MANGANESE BEHAVIOR IN THE SEDIMENTS OF DIVERSE SCOTTISH FRESH-WATER LOCHS

Mn concentrations were determined in sediments and pore waters of five freshwater lochs in Scotland. The aim was to establish whether the ge ochemical behavior of Mn differed in a range of limnological condition s: oligotrophic/mesotrophic Loch Lomond; eutrophic unstratified Loch L even; seasonally stratified eutrophic Balgavies Loch; oligotrophic, ac idified Round Loch of Glenhead; and oligotrophic, acid-sensitive Loch Coire nan Arr. Although redox-driven diagenesis of Mn was evident in a ll lochs, the diversity of conditions in the lochs resulted in some ca ses in modifications to behavior predicted by conceptual models for tr ansport of Mn at a redox boundary. These differences were reflected in the chemical associations and trends in concentration of Mn. A classi c example of Mn redox cycling in the sediments of Loch Lomond provided a basis for comparison with other systems. Higher inventories of Mn i n sediment collected from eutrophic Loch Leven during the summer, comp ared with sediment from autumn and winter sampling, were explained by the effects of enhanced primary productivity (high pH and dissolved ox ygen concentrations) during a summer algal bloom. In Balgavies Loch, w here the bottom water was hypoxic, at the time of sampling, previous d iagenetic enrichment of Mn in the surface sediment was subject to modi fication by dissolution and release from the solid phase to pore water , which was elevated in Mn. The increase in solid-phase Mn concentrati on with depth and lack of significant surface enhancement of Mn in Rou nd Loch contrasted markedly with results from other lochs. This was at tributed mainly to acidification, producing a pH-related decrease in M n sedimentation. Mn partitioning and porewater data for Loch Coire nan Arr were in some respects similar to those at Round Loch, with both l ochs having peaty sediment. However, Mn concentration patterns attribu ted to acidification effects at Round Loch were not found in Loch Coir e nan Arr.