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We determined the ␦ 13 C and ␦ 15 N of water-column particulate organic matter (POM), dissolved inorganic carbon, and nitrate, together with water chemistry and phytoplankton biomass and species composition every month in eutrophic Lake Lugano. As primary productivity increased during spring, the ␦ 13 C of photic-zone POM increased from Ϫ34‰ to Ϫ24‰. This 13 C enrichment reflects decreasing C-isotope fractionation between organic and inorganic carbon pools in response to decreasing surface water [CO 2 (aq)]. Variations in the ␦ 15 N of surface-water POM (ϩ2‰ to ϩ8‰) collected during the productive period were attributed to isotope effects associated with nitrate uptake, nitrogen fixation, and mixing of different organic matter sources. The apparent N-isotope enrichment () associated with nitrate assimilation varied with ϭ Ϫ1.0‰ Ϯ 0.9 for diatoms and ϭ Ϫ3.4‰ Ϯ 0.4 for green algae. The mechanisms controlling the N-isotopic composition of surface-water nitrate include the combined processes of nitrate assimilation, nitrification, mixing of water masses, and external nitrate loading. There was no consistent relation between the ␦ 15 N of POM, the ␦ 15 N of nitrate, and the nitrate concentration in surface waters. Low ␦ Stable carbon and nitrogen isotope measurements of autochthonous material from aquatic environments have proven to be a powerful tool to better understand biologically driven carbon and nitrogen cycles. Such studies helped to assess the sources and cycling of organic matter (e.g., Cifuentes et al. 1988;Bernasconi et al. 1997;Huon et al. 2002) and to identify microbial processes (e.g., Ostrom et al. 1997;Brandes et al. 1998). Through the carbon and nitrogen stable isotope analysis of sediments, insights may be gained into the trophic evolution of lakes, provided that the processes controlling isotope fractionation during organic matter synthesis and degradation are well understood. For example, the C-isotopic composition of lacustrine organic matter has been used as a proxy indicator for primary productivity, pCO 2 (aq) and CO 2 versus HCO uptake (Hollander and McKenzie Ϫ 3 1991;Ostrom et al. 1997; Hodell and Schelske1998).Variations in the isotopic composition of organic and inorganic nitrogen species in aquatic environments can be re-

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We investigated the annual changes in sediment fluxes at two depths in Lake Lugano, Switzerland, and the associated variations in carbon and nitrogen isotope composition of sedimenting organic matter. The organic carbon and nitrogen fluxes increased by 10 and 20% with depth, respectively, whereas particulate phosphorus fluxes showed an increase of 114% with depth. The 8°C and 615N of organic matter showed large seasonal changes ranging between -40 and -22%0 for C and 4 and 16%0 for N. The variations in SIC can be attributed to variations in primary productivity level, changes in the carbonate chemistry, and isotope discrimination during photosynthesis. Very heavy nitrogen isotope compositions of organic matter in winter may indicate an external source of organic N. Comparison of the C and N isotope composition of organic matter in the top sediment with the sediment traps indicated that the observed flux increases with depth were due to a combination of lateral organic matter transport, sediment reworking, and possibly a contribution of allochthonous organic matter.Organic matter (OM) is an important component of settling particles and sediments in lakes. It influences a variety of biogeochemical processes and is the most important factor controlling redox conditions, the oxygen budget of bottom waters, and the cycling of phosphorus, other nutrients, and trace metals. The quantification of fluxes and accumulation rates of OM are therefore important parameters for any model of lake restoration (Bloesch and Uehlinger 1986). Secondary processes such as resuspension from the bottom sediments, lateral transport within the water column (sediment focusing), or transport at depth of detrital matter through river input (Hilton et al. 1986), however, often impair the precise determination of sediment accumulation rates. By characterizing the C and N isotope composition of settling particles during an annual cycle, it may be possible to distinguish between these processes if the seasonal variability of SIC and 61sN in primary OM is large enough and contrasts with the isotopic composition of the bottom sediment and the allochthonous input from OM derived from the catchment area.The amount of OM stored in sediments and its chemical and isotopic composition are also valuable tools for reconstructing past changes in productivity, in C and N cycling, and biological community structure. Carbon isotope composition of bulk lacustrine OM has been widely used to reconstruct paleoenvironmental conditions (e.g. Hollander and McKenzie 199 1; Schelske and Hodell 1995). However, during sedimentation and deposition, OM is microbially transformed and decomposed, and many questions remain open AcknowledgmentsThis study was partially supported by Swiss National Science Foundation grant 5001-039146 for the Module 2 of the Priority Program Environment. We thank Manuela Simoni and Paola Da Rold for analytical support, Bill Anderson and Jane Teranes for reviewing an early version of the manuscript, and associate editor B. l? Boudreau and...

1. We carried out a coordinated survey of mountain lakes covering the main ranges across Europe (including Greenland), sampling 379 lakes above the local tree line in 2000. The objectives were to identify the main sources of chemical variability in mountain lakes, define a chemical classification of lakes, and develop tools to extrapolate our results to regional lake populations through an empirical regionalisation or upscaling of chemical properties. 2. We investigated the main causes of chemical variability using factor analysis (FA) and empirical relationships between chemistry and several environmental variables. Weathering, sea salt inputs, atmospheric deposition of N and S, and biological activity in soils of the catchment were identified as the major drivers of lake chemistry. 3. We tested discriminant analysis (DA) to predict the lake chemistry. It was possible to use the lithology of the catchments to predict the range of Ca 2+ and SO 4 2) into which a lake of unknown chemistry will decrease. Lakes with lower SO 4 2) concentrations have little geologically derived S, and better reflect the variations in atmospheric S loading. The influence of marine aerosols on lakewater chemistry could also be predicted from the minimum distance to the sea and altitude of the lakes. 4. The most remarkable result of FA was to reveal a factor correlated to DOC (positively) and NO 3 ) (negatively). This inverse relationship might be the result either of independent processes active in the catchment soils and acting in an opposite sense, or a direct interaction, e.g. limitation of denitrification by DOC availability. Such a relationship has been reported in the recent literature in many sites and at all scales, appearing to be a global pattern that could reflect the link between the C and N cycles. 5. The concentration of NO 3 ) is determined by both atmospheric N deposition and the processing capacity of the catchments (i.e. N uptake by plants and soil microbes). The fraction of the variability in NO 3 ) because of atmospheric deposition is captured by an independent factor in the FA. This is the only factor showing a clear pattern when mapped over Europe, indicating lower N deposition in the northernmost areas. 6. A classification has been derived which takes into account all the major chemical features of the mountain lakes in Europe. FA provided the criteria to establish the most important factors influencing lake water chemistry, define classes within them, and classify the surveyed lakes into each class. DA can be used as a tool to scale up the classification to unsurveyed lakes, regarding sensitivity to acidification, marine influence and sources of S.

Several research programs monitoring atmospheric deposition have been launched in the Alpine countries in the last few decades. This paper uses data from previous and ongoing projects to: (i) investigate geographical variability in wet deposition chemistry over the Alps; (ii) assess temporal trends of the major chemical variables in response to changes in the atmospheric emission of pollutants; (iii) discuss the potential relationship between the status of atmospheric deposition and its effects on forest ecosystems in the alpine and subalpine area, focusing particularly on nitrogen input. We also present results of studies performed at a local level on specific topics such as long-term changes in lead deposition and the role of occult deposition in total nitrogen input. The analysis performed here highlights the marked geographical variability of atmospheric deposition in the Alpine region. Apart from some evidence of geographically limited effects, due to local sources, no obvious gradients were identified in the major ion deposition. The highest ionic loads were recorded in areas in the foothills of the Alps, such as the pre-alpine area in North-Western Italy and the area of Canton Ticino, Switzerland. Trend analysis shows a widespread decrease in the acidity of precipitation in the last 15-20 years as a consequence of the reduced emission of S compounds. On the other hand, nitrate concentrations in rain have not changed so much, and ammonium has decreased significantly only at the Austrian sampling sites. The deposition of N is still well above the estimated critical loads of nutrient N at some forest sites in the alpine and subalpine areas, thus confirming the critical situation of both terrestrial and aquatic ecosystems regarding N inputs. Existing data highlights the importance of continuously monitoring atmospheric deposition chemistry in the Alpine area, taking account of acidifying elements, nutrients and other pollutants such as heavy metals and organic compounds. There is also a need for unifying sampling and analytical methods in order to obtain comparable data from the different regions of the Alps.

Summary 1. Despite long‐standing ecotoxicological evidence that episodes of acidification in streams are important biologically, there is still uncertainty about their effects on invertebrate communities. We surveyed 20 streams in an acid sensitive Alpine area (Canton Ticino, Switzerland), where episodes are driven by snowmelt in spring and by rainstorms at other times of the year. Samples of water and macroinvertebrates were collected in pre‐event conditions (winter and summer) and during periods of high flow (spring and autumn). 2. Using pH, [Ca2+] and [Aln+], streams were clustered into six acid–base groups that were either well buffered (groups 4–6), soft‐water with stable pH (group 3), or poorly buffered with low pH at high flow (groups 1 and 2). 3. Severe episodes occurred during snowmelt, when the group 1 streams became acidic with pH down to 5.0 and [Aln+] up to 140 μg L−1. pH declined to 6.2 in streams of group 2, but remained > 6.6 in groups 3–6. 4. Detrended canonical correspondence analysis showed that the streams sensitive to episodes (groups 1 and 2) had different invertebrate assemblages from well‐buffered sites (groups 4 and 5) or soft‐water stable streams (group 3), with faunal differences largest following spring snowmelt. Empididae, Isoperla rivulorum, Rhithrogena spp. and Baetis spp. were scarce in streams sensitive to episodes (groups 1 and 2). By contrast, Amphinemura sulcicollis was scarcer in hard‐water streams (groups 4–6). Taxonomic richness was lower in the episodic streams of group 1 than in other streams. 5. Together, these results indicate clear biological differences between acid‐sensitive streams with similar low‐flow chemistry but contrasting episode chemistry. Severe episodes of acidification appear to affect macroinvertebrate assemblages in streams in the southern Swiss Alps.

Lake Lugano is located at the border between Italy and Switzerland and is divided into three basins by two narrowings. The geomorphologic characteristics of these basins are very different. The catchment area is characterized by calcareous rock, gneiss and porphyry; the population amounts to approximately 290 000 equivalent inhabitants. The external nutrient load derives from anthropogenic (85%), industrial (10%) and agricultural (5%) sources. The limnological studies carried out by Baldi et al. (1949) and EAWAG (1964) revealed early signs of eutrophication, with a phosphorous concentration of about 30–40 mg m–3 and an oxygen concentration of less than 4 g m–3 in the deepest hypolimnion. Subsequently Vollenweider et al. (1964) confirmed these data and was the first to point out the presence of a meromictic layer in the hypolimnion of the northern basin. From the 1960s, as a result of an increase in the population and internal migration, the lake became strongly eutrophic with the P concentration reaching 140 mg m–3 and the oxygen in the hypolimnion reduced to zero. Fifty‐five per cent of the P was from metabolic sources and 45% from detergents and cleaning products. In 1976, a partial diversion of waste water from the northern to the southern basin was begun, and gradually eight waste water treatment plants came into operation using mechanical, chemical and biological treatments. In 1986, Italy and Switzerland began to eliminate the P in detergents and cleaning products. Since 1995, the main sewage treatment plants have improved their efficiency by introducing P post‐precipitation, denitrification and filtration treatments. The recovery of the lake is due to be completed by the year 2005. Altogether, during the last 20 years recovery measures have reduced the external P load from about 250 to 70–80 tonnes year–1; the goal to be reached is 40 tonnes year–1. In‐lake phosphorous concentrations have decreased from 140 to 50–60 mg m–3, with the target at 30 mg m–3. Dissolved oxygen concentration is satisfactory only between the depths of 0 and 50 m, falling rapidly to zero in the deepest layers. Below a depth of 90 m, high CH4, HS–, NH4+, Fe2+ and Mn2+ concentrations exist. Primary production has decreased from 420 to 310 g Cass m–2 year–1, notwithstanding an increase in the thickness of the trophogenic layer. Structure and dynamic biomass show marked changes: phytoplankton dry weight has decreased from 16 to 7 g m–2, while zooplankton dry weight has increased from 3 to 4.5 g m–2. Chlorophyll concentration has fallen from 14 to 9 mg m–3 and Secchi disk transparency has increased from 3.5 to 5.5 m. The current sources of the external load are uncollected small urban conglomerations, storm‐water overflows from outfall sewers, and the residual load from sewage treatment plants, particularly those without P post‐precipitation.