Trophic development of the deep lakes south of the Alps: a comparative analysis

This paper reports the results of long-term research carried out in the deep southern subalpine lakes (DSL: Maggiore, Lugano, Como, Iseo and Garda). The measurements performed since the 1970s at the time of the maximum overturn documented an increase of water temperature around 0.0 12-0.028 degrees C yr(-1), closely resembling the warming rate found in other lakes in Europe and North America.. The DSL showed an increase of total phosphorus after the 1960s and a complete recovery after the 1990s of Lake Maggiore, which reached mean values of TP close to those of the pristine conditions (<10 mu g P l(-1)). Present TP concentrations in the other lakes are above 100 mu g P l(-1) (Lugano), 30-40 mu g P l(-1) (Como), 50-60 mu g P l(-1) (Iseo) and c. 20 mu g P l(-1) (Garda), with only Lake Garda showing a clear tendency to increasing P-concentrations in recent years. The higher trophic status of lakes Lugano and Iseo has favoured hypolinmetic anoxia due to meromixis since the 1960s and the 1990s, respectively, from which there was a partial recovery in 2005. Monthly samplings carried out in the last decades showed a strong coupling between long-term variation in algal nutrients and phytoplankton. On a shorter temporal scale, a significant relationship between the extent of water mixing during the spring months and the replenishment of nutrients and algal biomass in the growing season has also been verified in lakes Iseo, Garda and Lugano.