A COMPARISON OF RECENT EPILITHIC DIATOM ASSEMBLAGES FROM THE INDUSTRIALLY ACIDIFIED AND COPPER POLLUTED LAKE ORTA (NORTHERN ITALY) WITH OLD LITERATURE DATA

Since 1926, the subalpine Lake Orta has become acidified through the nitrification of ammonia released by a cupro-ammonia rayon factory. The present pH is ca 5.2 and copper concentrations are ca 35 μg 1−1. In 1987–8 the most abundant epilithic diatoms at two stations were Eunotia exigua and Achnanthes minutissima, which are known to be tolerant to heavy metals, and Pinnularia subcapitata var. hiheana; the average pH inferred from the epilithic assemblages is 4.9. Critical examination of published accounts of the Lake Orta diatom flora, taking into account the taxonomical concepts used in old standard diatom texts, allows the general character of the diatom assemblages of the lake prior to pollution to be assessed. In 1884 and 1914 no acidobiontic taxa were present and the diatom assemblages were dominated by circumneutral and alkaliphilous taxa, in agreement with a directly measured pH-value of 7.1 in 1925. In cores taken by Corbella et al. (1958), the relative abundance of Achnanthes minutissima showed an increase from 7%.in ca 1880 to 60% in ca 1955, while that of Cycbtella comensis decreased from 64% to 21%, due to copper pollution. The diatom-inferred pH changed from 6.6 in ca 1880 to 7.0 in ca 1930, and 6.4 in ca 1958. The increase of acidobiontic taxa, mainly Eunotia exigua and Pinnularia subcapitata var. hiheana, apparently occurred after 1962, in agreement with the measured decrease in pH between 1958 and today.