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Multiproxy reconstruction of a large and deep subalpine lake's ecological history since the Middle Ages
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.
Department of Physics, University of Trento.
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2015 (English)In: Journal of Great Lakes research, ISSN 0380-1330, Vol. 41, no 4, 982-994 p.Article in journal (Refereed) Published
Abstract [en]

Two short sediment cores collected at the deepest points of the two sub-basins of Lake Garda (Northern Italy) were radiometrically dated and analyzed for geochemistry, spheroidal carbonaceous particles, photosynthetic pigments and diatoms aimed at reconstructing the lake's ecological evolution over the last ~700 years, and at defining its reference conditions. Both cores showed steady dominance of small Cyclotella spp. and oligotrophic diatom-inferred lake TP concentrations from the Middle Ages until the 1960s. During the successive decades, meso- to eutraphentic elongated Fragilariaceae increased at the expense of small centrics, and diatom-inferred TP concentrations increased. Independent records of subfossil pigments revealed higher pytoplankton biomass and abundance of cyanobacteria in both lake basins since the 1990s. Trends of biological proxies and reconstructed lake TP level agree with modern limnological data collected since the 1990s. Multivariate analyses outlined lake nutrient level as the principal driver of long-term trophic and diatom evolution of Lake Garda and suggested that decadal scale climate dynamics (i.e. air temperature, East Atlantic and North Atlantic Oscillation teleconnection indices) may indirectly modulate the nutrient-driven phytoplankton evolution. The comparison of the two cores revealed that only the larger lake basin responded to major hydrological changes in the catchment during the 1940s. The study emphasizes the vulnerability of large and deep subalpine lakes towards the steadily increasing anthropogenic pressures affecting such lakes, under the present context of global warming.

Place, publisher, year, edition, pages
Elsevier, 2015. Vol. 41, no 4, 982-994 p.
Keyword [en]
Diatoms, Subfossil pigments, Paleolimnology, Subalpine lake, Reference condition, Climate change
National Category
Earth and Related Environmental Sciences
URN: urn:nbn:se:umu:diva-115187DOI: 10.1016/j.jglr.2015.08.008ISI: 000367359800005OAI: diva2:899035
Available from: 2016-01-31 Created: 2016-01-31 Last updated: 2016-02-04Bibliographically approved
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.
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
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)
Available from: 2016-02-05 Created: 2016-01-31 Last updated: 2016-02-05Bibliographically approved

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Milan, ManuelaBigler, Christian
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