J. Limnol., 63(1): 77-89, 2004 The chemical and biological response of two remote mountain lakes in the Southern Central Alps (Italy) to twenty years of changing physical and chemical climate Aldo MARCHETTO*, Rosario MOSELLO, Michela ROGORA, Marina MANCA, Angela BOGGERO, Giuseppe MORABITO, Simona MUSAZZI, Gabriele A. TARTARI, Anna M. NOCENTINI, Alessandra PUGNETTI 1) , Roberta BETTINETTI 2) , Pierisa PANZANI, Michele ARMIRAGLIO, Pierluigi CAMMARANO and Andrea LAMI CNR Istituto per lo Studio degli Ecosistemi, Largo V. Tonolli 50, 28922 Verbania, Italy 1) CNR Istituto di Scienze Marine, Sezione di Venezia, 1364 S. Polo, 30125 Venezia, Italy 2) Università degli Studi dell'Insubria, Dip. Scienze Chimiche e Ambientali, Via Lucini 3, 22100 Como, Italy *e-mail corresponding author: a.marchetto@ise.cnr.it ABSTRACT Two small high mountain lakes in the Alps were monitored in 1984-2003 to follow their response to changes in human impact, such as deposition of atmospheric pollutants, fish stocking and climate change. The results were compared to occasional samplings performed in the 1940s, and to the remains found in sediment cores. When monitoring started, the most acid-sensitive of them, Lake Paione Superiore, was acidified, with evident effects in its flora and fauna: benthic diatoms assemblage was shifted towards aci- dophilous species, and zooplankton lost the dominant species, Arctodiaptomus alpinus. Palaeolimnological studies outlined that lake acidification paralleled the increasing input of long-range transported industrial pollutants, traced by spherical carbonaceous parti- cles. On the contrary, the biota of Lake Paione Inferiore appeared to be mainly affected by fish stocking. In the last twenty years, de- crease in acid load from the atmosphere led to an improvement in lake water quality, with an increase in both pH and alkalinity. First signs of biological recovery were identified, such as change in diatom flora and appearance of sensitive species among benthic insects. However, climate change and episodic deposition of Saharan dust were important driving factors controlling lake water chemistry. Further monitoring to assess the effects of climate change and of the increasing load of nitrogen and other pollutants is recommended. Key word: atmospheric deposition, acidification, plankton, benthos, diatoms 1. INTRODUCTION The Paione lakes are three cascade lakes of glacial origin located in a small valley in the southern part of Central Alps. They were first studied in the 1940s by Vittorio Tonolli, who carried out detailed studies on them, with a pioneering multidisciplinary approach in- tegrating climatic and morphometric properties of the watersheds and chemical analysis of lake water for an interpretation of the biological features of the lakes (Tonolli 1949). These studies represented a pilot re- search which led to the organisation of a survey cover- ing 170 lakes throughout the Italian Alps (Tonolli & Tonolli 1951), that produced a full picture of the plank- ton in Alpine lakes, and an account of the relationships between the biological communities and the main envi- ronmental parameters of the lakes and their watersheds. At that time, mountain lakes appeared to be far from human influence, and they were studied to establish ref- erence conditions for the lowland, more polluted lakes, or to examine ecological processes without taking into account the effects of the direct anthropogenic distur- bances, such as local pollution or land use change. It was in 1981, after a second survey covering 300 alpine lakes (Giussani et al. 1986), that human activity appeared to be a major factor affecting high mountain lakes: inappropriate fish stocking, without any scientific basis, was found to have the most striking impact on Alpine lakes, including Lake Paione Inferiore (LPI), the lower Paione lake. Acid deposition was a concern for a small, but significant number of the sampled lakes: 5% of them showed pH values below 6.0 (Mosello 1984). Lake Paione Superiore (LPS), the upper Paione lake, re- sulted to be one of the most acid lakes, because of the lithological characteristics of its watershed, and of the high acid load from atmospheric deposition (Mosello et al. 1999). For these reasons, both LPI and LPS were chosen as flag-ship sites in several research programmes aiming to quantify the effects of acid deposition and other anthro- pogenic impacts on mountain lakes and their biota, namely "Quantification of the susceptibility of Alpine lakes to acidification" (1988-1991), "AL:PE, Acidifica- tion of mountain Lakes: Palaeolimnology and Ecology" (1991-1994), "MOLAR: MOuntain LAkes Research" (1996-99) and "EMERGE: European Mountain lakes Ecosystems: Regionalisation, diaGnostic and socio-eco- nomic Evaluation" (2000-2003). These projects focused on high altitude environments, considered good indica- tors of the effects of atmospheric pollution on a regional