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-

High quality monitoring data are vital for tracking and understanding the causes of ecosystem change. We present a potentially powerful approach for phytoplankton and aquatic ecosystem monitoring, based on integration of scanning flow-cytometry for the characterization and counting of algal cells with multiparametric vertical water profiling. This approach affords high-frequency data on phytoplankton abundance, functional traits and diversity, coupled with the characterization of environmental conditions for growth over the vertical structure of a deep water body. Data from a pilot study revealed effects of an environmental disturbance event on the phytoplankton community in Lake Lugano (Switzerland), characterized by a reduction in cytometry-based functional diversity and by a period of cyanobacterial dominance. These changes were missed by traditional limnological methods, employed in parallel to high-frequency monitoring. Modeling of phytoplankton functional diversity revealed the importance of integrated spatiotemporal data, including circadian time-lags and variability over the water column, to understand the drivers of diversity and dynamic processes. The approach described represents progress toward an automated and trait-based analysis of phytoplankton natural communities. Streamlining of high-frequency measurements may represent a resource for understanding, modeling and managing aquatic ecosystems under impact of environmental change, yielding insight into processes governing phytoplankton community resistance and resilience.

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...

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.

was mainly inhabited by ultramicrobacteria related to the LD12-lineage of Alphaproteobacteria and to Actinobacteria; the latter group preferred the shallow regions. Cytophaga-Flavobacteria, in particular a population related to Fluviicola sp., were more frequent in and below the layer of maximal P. rubescens abundances. Betaproteobacteria on the other hand were highly abundant in the epi-and hypolimnion, but not in the P. rubescens layer. Four betaproteobacterial subpopulations with contrasting longitudinal and/or vertical habitat preferences were distinguished: Putatively methylotrophic bacteria of the LD28 lineage (beta IV) preferentially inhabited the hypolimnion, Polynucleobacter acidiphobus was found throughout the epilimnion, Limnohabitans (R-BT065) more in the shallow regions of the lake, and Polynucleobacter necessarius ssp. asymbioticus only in hypoxic waters. Our results stress the potential importance of spatial niche differentiation in freshwater bacterioplankton. This variability should be taken into account, e.g., in studies of seasonal community changes derived from single sampling locations and depths.