Effects of water-level fluctuations on lakes: An annotated bibliography

DIATOM COMMUNITY RESPONSE TO EXTREME WATER-LEVEL FLUCTUATIONS IN TWO ALPINE LAKES: A CORE CASE STUDY M. Leira: Centro de Geologia Universidade de Lisboa (CeGUL) Faculdade de Ciências. Campo Grande 1749-016 Lisboa, Portugal M.L. Filippi: Museo delle Scienze, Geology Section, Via Calepina 14, I-38122 Trento, Italy. Present address: 630Trafalgar Court, Dania Beach, 33004 FL, USA M. Cantonati: Museo delle Scienze, Limnology and Phycology Section, Via Calepina 14, I- 38122 Trento, Italy Keywords: Water-level Fluctuations; WLF; Resedimentation; Alpine lakes; diatoms; physiognomic groups 1 Abstract Water-level fluctuations (WLF) often represent one of the greatest impacts on the development of lake ecosystems. In the year 1931, the Hydroelectric Trentino Company (SIT) requested the hydroelectric use of water bodies in the Upper and Middle Sarca basin. The largest and deepest lakes were dammed to increase lake volume and exploited since mid 1950s. This research uses the sediment record of two small lakes in close proximity to each other (Garzonè and Serodoli) in northern Italy, use for hydroelectric power generation to determine the dependence of diatom-assemblage dynamics on WLF that have taken place over the last 60 years. Historical water-level fluctuations are clearly reflected in the lithological composition and grain-size variations of the sediment cores. During the regression and transgression phases, the boundaries between the Erosion, Transport and Accumulation zones fluctuated, causing redistribution of previously accumulated sediments, and their return into the lake’s cycling of biogeochemical matter. The water-level changes not only caused distinct taxonomic shifts in the diatom communities, which were dominated by different species in the core sections of each lake, but also significant shifts in the composition of the diatoms’ ecological, morphofunctional and life-form groups. Diatoms with a low- and high-profile attachment type were the dominant morphological forms in the upper core levels, where the most extreme fluctuations in water level had occurred. Increased turbulent mixing caused by WLF favours the presence of large, heavily-silicified centric diatoms, while more stable levels would select for smaller centric diatoms through thermal stratification. 2 Introduction Water-level fluctuations (WLF) often represent one of the greatest impacts on the development of lake ecosystems, particularly on the trophic state, underwater light climate, plankton dynamics and littoral development (Moos et al. 2005; Laird and Cumming 2008; Shuman et al 2009; Wolin and Stone 2010). WLF alter lake morphometry and directly affect sedimentation, resuspension, and biogeochemical dynamics (Håkanson 1977). Sediment properties as mineral matter content and grainsize provide additional insight into lake basin processes (Digerfeldt 1986; Boyle 2001) and water-level changes indicative of the hydrological balance of lakes that can be altered in case of changes in this balance (Shteinman and Parparov 1997; Punning et al. 2004a). Even small fluctuations in water level may result in large changes in assemblages of aquatic vegetation and macrophyte zones (Tarras-Wahlberg et al. 2002; Coops et al. 2003; Punning et al. 2004b). Water movement in freshwater environments have an important effect on sediment composition, particle size and sediment dynamics in and around submerged macrophyte beds (Madsen et al. 2001). Therefore, sediment content – above all organic matter – can reflect a number of factors, including aquatic productivity, input of detritus from terrestrial vegetation and the flow of mineral matter into lakes. Alpine lakes are a highly valuable resource. Focal points for tourism, they attract visitors who appreciate their natural beauty. These lakes have also been put to work – for hydroelectricity generation and water storage. Eleven lakes in the western, siliceous part of the Adamello-Brenta Nature Park were exploited for hydropower generation between the 1950s and 1991. One lake (L. Scuro al Mandrone) continues to be partially used to produce electricity for a mountain hut, and a second one (L. Serodoli) is still being used to provide drinking water. Exploiting lakes in these ways has altered lake ecosystems, and their littoral zones are often the first to suffer the effects of WLF. This zone is particularly important in such high altitude oligotrophic lakes, where the euphotic zone can reach a substantial part of the water column (O’Sullivan and Reynolds 2005). The littoral zone is a very dynamic region with high biodiversity and productivity, where important physical processes take place. Any significant change to a lake’s level, whether spatial or temporal, will affect not only the lake’s physical environment, but could also either affect individual plants and animals directly or populations and communities through 3 ecological processes (Leira and Cantonati 2008). Direct effects on the biological communities include slumping, which can impact on terrestrial and aquatic vegetation, and displacement of the shoreline, thus changing the zones affected by wave activity. Indirect effects include the reworking of substrates and alteration of the type and vertical zonation of vegetation around the edge of a lake, as this vegetation is dependent on the type of substrate as well as exposure to waves and water-level range. Moreover, in clear-water lakes, which have potentially large areas suitable for the growth of benthic algae, a substantial part of the photosynthesis occurs in benthic algal communities. It follows that any activity that affects the littoral zone can have a significant ecological effect. The composition and structure of lake sediments enable us to reconstruct the influence of extreme changes in water levels on lake ecosystems, and to provide a prognosis for lake evolution under changing environmental conditions. Given the scope of the study, we targeted the high mountain lakes of the Adamello-Brenta, as many of them have been involved in a huge hydroelectric project (Betti 2003). Lakes Garzonè and Serodoli were selected for this study (Fig. 1). Both were dammed to increase lake volume for hydroelectric power generation, although only the latter is still exploited for drinking water supply. The objective of our study was threefold: 1. Define the preimpact ecological reference conditions for those lakes that have been subjected to alteration; 2. Assess how current and past water level fluctuations have influenced the diatom community for selected lakes; and 3. Compare diatom life forms and morphofunctional groups. Environmental reconstructions based on a single-core study are often inadequate because changing bathymetry and WLF result in highly variable sedimentation. Digerfeldt (1986) advocated transects of coring sites from shallow to deep water. We applied the transect method to examine at least two sediment cores from each lake, one from the deepest part and the other from a shallow area close to the shore. Site description The Adamello-Brenta Regional Park is a protected area of about 600 km2 in NE Italy (Fig. 1). It comprises two mountain ranges with contrasting geology: the Adamello Massif formed by crystalline rocks, such as granites and tonalities, and the Brenta Massif, made up of sedimentary rocks (Cantonati et al. 2002). In the Adamello- Presanella mountain range high mountain lakes are only found in the western part of the 4 park. Few specifically limnological studies of lakes have been undertaken in this district (Baldi 1932; Zucchelli 1952; Cantonati et al. 2002; Corradini et al. 2005; Tardio et al. 2005). Most of the lake surveys have been carried out as part of general limnological investigations devoted to broader areas within the Italian Alps (Tonolli and Tonolli 1951; Tomasi 1962; Giussani et al. 1986). For this reason the primary objective of our research project was to select two high mountain lakes for a paleolimnological study, based on a first screening and morphological assessment of lakes in the park that were exploited for hydroelectricity generation. The two lakes are located above the tree line (1940-2371 m a.s.l., Table 1) in landscapes dominated by rocky outcrops, slope debris, grassland and shrub patches, in various proportions depending on altitude and substratum. The lakes are of glacial origin and have a median catchment area of 1.6 km2. Lake area ranges from 6.5 to 10.7 ha and lake depth varies between 27 and 33.2 m (Cantonati et al. 2002). The hydroelectric plant of S. Massenza, now the ENEL (Ente Nazionale per l'energia Elettrica), was built after the end of the Second World War, mainly between 1948 and 1957, under the initiative of the hydroelectricity company Sarca Molveno (SISM) (Betti 2003), following a project on the Sarca river basin. Lakes Toblino and S. Massenza are the centre of this massive hydropower system, which diverts waters at a high altitude (in the headwaters of the Sarca basin) and collects them in Lake Molveno, which was transformed in its turn into a hydropower reservoir. These waters, along with the residual flow coming from the middle course of the Sarca (Dam Bridge Pià) are whirled at the hydroelectric plant of S. Massenza and discharged into the lake of the same name, after which they flow into Lake Toblino. Next, they arrive at Lake Cavedine through the Rimone canal, feeding the resorts of Fies and, further downstream, Dro, and finally arriving at the resort of Torbole sul Garda. High altitude lakes were modified by artificially increasing either their volume, with damming, or their watershed, by means of water diversion. This complex of lakes between the Val di Genova, Val Nambrone and Val Nambino (located at altitudes between 1,900 and 2,700 m a.s.l.), has been affected by water being piped out during the winter, so as to increase the water volume discharged into the major bypass at the Val di Genova basin. The environmental impact of these operations has been, at times, very intense (Table 2). The effects were particularly great during construction work in the mid- 5 1950s, owing to the misuse of bypasses (between 1955 and 1995), and continue to this day because the dams and pipes that were put in place still exist. During construction of the tunnel for the water-diversion pipe under Lake Serodoli in 1954, the lake floor collapsed and 700,000 m3 of water poured out of the lake, reaching the tourist area of Madonna di Campiglio, where several hotels were damaged (luckily the accident didn’t cause victims). The project therefore made enormous changes to the ecosystem – particularly in the high-altitude lakes – which led to the disruption of their naturally highly stable physical and ecological setting, changes to plankton cycles and to the entire food web, and the reduction or extinction of fish populations that were of great interest from the natural history standpoint. Material and Methods Sediment coring Field observations and sediment coring in Lakes Garzonè and Serodoli were performed through winter ice in winter 2007. Coring was performed using a gravity corer (Aquatic Research Instruments; http://www.aquaticresearch.com/universal_core_head.htm). Sediment description was based on sedimentological features visible to the naked eye, using terminology proposed by Schnurrenberger (2003) and plates of Munsell colours (1954), with the preparation of some smear slides to increase detail. A total of two sediment cores were extracted from each lake, one core from the centre of the lake and a second one from a more littoral position (Table 3). Water depth was measured at each sampling location. Cores were split in the lab for sedimentological description. One half was archived while the other was used for sediment subsampling. Subsampling was done in the laboratory prior to the analysis, in 1 cm sections. All samples remained refrigerated in the dark until they could be analysed. Dating Chronological control in the current research is based on radiometrically determined sediment accumulation rates (Robbins et al. 1978) of the deep water cores. Relative concentrations of 210Pb (a naturally occurring isotope) and 137Cs in 10–14 sediment 6 samples per core were determined by high-resolution gamma spectrometry (Appleby 2001). Sediment chronologies and accumulation rates were used to calculate age using the constant rate of supply (CRS) of 210Pb model, and were verified using anthropogenic 137Cs activity profiles. The supported 210Pb was determined from concentrations of 214Pb and 214Bi in each sample. Levels of unsupported 210Pb (210Pbexcess) were then calculated as the difference between the total and the supported 210Pb concentrations. Limited resources meant that only one core per lake could be dated in this way, so 210Pb activities were only measured in the deep water cores. Chronological control was extended to other cores from the same lake by stratigraphic correlation using smear slide descriptions, visual assessment, and integration of mineralogical and grain size data, and bulk density properties. Samples for chronological control were dated in the Institute of Physics at University College Dublin (Ireland). Sediment analysis Wet sediment density and percentage water (as dry weight density) were determined from the weight of known volumes of fresh sediment samples after drying the samples in the oven at 50°C for 72 hrs. Organic content (loss on ignition, LOI, measured at 1cm intervals) was determined using standard procedures (Heiri 2001). The porosity of the sediment was calculated through the relationship between its wet bulk density and the wet-volume. Samples from sediment cores were prepared for diatom analysis and treated using standard procedures (Battarbee et al. 2001). Samples were heated with 30% H2O2 in a water-bath at 90ºC until all organic material was oxidised, then some drops of 10% HCl were added to remove carbonates. The suspensions were mounted on glass slides using Naphrax (r.i. 1.74). At least 400–500 diatom valves per sample were counted along random transects under oil immersion using a ZEISS microscope at 1000x magnification. Diatom taxonomy follows Krammer and Lange-Bertalot (1988–1991). Nomenclature was updated using Round et al. (1990) and Hofmann et al. (2011). Diatom abundance was expressed as a relative percentage. Diatoms were classified according to two habitat groups: planktic and periphytic. Periphytic taxa in this study include all epilithic, epipelic, epiphytic, and shallow-water benthic life forms. Diatom growth morphologies were grouped into three ecological guilds, i.e. low profile, high profile and motile species, according to Passy (2007). The low profile guild comprises 7 species of short stature, including prostrate (adhering to the substrate with the entire valve surface), adnate (apically attached but parallel to the substrate), erect (apically attached but perpendicular to the substrate), solitary centrics and slow moving species. The high profile guild encompasses species of tall stature, including erect, filamentous, branched, chain-forming, tube-forming, stalked, and colonial centrics. The motile guild includes comparatively fast-moving species from the genera Navicula, Nitzschia, Sellaphora, and Surirella. Diagrams were compiled using the C2 program (Juggins 2003). Statistical analysis Principal correspondence analysis (PCA) was used to identify the main patterns of variation in the diatom data, and to establish the direction and magnitude of changes in biological conditions at each coring location. Version 4.5 of CANOCO was employed (ter Braak and Ŝmilauer 2002). Prior to all analyses, diatom abundances were squareroot transformed in order to stabilise variance, and rare species were down-weighted. Finally, we tested the significance of the null hypothesis that there was no difference in the composition of the diatom assemblages between upper and deeper core sections, using a Monte Carlo randomisation procedure with 10,000 permutations. Results Lithological setting Three sections can be identified in SE07S4 (Fig. 2a). The upper layers of sediment in Lake Serodoli, from 0 to 2 cm sediment depth, are regularly laminated light brown to dark brown topped by a thin layer of sandy silt. The basal section of the core (4 to 27 cm sediment depth) consists of homogeneous dark brownish silt. A relatively thick structureless sandy layer occurred between the two sections (2-4 cm sediment depth) consisting of a distinctive lighter layer with a transition to dark brown and graded similarly to turbidite layers. The SE07S3 littoral core is characterised by a uniform dark brown silt (0-3 cm sediment depth) (Fig. 2a). The mid-section (3-9 cm sediment depth) consists of sediment richer in organic matter and vegetation macroremains. The bottom 8 of the core (9-10 cm sediment depth) is formed by a mineral greyish sediment with larger grain size. The core stratigraphy at GA07S4 has been divided into two sections (Fig. 2b). The upper (0-7 cm sediment depth) is regularly laminated with thin layers of grey to greenish-grey, with vegetation remains and sand grains intercalated throughout. A thin layer of organic brown silt, very rich in diatoms, capped this section (0-0.3 cm sediment depth). The second section, from 9 cm to at least 25 cm, consists primarily of diatom ooze and vegetation macroremains (9-10 cm sediment depth) with sand grains scattered throughout. A small layer (7-8 cm sediment depth) of organogenic micaceous sand topped the second section. The upper layers of the littoral core taken at Garzonè (GA07S6) consist of a homogeneous dark brown silt (0-1 cm sediment depth) (Fig. 2b), with intercalated layers of irregularly stratified sand with subangular feldspar grains (1 to 4.5 cm sediment depth) and plant fragments (a 4.5 cm piece of bark, a pine needle at 5 cm sediment depth). Below, from 5 to 7 cm sediment depth the core consists of a greyish brown silt. Chronology 210Pb chronology for SE07S4 core (Serodoli) The unsupported 210Pb (210Pbexcess) activity vs. depth profile shows a clear irregularity at about 3.5 cm (1.49 g cm–2), characterised by a virtual absence of 210Pbexcess (Table 4). This type of irregularity in the 210Pbexcess profile has been reported in other European mountain lakes by Appleby (2000), and can be interpreted as evidence of an episodic event, possibly associated with slumping from the catchment or from the margins of the lake. Application of the CRS model to the profile measured suggests that the topmost 3 cm of the core (corresponding to a cumulative mass of 1.18 g cm–2) contain sediments deposited over 60  10 years prior to sampling. Considering that the date of sampling was 2007, this dates the 3 cm horizon to ~ 1947, and suggests a date for the episodic event at around this time. Below ~ 4 cm, 210Pb decreases monotonically, reaching supported levels at a depth of ~ 8 cm. This represents the 210Pb dating horizon, corresponding to ~ 1865. Apart from the episodic event, sediment accumulation rates for each of the layers, based on the application of the CRS model, are low and relatively uniform, with values ranging between 0.006 – 0.032 g cm–2 y–1 (0.05–0.12 cm y–1). 9 occurrence mainly in the upper part of the cores retrieved from the deepest part of the lakes, while they decrease in the littoral cores. Achnanthidium minutissimum is distributed throughout the depth gradient; this is reflected in their stability throughout the sediment record. A. minutissimum is also reported as a primary coloniser in disturbed environments (Peterson and Stevenson 1992; Hodgson et al. 1997). Being pioneer species, Pseudostaurosira are indicative of shallow waters, and tend to colonise the bottom sediment of open, shallow waters with unstable conditions of fluctuating physical or chemical characteristics. They are also commonly found growing around the stems of emergent plants (Griffiths et al. 2002). Succession of Aulacoseira species can also be related to the water level changes experienced. The bottom sediment layers were dominated by Aulacoseira alpigena, a cosmopolitan species preferring oligotrophic lakes with low conductivity, while upper layers were dominated by the large A. valida and A. ambigua. Heavily silificated Aulacoseira spp. are common under conditions of increase turbulent mixing and silica content as well as low light conditions (Reynolds 2006; Kilhan et al. 1996), but since they are often associated with lake turnover, it is more likely the change in nutrient availability that's the more consistent control. Nutrients that remain in the hypolimnion are not available to the primary producers in the illuminated upper layers. Various processes related to WLF may lead to changes in diapycnal and boundary mixing and, thus, affect the upward flux of hypolimnetic nutrients and their supply into overlying water layers. These include boundary mixing (Imberger 1994; MacIntyre et al. 1999) and, at extreme water level drawdown, also full mixing events (Naselli-Flores 2003; Baldwin et al. 2008); or uplifting and mixing of the hypolimnion by large cold inflows that plunge under an existing hypolimnion (Vilhena et al. 2010). The presence of large diatom species with well-silicified valves during extreme water fluctuations may also reflect the potential influence of increasing nutrient levels with decreasing depth (Jónsson 1992; Hawes and Smith 1994). Aulacoseira ambigua is common and widespread in shallow, moderately nutrient-rich lakes during periods of mixing and lower light levels. According to Houk (2003), A. valida is a Nordic-alpine cosmopolitan species, relatively infrequent, occurring in dystrophic and oligotrophic lakes and pools. Mixed conditions are essential for the growth of large centric diatoms, while stable thermal stratification would select for smaller centric diatoms (Reynolds 1988), as the 16 latter are able to perform better in lakes and reservoirs characterised by strong physical and chemical gradients (Reynolds 2006; Clegg et al. 2007). Water-level changes have not only brought about distinct taxonomic shifts in diatom communities, different species of which dominated in each lake’s core sections, but have also generated significant shifts in the composition of the diatoms’ ecological, morphofunctional and life-form groups (Fig. 7). The low- and high-profile diatoms were the dominant morphological forms in the upper core levels, which suffered the most extreme fluctuations in water level. Water depth is a major factor that controls the processes of resuspension and redistribution of sediment. Compared to the motile diatoms, the low and high profiles displayed the strongest response to disturbance. Motile taxa increased in the lower levels of the core with water level stability. Birraphid motile diatoms are useful indicators of sediment since their shape and motility makes them able to exploit silted habitats. Differences in growth forms can lead to different assemblages, depending on the type of substratum. For example, stable rock surfaces favour attached (i.e. low profile) species such as Achnanthidium and Gomphonema, while less stable surfaces lead to different assemblages. Fine sediments favour motile species such as Nitzschia while sand grains bear a distinctive flora of attached diatoms such as Staurosira. The effects of drawdown rates (the speed at which the level of a lake is lowered) or ramping (the speed of rising water levels) on lake communities are likely to be very important. Rapid drawdown rates (leading to rapid changes in water level) resulted in a poor water-column light regime and a substantial reduction in colonisable substrate. Pioneer species (low profile) that are able to colonise highly disturbed environments increase in quantity over time. These pioneers are subject to competition from other species until a disturbance removes most of the latter, allowing pioneer species to re-establish. Shifts in the proportions of benthic and planktic diatoms over time also indicate fluctuations in water level and retention (Søballe and Kimmel 1987). Low-profile ecological guilds (dominated by prostrate taxa) became more abundant in accordance with their expected resistance to increased scouring forces during drawdown. In these periods the communities are often dominated by low-profile diatoms like Cocconeis or Cymbella (Poff et al. 1990). The scouring stress of rapid drawdown rates may have also restricted the proportion of the high-profile guild (Biggs and Thomsen 1995; Yang et al. 2009). However, these forms may have the advantage of being able to survive and maintain themselves under the low light conditions caused by 17 sediment resuspension and siltation while water levels are dropping. Increased turbidity and the dominance of a high-profile guild may indicate lower irradiances and increased shading, which would restrict the development of the low-profile guild. The most important strength of the low-profile guild is its resistance to high scour. Under high erosion conditions, species in this guild were able to use irradiance more effectively than others. However, the direct access of the high-profile guild to irradiance enabled this guild to thrive better in a turbid environment than the other guilds despite its sensitivity to scouring. In contrast, the motile guild was positively associated with more stable water levels, which indicated finer sediments covering the lake bottom, as these would favour a higher relative abundance of this guild. The low resource availability, light conditions, and high disturbance prevented the establishment of a complex three-dimensional biofilm. This was beneficial for the high-profile forms, whose access to light from the water column was unimpeded. By contrast, although favoured by the high levels of disturbance, the low-profile guild underwent a significant decline in relative abundance, most likely as a result of shading within the biofilm. Shifts from generally low-profile species, i.e. single-celled, prostrate, and erect to generally high-profile species, i.e. filamentous, chain-forming or large diatoms, were documented with the increase in light (Steinman and McIntire 1987; Steinman et al. 1989; Bourassa and Cattaneo 2000). However, this guild still showed high numbers. This is possibly a result of the complex nature of high-profile growth, as many representatives are colonial forms and, depending on local conditions, they may comprise multiple cells – thus possessing the tall stature characteristic of this guild – or just a few cells, in which case they should be classified as low-profile species. The variation among the three guilds across substrate grain size is also pronounced; e.g. the low-profile guild dominates the epipsammon, where it is significantly more numerous than in the other three habitats. The epipsammon is an inhospitable habitat where diatoms, mostly with short stature and a firm attachment, can withstand the abrasion and physical damage from moving sand grains (Krejci and Lowe 1987; Round and Bukhtiyarova 1996). The high-profile guild is significantly more abundant in the epilithon and epiphyton, i.e. in habitats that are comparatively stable, less prone to dislodgement than the soft sediments and/or provide shelter from the drag (among the macrophyte leaves). The motile guild is significantly more numerous in the epipelon than in the other three habitats. Algae frequently get buried in the unstable 18 epipelic substrates, so selection favours motile species (Bahls 1993). The instability and movement of the soft substrates can create enough disturbance to keep the motile guild in check and allow species from the other two guilds to establish viable populations. Disturbance is more severe in the epipelon, which is generally found in deep-water conditions. This can explain why motile guild diversity was significantly lower in the top of the cores, where the greatest range of water-level fluctuation took place. At the same time, the high-profile guild is favoured by macrophytes because they provide shelter from the damaging impact of water drawdown. This could explain the greater presence of high-profile diatoms in Lake Garzonè, which is at a lower altitude than Lake Serodoli and presents a better developed community of aquatic plants. Conclusions Lake-level fluctuations are responsible for essential changes in the composition and structure of sediments, and consequently in the cycling of biogeochemical matter in the lake, as well as for changes in diatom composition and ecological, morphofunctional and life-form groups. Diatom assemblages in the lower section of the cores were dominated by typical birraphid motile species and small centrics. The relative contribution of low- and high-profile and large centric diatoms, morphologically and physiologically adapted to growth in well-flushed and turbid environments, increased in the upper layers of the sediment record when the lakes were subjected to abrupt waterlevel changes. In summary, the response of the diatom assemblages to WLF can be used in interpretations of water-level changes in the past. Acknowledgments We appreciate the constructive and helpful comments from two anonymous reviewers that improved the manuscript. 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Limnological variables were measured once a month for water chemistry during the open-water season between 1996-1999 (March– October). In the winter of 2006-07 the lakes were sampled once approximately from the same location. Limnological and geomorphological parameters Garzonè Serodoli Latitude (N) 46º 08’ 34’’ 46º 14’ 54’’ Longitude (E) 10º 42’ 20’’ 10º 46’ 48’’ Altitude (m a.s.l.) 1940 2371 Length (m) 575 352 Width (m) 265 305 Maximum Depth (m) 27 33.2 Mean. Depth (m) 12.1 15.0 Lake area (ha) 10.7 6.5 Lake volume (m3) 1400000 965000 Catchment surface (ha) 172 145 Lithology Granite Granite and tonalite Secchi depth (m) 15 12.5 Tempeature (ºC) 9.5 5.8 Oxygen (%) 74 96 pH (20 ºC) 6.2 6.0 Conductivity (μS cm-1) 20 11 Alkalinity (μeq l-1) 93 42 Ca2+ (mg l-1) 2.3 1.2 Mg 2+ (mg l-1) 0.2 0.1 Na+ (mg l-1) 0.5 0.2 K+ (mg l-1) 0.3 0.2 SO42- (mg l-1) 2.5 1.3 Cl- (mg l-1) 0.3 0.5 SiO2 (mg l-1) 3.0 1.6 N-NO3- (g l-1) 187.2 178.9 N-NH4+ (g l-1) 23.2 <20 P-PO43-(g l-1) 2.0 1.6 TP (g l-1) 8 4 TDS (mg l-1) 12.7 7.2 34 Table 2 Principal characteristics of the hydrological management of lakes Grazonè and Serodoli (Betti, 2003). Hydrological features Garzonè Serodoli Dam heigth (m) 1 3 Altitude after artificial damming (m a.s.l.) 1941.5 2370.0 Volume after artificial damming (m3)140000 965000 Piped volume (m3) 1280000 890000 Altitude after maximum piped (m a.s.l.) 1920.5 2347.0 Maximum piped volume / Total volume (%) 91.43 92.23 Pipe diameter (cm) 200 200 Year of construction 1954 1954 35 Table 3 List of cores taken at lakes Garzonè and Serodoli. Core ID Longitude Latitude Water depth (m) Core length (cm) GA07 S2 46º08’26.19’’ 10º42’16.09’’ 8.4 8 GA07 S4 46º08’31.89’’ 10º42’15.85’’ 26.6 25 SE07 S3 46º14’52.08’’ 10º46’38.00’’ 7.6 10 SE07 S4 46º14’55.12’’ 10º46’41.90’’ 28.4 26 36 Table 4 210Pbtotal, 210Pbexcess, 137Cs and 40K concentrations in SE07S4 and GA07S4 core. Depth Activity concentration (Bq kg–1, dry wt.) CRS Core (cm) 210Pbtotal 210Pbexcess 137Cs 40Kdate SE07S4 0.5 404  18 372  19 1279  51 36  6 1978 1.5 140  14 90  15 366  17 75  10 1955 2.5 80  13 39  14 86  6 72  11 1947 3.5 31  12 1  13 19  7 20  13 1947 4.5 119  10 91  12 12  3 25  7 1919 5.5 61  16 25  17 ND 41  13 1909 6.5 59  11 32  13 ND ND 1890 8.5 44  13 6  15 ND 37  14 1836 12.5 49  17 3  18 ND ND 1802 GA07S4 0.5 1386  62 1201  62 4950  198 ND 1990 1.5 744  36 508  37 1310  53 ND 1973 2.5 690  35 400  37 443  22 ND 1956 3.5 343  18 90  20 124  6 ND 1947 4.5 305  17 94  19 64  6 ND 1935 5.5 289  16 110  19 5  1 ND 1922 6.5 279  20 70  25 ND ND 1906 10.5 163  12 32  14 ND ND 1873 12.5 154  17 12  20 ND ND 1860 37