INFLUENCES OF OLIGOMIXIS ON THE WATER AND SEDIMENT CHEMISTRY OF LAKE KUTUBU, PAPUA-NEW-GUINEA

Lake Kutubu in the Southern Highlands Province of Papua New Guinea is oligomictic. Seasonal depth profiles of temperature, oxygen, nutrients ana other physicochemical features usually indicated two distinct layers with a metalimnion between 10-25 m depth in a lake with a mean depth of 36 m. The epilimnion was well-oxygenated and was between 1.6 and 3.3-degrees-C warmer than the deoxygenated hypolimnion. Elevated concentrations of nitrite, ammonium, soluble reactive phosphorus, iron, manganese, major cations and silica in the hypolimnion indicated prolonged stability of the water column. Analysis, by x-ray fluorescence, of ashed sediment samples from fifty-five sites showed that elements affected by redox processes (Mn, Fe and P) had become concentrated in the deeper sediments. Conversely, Ca and Sr were concentrated in littoral sediments and concentrations of Zr, Ti, Mg, Ga, Ni, Y, Zn, Pb and Nd were highest near the major inflows. Stratification disruption occurs irregularly (and probably rarely) following the coincidence of cold weather and strong winds. In September 1990, a tilted oxycline, fish deaths and high concentrations of particulate iron were recorded in areas of hypolimnetic upwelling. The interplay of water column stability and disruption explains the distinct patterns of redox elements in the surface sediments. The patterns are further modified by the hydrology of the lake. Inflowing waters are significantly cooler than epilimnetic waters and therefore dissolved and suspended materials entering the lake are probably deposited directly in the hypolimnion. The proximity of the major inflows to the outflow and the shallowness of the outflow accentuate the role of the Kutubu basin as a materials sink. This conclusion is supported by the very high concentrations of iron (up to 19.8 % Fe2O3), phosphorus (up to 1.74 % P2O5) and manganese (up to 1.56 % MnO) in sediments from the deeper parts of the lake.