Journal of Paleolimnology 20: 333–345, 1998. 333 c 1998 Kluwer Academic Publishers. Printed in the Netherlands. Paleoecological evidence for diatom response to metal pollution in Lake Orta (N. Italy) Delio Ruggiu 1 , Antonella Lugli´ e 2 , Antonella Cattaneo 3 & Pierisa Panzani 1 1 C.N.R. Istituto Italiano di Idrobiologia, Largo Tonolli 50-52, 28922 Verbania Pallanza, Italy (e-mail: ruggiu@iii.to.cnr.it) 2 Dip. di Botanica e Ecologia Vegetale, Universit` a di Sassari, V. Muroni 25, 07100 Sassari, Italy ( e-mail: sechi@dbev.scienze.uniss.it) 3 D´ ep. Sci. Biologiques, Universit´ e de Montr´ eal, C.P. 6128, Succ. Centre Ville, Montr´ eal, Qu´ ebec, Canada H3C 3J7 (e-mail: cattanea@ere.umontreal.CA) Received 24 June 1997; accepted 22 November 1997 Key words: diatoms, sediment cores, industrial pollution, copper, teratology Abstract Diatom remains were analysed in two short sediment cores from a subalpine Italian lake (Lake Orta), known for its major industrial pollution dating from the late 1920s, which has only recently been stopped. Copper was recognised as the main toxic agent for diatoms during the first 30 years of pollution (peak value: 100 gl 1 in the late 1950s). A diatom community similar to other deep subalpine lakes existed in the past, and was disrupted by the pollution events. Acute and long-term effects of Cu contamination were tracked by changes in three distinct groups of species around the sharp boundary corresponding to the onset of the pollution. These groups were respectively composed of: (1) Species quickly extirpated by the discharge, mostly belonging to Fragilaria and Cyclotella and never reappearing; (2) Species apparently not affected, or not immediately affected, by the pollution, showing no definite trends with time. Synedra species, with various deformities, were conspicuous among these; (3) Species with accumulation rates increasing with time irrespective of pollution, mostly belonging to Achnanthes. Properties and tolerances of these groups (e.g. Synedra and Achnanthes) are discussed in detail. Introduction The literature on the effects of metals on algae is exten- sive, but most papers report laboratory experiments with one or a few species, which are often marine (Rai et al., 1981). In the field of paleolimnology, freshwater diatom remains from lake cores have been successful- ly used to reconstruct past history of acidification and eutrophication, but seldom of metal contamination. A diatom inference model including metals (Al, Ni, Ca) was proposed for the metal polluted Sudbury lakes in Canada (Dixit et al., 1991). However, known cases of both acute and persistent exposures of diatoms to met- als in the same site are difficult to find. Lake Orta (45 49 N, 8 24 E), an industrially polluted lake in Italy, provides an opportunity to study impact of metals on diatom assemblages. The lake is the westernmost among the deep Italian subalpine lakes, with a volume of 1.25 km 3 , an average depth of 69 m, a maximum depth of 143 m and a theoretical renewal time of about 9 years. It has been a sink of waste water from a rayon factory (Bemberg) since 1926. The effluent enters the lake in the south, opposite to the outlet, located in the northern area. Given the factory location and the lake’s volume, the discharged quantities were enormous, reaching 60–70 t Cu y 1 and 1000–3000 t N y 1 as ammonium in the first three decades after the onset of the pollution, and causing a sudden, massive extermination of life soon after the pollution began. Toxicity from copper was believed to be responsible for the biological collapse (Monti, 1930). Detailed descriptions on the pollution events that occurred in Lake Orta following the start