared to be a sink for the anthropogenic Pb exported from the mire through sedimentation. This study suggests that peat decomposition in peatlands affected by climate change may pose a risk to not only losing stored C, but also through release of heavy metals to the local environment.