Seasonal and interannual changes of chemical characteristics and phytoplankton in a mountain lake of the Eastern Italian Alps (Lake Calaita, Trentino)

Measurements at Lake Calaita, a small mountain lake located at 1605 m a.s.l. in a metamorphic catchment area in the south of the Trentino Region (Northern Italy), have revealed great seasonal variations in the chemical characteristics and phytoplankton community during the ice free period in 1992 and 1993. The acidity present in wet precipitations (H+, NH4+) was neutralised within the drainage basin by mineral dissolution which led to an increase of basic cations and alkalinity in the runoff. The dilution during periods of higher discharge, e.g. in spring and autumn, resulted in loa values of alkalinity (up to 60 mu eq 1(-1)), pH (mostly <6.7) and conductivity (<18 mu S cm(-1), 20 degrees C). In summer, the decrease in runoff caused higher alkalinity (>110 mu eq 1(-1)), pH (6.9-7.4) and conductivity values (up to 30 mu S cm(-1)). The phytoplankton showed a major development in summer (with biovolume values of up to 7000 mm(3) m(-3)), two different taxa being dominant in 1992 (Oocystis cf. lacustris) and 1993 (Synedra sp.). Unpredictable climatic conditions have a strong influence on the physical stability of the lake, which makes it more difficult to explain the evolution of the phytoplankton community as opposed to deeper lakes.