A biogeochemical comparison of sea loch sediments. Manganese and iron contents, sulphate reduction and oxygen uptake rates

The sediments of the upper basins of Lochs Goil, Fyne and Etive were high in both solid phase extractable manganese (up to 3.7% w/w), and high in pore water manganese (up to 600 mu M), while Loch Linnhe sediments were low in manganese. Solid phase manganese from the surface sediments at the deepest stations was highest in those lochs where the mean residence time of the bottom water is longest, indicating the importance of manganese cycling through the water column. Porewater iron concentrations in the top 2 cm of sediment were highest near the heads of the lochs, i.e. near the main freshwater inputs. Within sediments of Lochs Goil, Fyne and Etive high rates of total sulphate reduction were associated with high relative rates of formation of acid volatile sulphide, but this correlation did not occur in Loch Linnhe; here the formation of pyrite was more important. Loch Linnhe sulphate reduction rates were higher than those in Lochs Goil and Etive, but fixed sulphur concentrations in the sediments were lower, indicating a greater proportion of the products of sulphate reduction were being reoxidised. The sulphide burial rate was high in Loch Etive and low in Loch Linnhe. Oxygen uptake rates by sediments showed an increase near the heads of the lochs, implying the presence of a component of the terrestrial organic input which was readily degradable by marine sediment bacteria.