J. Limnol., 70(2): 177-185, 2011 DOI: 10.3274/JL11-70-2-03 Periphyton changes over 20 years of chemical recovery of Lake Orta, Italy: differential response to perturbation of littoral and pelagic communities Antonella CATTANEO*, Michèle DE SÈVE1), Giuseppe MORABITO2), Rosario MOSELLO2) and Gabriele TARTARI2) GRIL, Département de sciences biologiques, Université de Montréal, C.P. 6128, succ. Centre Ville, Montréal, QC, H3C 3J7, Canada 1)M.A. De Sève Consultants, 1232 Ducharme, app.1, Montréal, QC, H2V 1N4, Canada 2)C.N.R., Istituto Italiano per lo Studio degli Ecosistemi, L.go Tonolli 50, 28922 Pallanza, Italy *e-mail corresponding author:
[email protected] ABSTRACT 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 pre- liming period, progressively waned after the calcium carbonate addition.