Contribution of Chemical Stratification to the Extent of Water Renewal in a Deep Lake

Turnover events in deep lakes are crucial for water quality and trophic balance. If on one side there is wide consensus that climate change will strengthen the thermal stability of the epilimnion, on the other a smaller number of studies have investigated the stabilization operated by the chemical stratification, and the conditions under which it may occasionally fail. In this paper we investigate the role of thermal and trophic-induced stratification on the deep mixing of Lake Iseo, a pre-alpine lake that used to be monomictic in the past. The coupling of a site–specific density equation to 9 years of high-resolution temperature and wind data made it possible to compute the time series of the resistance to upwelling of deep waters and to quantify the role of the chemical stability. We show that the presence of calcium, bicarbonate and sulfate ions in the bottom layers, and the related increase in stability of about 103 Jm−2, has hindered the occurrence of a deep overturn, even in presence of a thermally uniform water column. A full upwelling was thus inhibited in 3 of the observed winters, when it would otherwise have occurred. On the other hand, whereas our computations show that a destabilizing effect of possible future stronger winds is unlikely, the progressive deep-water warming that followed the isolation of the monimolimnion has strongly decreased the lake's thermal stability, counteracting the chemical stratification and progressively increasing the probability of a future deep overturn.