Reduced effectiveness of wind and tributaries in the deep oxygenation of a chemically stratified lake.

A major concern of the scientific community working on deep lakes is the progressive isolation and consequent de-oxygenation that have been observed in the last decades. The distribution of the dissolved substances in a deep lake, such as oxygen and nutrients, is controlled by the action of wind-induced stresses, penetrative convection by surface cooling and density-driven plumes. The extent of deep circulation is thus the outcome of the competition between density stratification and the drivers of mixing, acting at the surface and at the boundary of the lakes.

Lake Iseo is a large (61 km²) and deep (256m) Italian subalpine lake, fed by two main tributaries with an overall average annual inflow of 55 m³/s. The first detailed scientific analysis documents a monomictic lake, characterized by deep water with 70% oxygen saturation. However, since the second half of the 1980ies the deep-water recirculation has been insufficient. The monimolimnion has become depleted of oxygen, has become enriched with solutes and had gradually warmed with rates that could be estimated approximately ~0.05°C/year.

In this contribution, we discuss the role of the chemical stratification of lake Iseo, induced by a gradient in calcium, bicarbonate and sulphate ions, in reducing the deep-water oxygenation.  At this purpose, we computed the stability of the lake, by coupling a site–specific density equation to the high-resolution time series of lake’s ware temperature and conductivity data, and we quantified the external forcings from high-resolution wind, discharge and tributaries’ temperature data. We thus estimated the time series of the resistance by the chemical stability to wind upwelling and to rivers’ underflows. We finally showed that the progressive deep-water warming that followed the isolation of the monimolimnion has strongly decreased the lake’s thermal stability, counteracting the chemical stratification in the last 8 years. We finally concluded that it does not seem that chemically stratified deep lakes are necessarily doomed to anoxia, but on the contrary to periods of longer isolation alternated by sporadic deep oxygenation triggered by deep warming.