Manganese Dynamics in the Water Column of the Upper Basin of Loch Etive, a Scottish Fjord

The concentrations of dissolved and particulate manganese and iron and of dissolved oxygen in the water column have been measured as a function of depth in the upper basin of Loch Etive, over a 17 month period. For most of this period the bottom water was isolated, which caused low dissolved oxygen concentrations within the water column and concomitant high levels of manganese. The highest concentrations of manganese were close to the bottom. During the study period an overturn event occurred which caused complete mixing of the water column, and flushing of the low oxygen, high manganese deep water. By using published rate constants for the oxidation of Mn2+ and by assuming steady state, we estimate that the oxidation of dissolved Mn2+ (and by implication the return of solid phase manganese to the sediment) occurred at the rate of 2·2–5·4 mmole m 2 d 1, on an areal basis during the period of isolation. Mn2+ efflux experiments gave values of 0·44 and 0·81 mmole m 2 d 1.