Lake Mondsee Research Articles

Optimierung der Klauswehrordnung des Mondsees, Teil I – Hydraulik

ZusammenfassungZiel dieser Arbeit ist die Quantifizierung der hydraulischen Leistungsfähigkeit des Abflusssystems am Mondsee, Österreich. Die hier gewonnenen Ergebnisse dienen als hydraulische Grundlage im Prozess der Optimierung der Seesteuerung und definieren die technisch-hydraulischen Grenzen für Maßnahmen zur Verbesserung des Hochwasserrisikomanagements. Mithilfe eines hybriden Modellierungsansatzes – bestehend aus numerischer und physikalischer Modellierung sowie Feldmessungen zur Kalibrierung und Validierung – konnte gezeigt werden, dass die bestehenden hydraulischen Verhältnisse abflussbegrenzend wirken und die Seewasserstände bei Hochwasserereignissen erhöhen können. Die verringerte hydraulische Leistungsfähigkeit des Wehrzulaufs schränkt die derzeitige Seesteuerung am Klauswehr ein und limitiert die Möglichkeit einer Optimierung. Eine deutliche Erhöhung des Abflusses ist durch die Vergrößerung des effektiven Fließquerschnitts, Verringerung des Strömungswiderstands im Wehrzulauf und Anpassung der Klappenstellung am Wehr möglich. Die hydraulischen Analysen wurden von einer Stakeholderbefragung begleitet, aus der Einschränkungen und nutzerspezifische Kriterien für die Überarbeitung der Seesteuerung am Klauswehr abgeleitet werden konnten. Die Einbeziehung der Interessen der Betroffenen werden als entscheidender Schritt zu einer allgemein akzeptierten und damit realisierbaren optimierten Seesteuerung für ein verbessertes Hochwasserrisikomanagement am Mondsee angesehen.

Long-term adjustment of phytoplankton structure to environmental traits at timescales during lifetime development and over generations

Long-term ecological research of deep Lake Mondsee covers over more than six decades of phytoplankton observation. According to our study, phytoplankton groups of various phenological traits are suitable to address the impact of two major environmental stressors: nutrient surplus by eutrophication from 1968 to 1998 and warming by climate change. Here, we focus on phytoplankton biovolume, phytoplankton assemblage structure, net changes rates, and phytoplankton biodiversity. Biweekly net change-dependent persistence of phytoplankton assemblages followed a dome-shaped relationship of observations. A short-term persistence of taxonomic traits along weeks is predominant and contributes to the structural stabilization of phytoplankton assemblages. This short-term persistence is interpreted by the benefit of lifetime adjustment of phytoplankton organisms. The long-term development phytoplankton structure is discussed as an alignment of organisms over generations. Single key taxa, as Planktothrix rubescens (De Candolle ex Gomont) Anagnostidis &Komárek 1988, which are omnipotent players in phytoplankton assemblages and occur during different environmental scenarios in the long-term, are most suitable for long-term ecological research. Our findings interpret that phytoplankton taxa are excellent organisms to track the impact of environmental constraints due to their short generation time (1), their lifetime adjustment (2), and the re-occurrence in the long-term over generations (3).

A First Detailed Study of an Austrian Neolithic Pile Dwelling’s Bryophyte Flora

ABSTRACT The circum-alpine pile dwellings, ranging from the Neolithic period to the Bronze Age, offer a vast amount of well-preserved botanical material for archaeobotanical studies. Mosses are a common find in these waterlogged sediments, yet they are rarely analysed in detail. The case study of the recently excavated Mooswinkel pile dwelling at the Austrian lake Mondsee will give an insight into the study of moss remains. 16 different species from various habitats have been identified in the layers of the late Neolithic site. The analysis shows what possible uses the mosses could have served and from where they were gathered by the pile dwellers.

Visible or not? Reflection of the 8.2 ka BP event and the Greenlandian–Northgrippian boundary in a new high-resolution pollen record from the varved sediments of Lake Mondsee, Austria

This paper presents a new pollen record from Lake Mondsee in the north-eastern European Alps and discusses changes in vegetation composition between 9000 and 7000 a BP in response to the 8.2 ka BP cooling event, which is clearly reflected in a stable oxygen isotope record from the same lake. Pollen and microcharcoal analyses performed at bi-decadal resolution provide detailed insights into the vegetation history and fire regime in the study area. The late Greenlandian and early Northgrippian between ca. 9000 and 8000 a BP were mainly characterised by the gradual expansion of spruce, accompanied by the retreat of hazel. Results of the pollen-based biome reconstruction show that the Greenlandian–Northgrippian boundary at ca. 8280 a BP was marked by a shift in the affinity scores of regional forest biomes, reflecting a decreasing role of temperate broadleaved trees and shrubs and a strengthening of boreal conifers in the study area. During the early Northgrippian between ca. 8000 and 7000 a BP, fir and beech spread around Lake Mondsee, which is in line with other pollen records from the northern Alpine Foreland, indicating cooler and moister conditions. This long-term trend is likely a response to decreasing summer insolation (i.e. decreasing summer temperatures) and the weakening of the Siberian High, which promoted westerly-driven moisture transport from the Atlantic region. Visual inspection of the Lake Mondsee pollen data does not reveal a clear signal of the 8.2 ka BP event. However, biomization results show a distinct minimum in the temperate deciduous biome scores and summed scores of plant functional types representing warm-loving tree and shrub taxa at ca. 8165–8135 a BP.

How animal dung can help to reconstruct past forest use: a late Neolithic case study from the Mooswinkel pile dwelling (Austria)

Animal dung analyses are a useful tool for vegetational studies. Preserved ruminant dung from archaeological layers offers a unique possibility for the reconstruction of past fodder management strategies, and further for studying the impact of fodder acquisition and pasturing on forests. In this case study we investigate the impact of Late Neolithic livestock keeping on the forest around the “Mooswinkel” pile dwelling at the Austrian lake Mondsee through the analysis of botanical macroremains, insect remains as well as microhistological analyses of botanical remains in animal dung. Seasonal plant parts in the dung point out that cattle, goats, and/or sheep were evidently kept inside the settlement during the winter for protection. During the daytime, they were allowed to forage around the settlement. Winter fodder consisted of dried leaf hay, hay from grasses and herbs, male flowers of early blooming bushes, and fresh twigs of evergreen species, such as fir (Abies alba), ivy (Hedera helix), and mistletoe (Viscum album).

Can 7000 Years of flood history inform actual flood risk management? A case study on Lake Mondsee, Austria

Flood risk models typically rely on discharge records of time series no longer than a few hundred years, which leads to high uncertainty in the estimates for major flood events. In this paper, we investigate the capability of geological flood records retrieved from lake sediment cores to prolong available data sets, lower the uncertainty of flood risk analysis, and detect changes in flood patterns by releasing the common assumption of stationarity. Paleoflood records of Lake Mondsee, Austria, covering 7000 years are linked to flood damage data of the considered catchment area to calculate (1) the average annual damage, i.e. the fair premium and (2) the damage at 99.5% Value at Risk, i.e. the Solvency II capital requirement. To illustrate our results, the performance of a hypothetical catastrophe fund for these 7000 years is studied at current vulnerability levels. Our findings show that high resolution paleoflood records with robust chronologies and flood information have significant impacts on flood model outcomes for disaster risk management and that damage potentials derived merely from recent damage data may overestimate actual risks.

Aquatic food webs in deep temperate lakes: Key species establish through their autecological versatility

Microbial planktonic communities are the basis of food webs in aquatic ecosystems since they contribute substantially to primary production and nutrient recycling. Network analyses of DNA metabarcoding data sets emerged as a powerful tool to untangle the complex ecological relationships among the key players in food webs. In this study, we evaluated co-occurrence networks constructed from time-series metabarcoding data sets (12months, biweekly sampling) of protistan plankton communities in surface layers (epilimnion) and bottom waters (hypolimnion) of two temperate deep lakes, Lake Mondsee (Austria) and Lake Zurich (Switzerland). Lake Zurich plankton communities were less tightly connected, more fragmented and had a higher susceptibility to a species extinction scenario compared to Lake Mondsee communities. We interpret these results as a lower robustness of Lake Zurich protistan plankton to environmental stressors, especially stressors resulting from climate change. In all networks, the phylum Ciliophora contributed the highest number of nodes, among them several in key positions of the networks. Associations in ciliate-specific subnetworks resembled autecological species-specific traits that indicate adaptions to specific environmental conditions. We demonstrate the strength of co-occurrence network analyses to deepen our understanding of plankton community dynamics in lakes and indicate biotic relationships, which resulted in new hypotheses that may guide future research in climate-stressed ecosystems.

Anthropogenic and climate controls on vegetation changes between 1500 BCE and 500 CE reconstructed from a high-resolution pollen record from varved sediments of Lake Mondsee, Austria

This study reports a precisely dated pollen record with a 20-year resolution from the varved sediments of Lake Mondsee in the north-eastern European Alps (47°49′N, 13°24′E, 481 m above sea level). The analysed part of core spans the interval between 1500 BCE and 500 CE and allows changes in vegetation composition in relation to climatic changes and human activities in the catchment to be inferred. Intervals of distinct but modest human impact are identified at ca. 1450–1220, 740–490 and 340–190 BCE and from 80 BCE to 180 CE. While the first two intervals are synchronous with prominent salt mining phases during the Bronze Age and Early Iron Age at the nearby UNESCO World Heritage Site of Hallstatt, the last two intervals fall within the Late Iron Age and Roman Imperial Era, respectively. Comparison with published records of extreme runoff events obtained from the same sediment core shows that human activities (including agriculture and logging) around Lake Mondsee were low during intervals of high flood frequency as indicated by a higher number of intercalated detrital event layers, but intensified during hydrologically stable intervals. Comparison of the pollen percentages of arboreal taxa with the stable oxygen isotope and potassium ion records of the NGRIP and GISP2 ice cores from Greenland reveals significant positive correlations for Fagus and negative correlations for Betula and Alnus. This underlines the sensitivity of vegetation around Lake Mondsee to temperature fluctuations in the North Atlantic as well as to moisture fluctuations controlled by changes in the intensity of the Siberian High and the North Atlantic Oscillation (NAO) regime.

Stable oxygen and carbon isotopes of carbonates in lake sediments as a paleoflood proxy

Abstract Lake sediments are increasingly explored as reliable paleoflood archives. In addition to established flood proxies including detrital layer thickness, chemical composition, and grain size, we explore stable oxygen and carbon isotope data as paleoflood proxies for lakes in catchments with carbonate bedrock geology. In a case study from Lake Mondsee (Austria), we integrate high-resolution sediment trapping at a proximal and a distal location and stable isotope analyses of varved lake sediments to investigate flood-triggered detrital sediment flux. First, we demonstrate a relation between runoff, detrital sediment flux, and isotope values in the sediment trap record covering the period 2011–2013 CE including 22 events with daily (hourly) peak runoff ranging from 10 (24) m3 s−1 to 79 (110) m3 s−1. The three- to ten-fold lower flood-triggered detrital sediment deposition in the distal trap is well reflected by attenuated peaks in the stable isotope values of trapped sediments. Next, we show that all nine flood-triggered detrital layers deposited in a sediment record from 1988 to 2013 have elevated isotope values compared with endogenic calcite. In addition, even two runoff events that did not cause the deposition of visible detrital layers are distinguished by higher isotope values. Empirical thresholds in the isotope data allow estimation of magnitudes of the majority of floods, although in some cases flood magnitudes are overestimated because local effects can result in too-high isotope values. Hence we present a proof of concept for stable isotopes as reliable tool for reconstructing flood frequency and, although with some limitations, even for flood magnitudes.

Flood occurrence change-point analysis in the paleoflood record from Lake Mondsee (NE Alps)

Knowledge about changes of flood occurrence patterns is important for risk estimation of the future. Robust and well-calibrated paleoflood records, derived e.g. from lake sediments, are excellent natural archives to investigate flood variability of the past and to use the data for further modelling. In this paper, we analyse a 7100 year summer flood record recovered from Lake Mondsee (NE Alps), using a statistical approach. We identify a point process of renewal type, with a significant change-point of the occurrence pattern around 350 CE, switching from the overlay of two mechanisms of event recurrences of 5 and 50 years before to 2 and 17 years after this change-point. This change-point approach enables a comparison to other flood records, and possibly to relate event frequencies to climatic conditions. We also highlight how lower temporal resolution of flood records can hamper the analysis of relations to climatic signals. Hence high-resolution records with robust chronologies and flood information (e.g. seasonality and event characteristics) are essential to improve the understanding of the interplay between climatic signals and flood occurrences, which is an important ingredient for proper risk estimation and risk management.

Late Glacial and Holocene sedimentary infill of Lake Mondsee (Eastern Alps, Austria) and historical rockfall activity revealed by reflection seismics and sediment core analysis

Abstract Glacigenic perialpine lakes can constitute continuous post-last glacial maximum (LGM) geological archives which allow reconstruction of both lake-specific sedimentological processes and the paleoenvironmental setting of lakes. Lake Mondsee is one among several perialpine lakes in the Salzkammergut, Upper Austria, and has been previously studied in terms of paleoclimate, paleolimnology and (paleo)ecology. However, the full extent and environment of Late Glacial to Holocene sediment deposition had remained unknown, and it was not clear whether previously studied core sections were fully representative of 3D sediment accumulation patterns. In this study, the sedimentary infill of Lake Mondsee was examined via high-resolution seismic reflection survey over a 57-km extent (3.5 kHz pinger source) and a sediment core extracted from the deepest part of the lake, with a continuous length of 13.76 m. In the northern basin, seismic penetration is strongly limited in most areas because of abundant shallow gas (causing acoustic blanking). In the deeper areas, the acoustic signal reaches depths of up to 80 ms TWT (two-way travel time), representing a postglacial sedimentary sequence of at least 60-m thickness. Holocene deposits constitute only the uppermost 11.5 m of the sedimentary succession. Postglacial seismic stratigraphy of Lake Mondsee closely resembles those of well-studied French and Swiss perialpine lakes, with our data showing that most of Lake Mondsee’s sedimentary basin infill was deposited within a short time period (between 19,000 BP and 14,500 BP) after the Traun Glacier retreated from the Mondsee area, indicating an average sedimentation rate of about 1.4 cm/yr. Compared to other perialpine lakes, the seismic data from Lake Mondsee reveal little indication of mass movement activities during the Holocene. One exception, however, is rockfalls that originate from a steep cliff, the Kienbergwand, situated on the southern shore of Lake Mondsee, where, in the adjacent part of the lake, seismic profiles show mass transport deposits (MTDs), which extend approximately 450 m from the shore and are mappable over an area of about 45,300 m2. Sediment cores targeting the MTDs show two separate rockfall events. The older event consists of clast-supported angular dolomitic gravels and sands, showing high amounts of fine fraction. The younger event exhibits dolomitic clasts of up to 1.5 cm in diameter, which is mixed within a lacustrine muddy matrix. Radiocarbon dating and correlations with varve-dated sediment cores hint at respective ages of AD 1484 ± 7 for Event 1 and AD 1639 ± 5 for Event 2. As our data show no evidence of larger-scale mass movements affecting Lake Mondsee and its surroundings, we infer that the current-day morphology of the Kienbergwand is the result of infrequent medium-scale rockfalls.

Rapid detection and quantification of the potentially toxic cyanobacterium Planktothrix rubescens by in-vivo fluorometry and flow cytometry

We combined profiling of the bloom-forming and potentially toxigenic cyanobacterium Planktothrix rubescens using a multiparameter probe equipped with a phycoerythrin sensor (in vivo fluorometry, IVL) in Lake Mondsee, Austria, with flow cytometric live analyses of discrete samples taken from several depths in the upper 20 m of the water column. Results obtained by IVL and acoustic flow cytometry (AFC) were compared to microscopic analyses of integrated (0–21 m) water samples using fixed material. This comparison was made because the integrated samples are used for the Austrian monitoring programme according to the EU Water Framework Directive. We demonstrate that AFC provides quantitative analyses of the filaments of P. rubescens that are significantly correlated to IVF and microscopic analyses, allowing rapid (within hours) and more precise calculation of P. rubescens biomass than estimates derived from IVL. Our analysis shows that vertically integrated water samples provide unreliable information on the concentration of P. rubescens in the upper surface waters and on the peak concentration of P. rubescens within the water column. We conclude that the protocol that we developed is superior to the current monitoring practice.

Do current European lake monitoring programmes reliably estimate phytoplankton community changes?

Many European lakes are monitored according to the EU Water Framework Directive (WFD), with focus on phytoplankton biomass and species composition. However, the low-frequency WFD monitoring may miss short-term phytoplankton changes. This is an important issue because short-term extreme meteorological events (heat waves and heavy rain) are predicted to increase in frequency and intensity with climate change. We used records from Lake Mondsee (Austria) from 2009 to 2015 to test if a reduction from monthly to seasonal sampling affected the average annual phytoplankton biovolume. Furthermore, we combined inverted light microscopy, FlowCAM and flow cytometry to estimate the effect of sampling during extreme events on average phytoplankton biovolume. Relative to monthly sampling, seasonal sampling significantly overestimated phytoplankton biomass. A heat wave in 2015 and two episodes of heavy rain in 2015 and 2016 caused species-specific changes; biovolumes of chlorophytes and the filamentous cyanobacterium Planktothrix rubescens (De Candolle ex Gomont) Anagnostidis & Komárek increased significantly during the heat wave. Using live material with FlowCAM and flow cytometry, we detected small and fragile cells and colonies that were either ignored or underrepresented by analysing fixed samples with light microscopy. We suggest a modified sampling and analysis strategy to capture short-term changes within the phytoplankton community.

Phytoplankton response to the summer 2015 heat wave – a case study from prealpine Lake Mondsee, Austria

Phytoplankton response to a heat wave in deep prealpine Lake Mondsee, Austria, was minor overall, although temperature significantly affected the epilimnetic phytoplankton assemblage at the deepest site of the lake. We detected no significant effect of nutrients. Using 3 complementary optical methods—light microscopy (Utermöhl technique), FlowCAM, and acoustic flow cytometry (AFC)—we found relatively low horizontal variation in the epilimnion but significant changes in phytoplankton community composition integrated over the 0–20 m water layer at the central station of the lake. These changes were mainly caused by a vertical shift in the Planktothrix rubescens peak from 11 to 16 m during the heat wave. Air temperatures reaching or exceeding 30 °C were measured in the area during 6 consecutive days at the end of June to the beginning of July, the first of 3 heat waves recorded during summer 2015; in-shore surface water temperature exceeded 27 °C. We sampled 9 stations across Lake Mondsee on 4 occasions during the heat wave and analysed temperature, nutrient levels, conductivity, pH, and the phytoplankton community. In addition to reporting the implications of increasing water temperatures for the algal assemblage in a deep stratified lake, we discuss the pros and cons of the different optical methods for phytoplankton identification and counting. For future field campaigns similar to the present study, we recommend using light microscopy to assess large or rare species, such as Ceratium hirundinella, and AFC, FlowCAM, or similar semi-automated devices for abundant small- to medium-sized species.

Hydrological and sedimentological processes of flood layer formation in Lake Mondsee

AbstractDetrital layers in lake sediments are recorders of extreme flood events. However, their use for establishing time series of past floods is limited by lack in understanding processes of detrital layer formation. Therefore, we monitored hydro‐sedimentary dynamics in Lake Mondsee (Upper Austria) and its main tributary, Griesler Ache, over a 3‐year period from January 2011 to December 2013. Precipitation, discharge and turbidity were recorded continuously at the river outlet to the lake and compared to sediment fluxes trapped with 3 to 12 days resolution at two locations in the lake basin, in a distance of 0·9 (proximal) and 2·8 km (distal) to the Griesler Ache inflow. Within the 3‐year observation period, 26 river floods of different magnitude (10 to 110 m3 s−1) have been recorded resulting in variable sediment fluxes to the lake (4 to 760 g m−2 d−1) including the ‘century‐scale’ flood event in June 2013. The comparison of hydrological and sedimentological data revealed (i) a rapid sedimentation within 3 days after the peak runoff in the proximal and within 6 to 10 days in the distal lake basin; (ii) empirical flood thresholds for triggering sediment flux at the lake floor increasing from the proximal (20 m3 s−1) to the distal lake basin (30 m3 s−1) and (iii) various factors that control the detrital sediment transport in the lake. The amount of sediment transported to the lake is controlled by runoff and catchment sediment availability. The distribution of detrital sediment within the lake basin is mainly driven by mesopycnal interflows and closely linked to flood duration and the season in which a flood occurred. The combined hydro‐sedimentary monitoring revealed detailed insights into processes of flood layer formation in a meso‐scale peri‐Alpine lake and, thereby, improves the interpretation of the depositional record of flood layers.

Palaeoecology of Late Glacial and Holocene profundal Ostracoda of pre-Alpine lake Mondsee (Austria) — A base for further (palaeo-)biological research

Succession of profundal ostracod palaeoassemblages in response to environmental changes during the Late Glacial and Holocene was studied in a~15-m-long sediment sequence from pre-Alpine lake Mondsee (Austria). First local ostracod assemblage zone LOAZ-1 (prior to 15,700 a BP), with low abundances of Leucocythere mirabilis and Limnocytherina sanctipatricii followed by Cytherissa lacustris, corresponds to the Pleniglacial phase of clastic-detrital sedimentation at relatively high rates. Most of the key species of LOAZ-2 (15,590 to 13,940 a BP, including the Pleniglacial–Late Glacial transition), i.e. limnocytherids, Fabaeformiscandona cf. harmsworthi, F. tricicatricosa, C. lacustris and Candona candida, reveal the significant association with high Al contents and low sedimentation rates and are classified as preferring low-productivity conditions. In contrast, Candona neglecta, dominant in LOAZ-3 (13,820 to 9960 a BP, palynologically defined as Bölling–Early Holocene) and in the last LOAZ-4 (9780 a BP to present) as well as Cypria ophtalmica (second key species in LOAZ-4) show relationship with higher productivity, elevated sedimentation rates and decreasing Al contents.Furthermore, valve biodegradation and shape disparity at different time periods and in relation to sedimentation rate changes were investigated in A-3 juveniles of Candona neglecta. The highest percentage of valves biodegraded by Actinobacteria was recorded during the Allerød, Younger Dryas and Early Holocene, when the lowest sedimentation rates occurred, whereas significantly lower frequencies of biodegraded valves were recorded during the mid-Holocene and Late Holocene, when sediment accumulation was much higher. It is also hypothesised that the degree of the valve shape variation was related to the lake productivity. During the Allerød, Younger Dryas and Early Holocene a valve shape variation significantly lower than that recorded during Late Holocene intervals, characterised by intensified lake productivity, was observed. Additionally, an agenda of potential questions and approaches that should be considered and form the core of further (palaeo-)biological research projects is offered.

Processes of flood‐triggered detrital layer deposition in the varved Lake Mondsee sediment record revealed by a dual calibration approach

ABSTRACTA succession of 23 sub‐millimetre to maximum 12‐mm‐thick, mostly flood‐triggered detrital layers, deposited between 1976 and 2005, was analysed in 12 varved surface sediment cores from meso‐scale peri‐alpine Lake Mondsee applying microfacies and high‐resolution micro X‐ray fluorescence analyses. Detailed intrabasin comparison of these layers enabled identification of (i) different source areas of detrital sediments, (ii) flood‐triggered sediment flux and local erosion events, and (iii) seasonal differences of suspended flood sediment distribution within the lake basin. Additional calibration of the detrital layer record with river discharge and precipitation data reveals different empirical thresholds for flood layer deposition for different parts of the basin. At proximal locations detrital layer deposition requires floods exceeding a daily discharge of 40 m3 s−1, whereas at a location 2 km more distal an hourly discharge of 80 m3 s−1 and at least 2 days of discharge above 40 m3 s−1 are necessary. Furthermore, we observe a better correlation between layer thickness and flood amplitude in the depocentre than in distal and proximal areas of the basin. Although our results are partly site‐specific, the applied dual calibration approach is suitable to precisely decipher flood layer formation processes and, thereby, improve the interpretation of long flood time series from lake sediments.

A Buoy for Continuous Monitoring of Suspended Sediment Dynamics

Knowledge of Suspended Sediments Dynamics (SSD) across spatial scales is relevant for several fields of hydrology, such as eco-hydrological processes, the operation of hydrotechnical facilities and research on varved lake sediments as geoarchives. Understanding the connectivity of sediment flux between source areas in a catchment and sink areas in lakes or reservoirs is of primary importance to these fields. Lacustrine sediments may serve as a valuable expansion of instrumental hydrological records for flood frequencies and magnitudes, but depositional processes and detrital layer formation in lakes are not yet fully understood. This study presents a novel buoy system designed to continuously measure suspended sediment concentration and relevant boundary conditions at a high spatial and temporal resolution in surface water bodies. The buoy sensors continuously record turbidity as an indirect measure of suspended sediment concentrations, water temperature and electrical conductivity at up to nine different water depths. Acoustic Doppler current meters and profilers measure current velocities along a vertical profile from the water surface to the lake bottom. Meteorological sensors capture the atmospheric boundary conditions as main drivers of lake dynamics. It is the high spatial resolution of multi-point turbidity measurements, the dual-sensor velocity measurements and the temporally synchronous recording of all sensors along the water column that sets the system apart from existing buoy systems. Buoy data collected during a 4-month field campaign in Lake Mondsee demonstrate the potential and effectiveness of the system in monitoring suspended sediment dynamics. Observations were related to stratification and mixing processes in the lake and increased turbidity close to a catchment outlet during flood events. The rugged buoy design assures continuous operation in terms of stability, energy management and sensor logging throughout the study period. We conclude that the buoy is a suitable tool for continuous monitoring of suspended sediment concentrations and general dynamics in fresh water bodies.

Mid- to late Holocene flood frequency changes in the northeastern Alps as recorded in varved sediments of Lake Mondsee (Upper Austria)

Annually laminated (varved) lake sediments with intercalated detrital layers resulting from sedimentary input by runoff events are ideal archives to establish precisely dated records of past extreme runoff events. In this study, the mid- to late Holocene varved sediments of Lake Mondsee (Upper Austria) were analysed by combining sedimentological, geophysical and geochemical methods. This approach allows to distinguish two types of detrital layers related to different types of extreme runoff events (floods and debris flows) and to detect changes in flood activity during the last 7100 years. In total, 271 flood and 47 debris flow layers, deposited during spring and summer, were identified, which cluster in 18 main flood episodes (FE 1–18) with durations of 30–50 years each. These main flood periods occurred during the Neolithic (7100–7050 vyr BP and 6470–4450 vyr BP), the late Bronze Age and the early Iron Age (3300–3250 and 2800–2750 vyr BP), the late Iron Age (2050–2000 vyr BP), throughout the Dark Ages Cold Period (1500–1200 vyr BP), and at the end of the Medieval Warm Period and the Little Ice Age (810–430 vyr BP).Summer flood episodes in Lake Mondsee are generally more abundant during the last 1500 years, often coinciding with major advances of Alpine glaciers. Prior to 1500 vyr BP, spring/summer floods and debris flows are generally less frequent, indicating a lower number of intense rainfall events that triggered erosion. In comparison with the increase of late Holocene flood activity in western and northwestern (NW) Europe, commencing already as early as 2800 yr BP, the hydro-meteorological shift in the Lake Mondsee region occurred much later. These time lags in the onset of increased hydrological activity might be either due to regional differences in atmospheric circulation pattern or to the sensitivity of the individual flood archives. The Lake Mondsee sediments represent the first precisely dated and several millennia long summer flood record for the northeastern (NE) Alps, a key region at the climatic boundary of Atlantic, Mediterranean and East European air masses, aiding a better understanding of regional and seasonal peculiarities of flood occurrence under changing climate conditions.

Late Neolithic Mondsee Culture in Austria: living on lakes and living with flood risk?

Abstract. Neolithic and Bronze Age lake dwellings in the European Alps became recently protected under the UNESCO World Heritage. However, only little is known about the cultural history of the related pre-historic communities, their adaptation strategies to environmental changes and particularly about the almost synchronous decline of many of these settlements around the transition from the Late Neolithic to the Early Bronze Age. For example, there is an ongoing debate whether the abandonment of Late Neolithic lake dwellings at Lake Mondsee (Upper Austria) was caused by unfavourable climate conditions or a single catastrophic event. Within the varved sediments of Lake Mondsee, we investigated the occurrence of intercalated detrital layers from major floods and debris flows to unravel extreme surface runoff recurrence during the Neolithic settlement period. A combination of detailed sediment microfacies analysis and μXRF element scanning allows distinguishing debris flow and flood deposits. A total of 60 flood and 12 debris flow event layers was detected between 7000 and 4000 varve years (vyr) BP. Compared to the centennial- to millennial-scale average, a period of increased runoff event frequency can be identified between 5900 and 4450 vyr BP. Enhanced flood frequency is accompanied by predominantly siliciclastic sediment supply between ca. 5500 and 5000 vyr BP and enhanced dolomitic sediment supply between 4900 and 4500 vyr BP. A change in the location and the construction technique of the Neolithic lake dwellings at Lake Mondsee can be observed during the period of higher flood frequency. While lake dwellings of the first settlement period (ca. 5800–5250 cal. yr BP) were constructed directly on the wetlands, later constructions (ca. 5400–4700 cal. yr BP) were built on piles upon the water, possibly indicating an adaptation to either increased flood risk or a general increase of the lake level. However, our results also indicate that other than climatic factors (e.g. socio-economic changes) must have influenced the decline of the Mondsee Culture because flood activity generally decreased since 4450 vyr BP, but no new lake dwellings have been established thereafter.