Evolution towards meromixis of Lake Iseo (Northern Italy) as revealed by its stability trend

Temperature, oxygen concentration, conductivity and ionic composition of the water of Lake Iseo (zmax 251 m) between 1978 and 2003 were used to analyze the lake's evolution towards meromixis, through stability calculations following Walker (1974). Total, thermal and chemical stability is assessed, both as an integrated quantity on the whole water column and in its vertical distribution. The results show an increase over time of total stability in winter, caused by the increase of solutes present in the deep layers, the result of the isolation of these layers due to a lack of complete vertical mixing, and in summer by an increase in the thickness of the surface water layer involved in the annual heat exchange. It emerges clearly that the anoxia of the deep layers, which first occurred in 1994, is related to the increase in the solutes present in the deep hypolimnion, and that the solutes, via chemical stability, are related to the depth of the vertical winter mixing. In addition, the high density present in the deep layers due to the solutes also prevents them from being oxygenated by hydro-meteorological mechanisms resulting from the inflow of river water to the deep layers, mechanisms which are common in deep lakes like Iseo. The vertical distribution of the stability values makes it possible to identify the water layers that present the greatest resistance to mixing, and to evaluate how their volume has continually expanded over the years, so that now more than 50% of the lake volume is affected. As a result the deep layers are increasingly stable and isolated. The vertical distribution of the chemical stability reveals that at the end of winter Lake Iseo still has below 50 m a level with a chemical stability of more than 0.02 J m-2, which means that a full circulation cannot occur.