The Influence of Iron and Manganese on Nutrient Cycling in Shallow Freshwater Antarctic Lakes

Two lakes, mesotrophic Heywood Lake and oligotrophic Sombre Lake, at Signy Island, have been studied over 1 full yr (1980-1981). The development of deep water anoxia in Sombre Lake is accompanied by release, from the sediments, of nitrogen, silicon, phosphorus, iron, and manganese. Iron is released from anoxic sediments in massive amounts, but is quickly returned through oxidation to iron oxides, near the anoxic-<>xic boundary and through possible ferrous carbonate precipitation, within the anoxic region. Phosphorus is also quickly returned to sediments, at the oxic-anoxic boundary, which suggests efficient co-precipitation-sorption by the recently oxidized ferrous. Phosphorus is therefore unable to enter the main waterbody. Melt water inflows are the source of P supply to the surface waters. Nitrogen and Si appear to have little association with axides and, therefore, freely pass into the oxic main water-body of the oligotrophic system. There are strong seasonal correlations between the vertical profiles of Fe and P, under the ice cover in Heywood Lake. Nitrogen profiles appear independent of the other determinants, Si, P, Fe, and Mn. Towards the end of the period of bottom-water anoxia, during the spring melt, high concentrations of ferrous, released from the anoxic sediments, have been detected in oxygenated waters. This apparent stability, in the presence of oxygen, may be attributable to a combination of low pH and the possible influence of high concentrations of humic acids from the melt inflows. Sulphide concentrations may become sufficiently high to influence Fe transport significantly, probably through ferrous sulphide precipitation, within the anoxic region.