Unusual deep mixing of Lago Maggiore during the winter 1980-1981

Abstract

The main hydrodynamic characteristic of L. Maggiore, as well as the other deep sudalpine lakes, is their oligomixis, i.e., full circulation, with the consequent thermal and chemical homogeneity of the whole water column, does not occur every year. Since 1960, this situation has been observed only in the late winter of 1963, 1970 and 1981. The last occurrence was particularly well studied, with weekly measurements of the vertical distribution of temperature and oxygen concentrations, supplemented by monthly analyses of the main chemical variables. The results showed at the beginning of February a wind induced drift of superficial water and a deep water counter current with the formation of a three layers situation: the superficial mass, from a depth of 0 to 120 m, with an oxygen concentration of 8,5-9,0 mg l^-1; a middle layer (120-220 m), with a concentration of 6,5 mg l^-1, and a lower layer (220- 370 m), with a concentration of 9,0 mg l^-1, well above the values measured on 23rd. January, which ranged from 5,7 mg and 6.6 mg l^-1. This situation was observed also at other points of the lake, which indicates a “conveyor belt” mechanism of circulation. In the subsequent days the wind and the convective movements caused a progressive oxygenation of the water mass down to 200m. In this paper the water mass dynamics is discussed in relation to the main meteorological and hydrological variables, as well as the morphology of the lake.