Long-term Variation in the Stability of the Meromictic Lake Nordbytjernet Caused by Groundwater Fluctuations

A zone with permanent anoxic conditions prevailed in the deep waters of Nordbytjernet before 1978 due to a significant depth gradient in dissolved substances. High concentrations of manganese and bicarbonate in the deep waters were most important for the lake's stability. The electrolyte concentrations in the mixed layer increased between 1978 and 1991, wiping out the depth gradient. A subsequent study showed a major variation in the solute concentration in the most important surface tributary that depends on fluctuations in the elevation of the groundwater table. A rising groundwater table seems to enhance oxidation of iron sulphides and dissolution of carbonates. Increasing solute concentrations in the mixed zone contributes to a weakening of the density gradient and reduced stability. Mineralised stream water may also intrude into the anoxic zone when the lake is not thermally stratified and cause precipitation of iron and manganese. Thus, a temporary rise of the groundwater table seems to be the reason why the lake changed from meromixis to holomixis. Lately the stream water concentrations have decreased and the lake has returned to meromixis. This periodic shift may be normal for this kind of lake.