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Tracing recent geochemical sediment properties in subalpine lakes using wavelength-dispersive X-ray fluorescence spectroscopy (XRF): understanding regional patterns of recent lake development
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. IASMA Research and Innovation Centre, E. Mach Foundation.ORCID iD: 0000-0003-1637-304X
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
IASMA Research and Innovation Centre, E. Mach Foundation–Istituto Agrario di San Michele all'Adige, Italy.
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The sedimentary geochemical records of two subalpine lakes (Lake Garda and Lake Ledro) in northern Italy were analyzed and explored using wavelength-dispersive X-ray fluorescence spectroscopy (XRF). The aim of the study was to investigate how human impact and environmental changes have affected the geochemical record of the two lakes and their catchments, in comparison to existing paleolimnological evidence based on biological proxies. Sediment cores were collected from the deepest points of the two basins of Lake Garda (Brenzone and Bardolino) and Lake Ledro, and chronologies were established using radioisotopic dating (210Pb and 14C). In Brenzone, the main basin of Lake Garda, a pronounced shift in elemental composition occurred in the mid-1900s, when major elements (Mg, Al, K) and lithogenic tracers (Ti) start to decrease, while some elements related to redox conditions (Mn, P) and other (contaminant) trace elements (Pb) increase. Bardolino, the shallower and smaller basin of Lake Garda, shows to some extent comparable shifts (Mg, Al, K, Ti, Pb), but some elements show deviating patterns such as those influenced by changing redox (Mn, P). Lake Ledro shows in general higher short-term variability for most elements, even though some features are comparable (Pb) to Brenzone and Bardolino. Overall, the geochemical record reveals a general change in Lake Garda since the mid-1900s, which is in good agreement with the biological records (diatoms, Cladocera, pigments). The differences recorded in the two basins of Lake Garda reflect the effects of local conditions, both related to hydrology and sedimentation patterns. In contrast, the more variable geochemical record of Lake Ledro is mainly affected by human-induced lake-level fluctuations and the effects of the relatively large catchment. This study reveals the importance of the ratio between lake area and catchment area on the lake geochemical records. The sediment records from large and deep lakes seem to be more affected by direct impacts of nutrient enrichment and/or climate change than the changes within the catchment area. On the other hand, small lakes with larger catchment areas are to a larger extent influenced by the modifications occurring in the drainage basin.

Keyword [en]
Paleolimnology, Geochemistry, Wavelength-dispersive X-ray fluorescence spectroscopy, Lake Garda, Lake Ledro
National Category
Earth and Related Environmental Sciences
Identifiers
URN: urn:nbn:se:umu:diva-115193OAI: oai:DiVA.org:umu-115193DiVA: diva2:899043
Available from: 2016-01-31 Created: 2016-01-31 Last updated: 2016-02-04
In thesis
1. Long-term development of subalpine lakes: effects of nutrients, climate and hydrological variability as assessed by biological and geochemical sediment proxies
Open this publication in new window or tab >>Long-term development of subalpine lakes: effects of nutrients, climate and hydrological variability as assessed by biological and geochemical sediment proxies
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Sediment records of two Italian subalpine lakes (Lake Garda and Lake Ledro) were analyzed in order to reconstruct their ecological evolution over the past several hundred years. A multi-proxy and multi-site approach was applied in order to disentangle the effects of local anthropogenic forcings, such as nutrients, and climate impacts on the two lakes and their catchments. Biological indicators (sub-fossil pigments, diatoms and Cladocera) were used to reconstruct changes in the aquatic food web and to define the lake reference conditions, while geochemical methods, i.e. wavelength-dispersive X-ray fluorescence spectroscopy (WD-XRF), were used to provide quantitative information on the different physical or chemical processes affecting both lake and catchment systems.

Sub-fossil pigments and diatoms, together with their respective inferred TP values, suggested very stable oligotrophic conditions in both lakes until the 1960s. The period following was affected by nutrient enrichment, which led to a drastic shift in the phytoplanktonic community. The response of sub-fossil pigments and diatoms to major climatic anomalies such as the Medieval Climatic Anomaly (MCA) and the Little Ice Age (LIA) were not pronounced, and the taxonomic composition remained relatively stable. On the contrary, these proxies showed an indirect response to climate variability since the beginning of the nutrient enrichment phase in the 1960s. In Lake Garda, the winter temperature regulates the water column mixing, which in its turn controls the degree of nutrient fertilization of the entire water column, and the related phytoplankton growth. In Lake Ledro a rapid reorganization of planktonic diatoms was observed only during the temperature recovery after the LIA, while recent temperature effects are masked by the prevailing nutrient effects. In Lake Garda, Cladocera remains responded in quantitative and qualitative terms to climatic changes, whereas in Lake Ledro they appeared to be mainly affected by variations in hydrological regimes, i.e. flood events. Cladocera remains corroborated the nutrient enrichment after the 1960s in both lakes as inferred by diatoms and pigments.

In Lake Garda, the geochemical data showed a pronounced shift in elemental composition since the mid-1900s, when major elements and lithogenic tracers started to decrease, while some elements related to redox conditions and other (contaminant) trace elements increased. The general trends since the mid-1900s agree with the biological records. However, some differences recorded in the two different basins of Lake Garda reflected the effects of local conditions, both related to hydrology and sedimentation patterns. Lake Ledro showed higher short-term variability for most elements, even though some features were comparable to Lake Garda. The geochemical record of Lake Ledro revealed a major influence of human-induced lake-level fluctuations and catchment properties.

This paleolimnological study allows us to place temporally restricted limnological surveys into a longer-term secular perspective, which is highly valuable for the definition of lake reference conditions. Because the restoration targets are usually based on the lake reference conditions, this study highlighted also the necessity to pay particular attention to the lake-specific sensitivity patterns. The multi-proxy and multi-site approach showed that the lake conditions of large and deep lakes in northern Italy, such as Lake Garda, are mainly driven by nutrient enrichment and/or climate change. In contrast, smaller lakes with larger catchment areas, such as Lake Ledro, are seemingly more impacted by conditions and processes occurring in the drainage basin.

Place, publisher, year, edition, pages
Umeå: Umeå University, 2016. 22 p.
Keyword
Paleolimnology, diatoms, Cladocera, sub-fossil pigments, geochemistry, wavelengthdispersive X-ray fluorescence spectroscopy, Lake Garda, Lake Ledro, reference conditions, nutrient enrichment, climate change, hydrological regime.
National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:umu:diva-115188 (URN)978-91-7601-396-0 (ISBN)
Public defence
2016-02-26, KBC-huset, Lilla Hörsalen, KB3A9, Umeå universitet, Umeå, 10:00 (English)
Opponent
Supervisors
Available from: 2016-02-05 Created: 2016-01-31 Last updated: 2016-02-05Bibliographically approved

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