Manganese behavior in the sediments of diverse Scottish freshwater lochs

Abstract

Mn concentrations were determined in sediments and pore waters of five freshwater lochs in Scotland. The aim was to establish whether the geochemical behavior of Mn differed in a range of limnological conditions: oligotrophic/mesotrophic Loch Lomond; eutrophic unstratified Loch Leven; seasonally stratified eutrophic Balgavies Loch; oligotrophic, acidified Round Loch of Glenhead; and oligotrophic, acid-sensitive Loch Coire nan Arr. Although redox-driven diagenesis of Mn was evident in all lochs, the diversity of conditions in the lochs resulted in some cases in modifications to behavior predicted by conceptual models for transport of Mn at a redox boundary. These differences were reflected in the chemical associations and trends in concentration of Mn. A classic example of Mn redox cycling in the sediments of Loch Lomond provided a basis for comparison with other systems. Higher inventories of Mn in sediment collected from eutrophic Loch Leven during the summer, compared with sediment from autumn and winter sampling, were explained by the effects of enhanced primary productivity (high pH and dissolved oxygen concentrations) during a sumer algal bloom. In Balgavies Loch, where the bottom water was hypoxic, at the time of sampling, previous diagenetic enrichment of Mn in the surface sediment was subject to modification by dissolution and release from the solid phase to pore water, which was elevated in Mn. The increase in solid-phase Mn concentration with depth and lack of significant surface enhancement of Mn in Round Loch contrasted markedly with results from other lochs. This was attributed mainly to acidification, producing a pH-related decrease in Mn sedimentation. Mn partitioning and porewater data for Loch Coire nan Art were in some respects similar to those at Round Loch, with both lochs having peaty sediment. However, Mn concentration patterns attributed to acidification effects at Round Loch were not found in Loch Coire nan Arr.