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Effects of long term nutrient and climate variability on subfossil Cladocera in a deep, subalpine lake (Lake Garda, northern Italy)
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.
Institute of Geological Sciences, Polish Academy of Sciences, Poland.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

While subfossil Cladocera remains are considered to be a reliable proxy for tracking historical lake development, they have hardly been studied in large and deep lakes. In this study, subfossil Cladocera remains from Lake Garda in northern Italy were analyzed to track environmental changes since the Middle Ages. One core was retrieved from the largest sub-basin of Lake Garda (Brenzone, 350 m deep) and another two were retrieved from the profundal and littoral zones of a smaller sub-basin (Bardolino; 80 and 40 m deep, respectively). The species distribution of Cladocera remains in recent sediment layers was similar to that of the contemporary assemblages observed in the water column. The deepest sections of the three cores were characterized by species sensitive to water temperature, suggesting a lake response to the major climatic events that occurred during the late Holocene. A common and evident change in Cladocera assemblages was observed in the studied cores during the 1960s, when planktonic taxa such as Daphnia spp. and Bosmina spp. became the dominant group at the expense of littoral taxa. Despite the highly coherent general pattern within Cladocera assemblages, the cores showed minor differences, which were attributed to different morphological and hydrological features of the two basins forming Lake Garda. Multivariate analysis revealed a clear response of Cladocera to climate variability during periods of low lake nutrient levels (i.e. before the 1960s). This stands in contrast to conclusions drawn previously based on data for diatoms and pigments, which did not fully capture effects of climate change during these periods. Since the beginning of the lake’s nutrient enrichment phase (i.e. after the 1960s), diatoms and Cladocera exhibited concomitant shifts in assemblage composition due to a pronounced increase and then a less pronounced decrease in nutrient concentrations. During this period, the response of the Cladocera to climate shifts was overridden by shifting nutrient levels. This work highlights the value of a multi-proxy approach for disentangling the multifaceted biological responses to multiple environmental stressors in large and deep lakes.

Keyword [en]
Cladocera, Paleoecology, Sediment, Climate change, Trophy, Lake Garda
National Category
Earth and Related Environmental Sciences Biological Sciences
Identifiers
URN: urn:nbn:se:umu:diva-115191OAI: oai:DiVA.org:umu-115191DiVA: diva2:899037
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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