| 1. |
- Milan, Manuela, et al.
(författare)
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Combining sediment Cladocera remains and geochemistry to reveal the role of a large catchment in driving changes in a small subalpine lake (Lake Ledro, N-Italy)
- 2016
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Ingår i: Advances in Oceanography and Limnology. - : PAGEPress Publications. - 1947-5721 .- 1947-573X. ; 7:2, s. 220-234
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Tidskriftsartikel (refereegranskat)abstract
- Sediment Cladocera remains and geochemistry were analyzed at Lake Ledro, a small subalpine lake with a large catchment area located in northern Italy. The aim of the study was to investigate human, climate and hydrological impacts on the Cladocera community and on the geochemical components during the last few centuries. A sediment core was collected from the deepest point of Lake Ledro and radiometrically dated. Cladocera remains were analyzed to track the trophic lake evolution. The core bottom section revealed the dominance of Bosminidae in concomitance with nutrient pulses entering into the lake during major flood events. The abundance of species preferring cold water temperatures confirmed the deposition of this core section during the Little Ice Age. The flood event occurred in the first half of the 19th century produced a drastic increase in littoral species, due to the development of new habitats. The decrease in Cladocera densities during the following lake stage was followed by a rapid increase in planktonic species during the nutrient enrichment after the 1960s. Statistical analyses revealed a clear response of Cladocera community to climate variability during oligotrophic periods, while no relation to temperature changes was recorded during high nutrient levels. A preliminary study on Bosminidae and Daphnidae body size and appendages length was carried out to reconstruct major changes in the lake food web. Only Bosmina spp. revealed clear body size changes: minor shifts were recorded before the 1930s in relation to the low nutrient concentrations, while the major changes occurred during the 1980s were interpreted as related to the appearance of Cladocera invertebrate predators. Geochemical components were studied using X-ray fluorescence spectroscopy (XRF) analysis in order to recognize the impact of the large catchment area and from the lake-level regulations on the lake hydrology. Moreover the Si:Al ratios profile confirmed the increase in lake productivity after the 1960s. Although both Cladocera and geochemical analysis indicate major changes since the 1960s, they also revealed diverse responses to common external and local forcing, thus confirming the value of a multi-proxy approach for disentangling the lake responses to different environmental stressors. Moreover, it outlined the importance of larger catchment areas on small lakes as they are to a larger extent influenced by the modifications occurring in the drainage basin.
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| 2. |
- Milan, Manuela, et al.
(författare)
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Effects of long term nutrient and climate variability on subfossil Cladocera in a deep, subalpine lake (Lake Garda, northern Italy)
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 3. |
- Milan, Manuela, et al.
(författare)
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Effects of long term nutrient and climate variability on subfossil Cladocera in a deep, subalpine lake (Lake Garda, northern Italy)
- 2017
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Ingår i: Journal of Paleolimnology. - : SPRINGER. - 0921-2728 .- 1573-0417. ; 58:3, s. 335-351
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Tidskriftsartikel (refereegranskat)abstract
- Albeit subfossil Cladocera remains are considered as a reliable proxy for tracking historical lake development, they have been scarcely studied in large subalpine lakes south of the Alps. In this study, subfossil Cladocera remains from Lake Garda in northern Italy were analyzed to track the lake's environmental changes since the Middle Ages. One core was retrieved from the largest sub-basin of Lake Garda (Brenzone, 350 m deep) and two cores were retrieved from the profundal and littoral zone of the smaller lake sub-basin (Bardolino, 80 and 40 m deep, respectively). The species distribution of Cladocera remains in recent sediment layers was similar to that observed in contemporary water samples. The deepest sections of the three cores were characterized by species sensitive to water temperature, which suggest a key role of major climatic events in driving the lake response during the late Holocene. The most evident change in Cladocera assemblages in the studied cores was observed during the 1960s, when planktonic taxa such as Daphnia spp. and Bosmina spp. became dominant at the expense of littoral taxa. Despite the highly coherent general pattern of subfossil Cladocera, the cores showed a set of minor differences, which were attributed to different morphological and hydrological features of the two basins forming Lake Garda. Multivariate analysis revealed a clear relation of Cladocera to climate variability during periods of low lake nutrient levels (i.e. before the 1960s). This provides additional information on past ecological responses of Lake Garda, as contrast previous data from subfossil diatom and pigment analyses did not fully capture effects of climate change during the same period. Since the 1960s shifts in assemblage composition of Cladocera and diatoms were highly coherent, in response to nutrient increase and the following, less pronounced, decrease in nutrient concentrations. During the last five decades, the response of the Cladocera to climate variability appeared to be masked by nutrient change. This work points up the potential of the multi-proxy approach for disentangling the multifaceted biological responses to multiple environmental stressors in large and deep lakes.
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| 4. |
- Milan, Manuela, 1983-
(författare)
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Long-term development of subalpine lakes : effects of nutrients, climate and hydrological variability as assessed by biological and geochemical sediment proxies
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 5. |
- Milan, Manuela, et al.
(författare)
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Multiproxy reconstruction of a large and deep subalpine lake's ecological history since the Middle Ages
- 2015
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Ingår i: Journal of Great Lakes research. - : Elsevier. - 0380-1330. ; 41:4, s. 982-994
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 6. |
- Milan, Manuela, et al.
(författare)
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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
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 7. |
- Nava, Veronica, et al.
(författare)
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Plastic debris in lakes and reservoirs
- 2023
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Ingår i: Nature. - : Springer Nature. - 0028-0836 .- 1476-4687. ; 619:7969, s. 317-322
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Tidskriftsartikel (refereegranskat)abstract
- Plastic debris is thought to be widespread in freshwater ecosystems globally(1). However, a lack of comprehensive and comparable data makes rigorous assessment of its distribution challenging(2,3). Here we present a standardized cross-national survey that assesses the abundance and type of plastic debris (>250 mu m) in freshwater ecosystems. We sample surface waters of 38 lakes and reservoirs, distributed across gradients of geographical position and limnological attributes, with the aim to identify factors associated with an increased observation of plastics. We find plastic debris in all studied lakes and reservoirs, suggesting that these ecosystems play a key role in the plastic-pollution cycle. Our results indicate that two types of lakes are particularly vulnerable to plastic contamination: lakes and reservoirs in densely populated and urbanized areas and large lakes and reservoirs with elevated deposition areas, long water-retention times and high levels of anthropogenic influence. Plastic concentrations vary widely among lakes; in the most polluted, concentrations reach or even exceed those reported in the subtropical oceanic gyres, marine areas collecting large amounts of debris(4). Our findings highlight the importance of including lakes and reservoirs when addressing plastic pollution, in the context of pollution management and for the continued provision of lake ecosystem services.
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| 8. |
- Salmaso, Nico, et al.
(författare)
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Historical colonisation patterns of dolichospermum lemmermannii (Cyanobacteria) in a deep lake south of the alps
- 2015
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Ingår i: Advances in Oceanography and Limnology. - : PAGEPress Publications. - 1947-5721 .- 1947-573X. ; 6:1-2, s. 33-45
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Tidskriftsartikel (refereegranskat)abstract
- Since the beginning of the 1990s, Lake Garda showed the appearance of extended surface water blooms of Dolichospermum lemmermannii (Nostocales, Cyanobacteria). Between the 1990s and the 2000s, the blooms appeared also in the other large lakes south of the Alps (Iseo, Como and Maggiore). Despite the sudden appearance of the blooms, the correct identification of the establishment time of the populations of Dolichospermum in the southern subalpine lake district remained unclear. In this work, the establishment of the populations of D. lemmermannii in Lake Garda has been evaluated by the direct counting of sub-fossil akinetes extracted from sediment cores, and by estimating the abundance of filaments germinated from sub-fossil viable akinetes. The two techniques provided comparable results, allowing locating the beginning of the establishment of Dolichospermum around the middle of the 1960s. Four strains of Dolichospermum germinated from akinetes isolated from the core sediments between around the 1989 and 2012 did not show any mutation or recombination signal in the rpoB gene sequences, suggesting a strong founder effect. The establishment of Dolichospermum coincided with the beginning of the rapid increase of total phosphorus as inferred from the distribution of sub-fossil diatoms in the sediment core. These results supported the hypothesis of a strong link between the shift of Lake Garda from ultraoligotrophy/oligotrophy to oligo-mesotrophy and the development of Dolichospermum. This colonisation pattern was possibly reinforced by the increase in the water temperatures in the subalpine lake district during the last 3 decades. In warmer lakes, gas-vacuolated Nostocales are favoured by high replication rates and, in particular, by their ability to control vertical movements in stratified water columns. This allows these species to exploit the gradients of light and nutrients, giving them a competitive advantage compared to other species. From a management point of view, the control and decrease of Dolichospermum should be obtained through the reduction and control of nutrient loads to the lake.
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| 9. |
- Tolotti, Monica, et al.
(författare)
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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
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 10. |
- Tolotti, Monica, et al.
(författare)
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Subfossil cladocera as a powerful tool for paleoecological reconstruction
- 2016
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Ingår i: Advances in Oceanography and Limnology. - : PAGEPress Publications. - 1947-5721 .- 1947-573X. ; 7:2, s. 125-130
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Tidskriftsartikel (refereegranskat)abstract
- Subfossil Cladocera (Crustacea, Branchiopoda) represent one of the most valuable biological proxies preserved in lake sediments that can be studied for reconstruction purposes. They are widespread in both the pelagic and littoral zones of lakes of different geographical distribution, altitude and typology, where they often represent the dominant component of zooplankton in terms of biomass. The chitinous parts of their body are well preserved in lake sediments, and the taphonomic taxonomy is well established, thanks to the numerous studies that followed the first pioneer works by Frey (1960).
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