Eight high mountain lakes located above 2000 m a.s.l. in the watershed of Lago Maggiore, with alkalinity between 10 and 100 µeq l -1 , were studied during summer 1997. The survey revealed that some of the lakes had a saline density gradient which might hinder the mixing of water and restrict the oxygenation in hypolimnion. Following acidification, aluminium was present in some lakes in the form of Al(OH) 2 + , with pH around 5.9, and as Al(OH) 4 -, with pH around 6.6. The negative correlation

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... served between pH and nitrate concentrations seems to be linked to vegetal activity. In fact, acidity in soil and water, combined with the presence of aluminium, can partially inhibit the metabolism of plants and algae, and reduce the assimilation of NO 3 -. Major biological differences emerged among the lakes, both at the level of plankton composition and biomass, and in seasonal dynamics. Dinophyceans and chlorophyceans predominated quantitatively in the lakes with low pH and alkalinity values, whereas diatoms were present in the lakes with higher values of these parameters. The phytoplankton biodiversity index showed a considerable variation from lake to lake. Higher alkalinity and salinity was not coupled to a higher biocenosis diversity and one or two species tend to dominate the others.