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Different performances of independent sediment biological proxies in tracking ecological transitions and tipping points of a small sub-alpine lake since the Little Ice Age
IASMA Research and Innovation Centre, E. Mach Foundation–Istituto Agrario di San Michele all'Adige, Italy.
Institute for Ecosystem Study (CNR-ISE).
Environmental Change Research Centre (ECRC), University College London.
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
(English)Manuscript (preprint) (Other academic)
Abstract [en]

A comparative study of independent geochemical and biological proxies was carried out on a short (83 cm) sediment core collected in 2011 from the deepest point of a small subalpine Lake Ledro (Trentino, N-Italy). The aim of the study is to compare the capability of subfossil photosynthetic pigments, diatoms and Cladocera in tracking lake ecological transitions and tipping points related to major environmental perturbations occurred during the last three centuries, i.e. after the culmination of the Little Ice Age in the Alpine region. In relation to the sparse neo-limnological and climate data available for the lake, the study aims also at defining of the lake trophic and ecological reference conditions, at improving the reconstruction of the nutrient enrichment process during the last decades, and at evaluating the effects of restoration measures initiated in the 1990s. The analysis of the selected proxies outlined a pronounced sensitivity of Lake Ledro to hydrological variability throughout the whole time span considered, especially during the 18th and 19th century, and revealed two major stages in the ecological evolution of the lake, which were mainly controlled by climate related hydrological variability and lake nutrients. The results largely agree with the hypothesis that responses of sediment biological proxies to different natural and human stressors may differ in type, timing and magnitude. Subfossil pigments, diatoms and Cladocera showed a comparable capability in tracking ecological transitions and tipping points related to lake hydrology and nutrient variability, while only diatoms demonstrated a certain capability to track changes in water temperature of the lake studied. The strong response of planktonic organisms to hydrological variability depends on the peculiar catchment and lake morphology, and confirmed that planktonic organism principally respond to climate variability in an indirect way. The reconstruction of the trophic development of Lake Ledro during the last decades revealed that the vulnerability of the lake toward climate and land use driven hydrological variability is congenital for the lake, though at present it is masked by nutrients. This stresses the necessity to maintain and improve the control of nutrient inputs also in reoligotrophicated subalpine lakes, in relation to the present context of human use and climate change, and paying particular attention to the lake-specific sensitivity to local forcings.

Keyword [en]
Paleoecology, Multi-proxy, Diatoms, Cladocera, Subfossil pigments, Ecological transitions, Lake Ledro
National Category
Earth and Related Environmental Sciences Biological Sciences
Identifiers
URN: urn:nbn:se:umu:diva-115192OAI: oai:DiVA.org:umu-115192DiVA: diva2:899042
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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