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Lake Orta, a large Italian subalpine lake, has a long history of industrial pollution by acid, ammonia, and metals. A whole-lake liming treatment in 1989-1990 led to a gradual increase in pH (from 4.4 to 7) and a concomitant decline in ammonia (from 2.5 to 0.05 mg L-1) and metal concentrations (e.g., copper: from 35 to 3 μg L-1). In this study, we examine the response of the littoral zone, in terms of chemistry and periphyton assemblages, to contamination and recovery of Lake Orta. We compare these findings with a long-term data set of chemistry and phytoplankton collected in the pelagic zone. We sampled periphyton at two sites from the liming period (1989) through 2010 when the lake approached chemical equilibrium. Chemical variables collected in the littoral zone near the periphyton samples followed the same temporal trends observed in the pelagic zone. Chlorophytes, the dominant algae in the preliming period, progressively waned after the calcium carbonate addition. Diatom importance gradually increased to reach up to 80% of the total biovolume in the most recent years. There was a clear shift in the diatom taxonomic composition over the study period. Acidophilous Pinnularia subcapitata var. hilseana and metal-tolerant Achnanthes minutissima were dominant before liming. With the progressive increase in pH and decline in metal concentrations, there was successively the dominance of Fragilaria nanoides (formerly identified as Synedra tenera) and Cymbella microcephala in the transition period and of circumneutral Navicula cryptocephala, N. radiosa, and Synedra ulna in the most recent period. Pelagic diatoms were more affected than benthic diatoms by the industrial pollution and their recovery after the liming intervention was slower. Whereas present periphyton diatom assemblages are similar to those observed in nearby unpolluted Lake Mergozzo, pelagic assemblages are still in transition from severe disturbance towards characteristics reported before pollution. Our results suggest that the littoral zone may have acted as a refugium during contamination providing an inoculum for pelagic recolonization.
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