TEMPORAL VARIATIONS OF FOSSIL CLADOCERA IN THE SEDIMENTS OF LAKE ORTA(NORTH ITALY) OVER THE LAST 400 YEARS

We use Cladocera remains in the sediments to reconstruct the longterm history of Lake Orta, a lake which experienced severe pollution from c opper and ammonium sulphate, and started to recover during the last 15 years. Both human and natural impacts were detected over almost 400 y ears. Pollution was manifested by a dramatic decrease in the number of remains, in the planktonic/littoral (Pn) ratio, and in chydorid speci es diversity. Most species, even those most tolerant to stress, disapp eared. The recolonization of biota was initially sustained by one spec ies of chydorid, Chydorus sphaericus, which had a three-fold increase at the beginning of the 1980s, when studies on plankton recorded the d evelopment of a pelagic population. This fact, and the appearance in t he sediments of Alona quadrangularis, attest to the development of fil amentous algae which was reported in studies on the plankton. The colo nization of the lake by pelagic species came only in the last twelve y ears and was manifested also as a stabilization of the P/L ratio. Two other periods of disturbance were detected in the sediments: the first , at the end of the 19th century, was related to the introduction of e xotic fish species; the second occurred during the second half of the 17th century. This event probably cannot be ascribed to human impact, but may be related to a decrease in temperature in that period (the Ma under Minimum).