Understanding deep oligotrophic subalpine lakes for efficient management

Lake Garda - along with other large lakes located on the southern edge of t he Alps - belongs to a distinct typology of lakes characterised by high dep ths and large volumes. Owing to its morphometric features (max. depth 350 m ), the water column of L. Garda undergoes complete mixing only in harsh win ters. During the period 1991-1995 this was only once, in 1991; in other yea rs the mixing depth lay between 140 to 180 m. There is a correlation betwee n lake volume at overturn with algal abundances and water transparencies. T he extent of the overturn appears to control the interannual variations of the algal crop around a baseline trophic status determined by the whole tot al phosphorus content in the water column. Full understanding of the limnol ogical features of the lake are thus important for the most suitable manage ment practices. Future plans should try to reduce the discharge of sewage i nto deep waters to a minimum; it is stressed that the only reliable solutio n for preservation of water quality in deep lakes of this typology is reduc tion of nutrient loads. As far as L. Garda is concerned, this appears fairl y pressing, considering the increase of total phosphorus concentrations in the water column observed since the 1970s.