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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.