| 1. |
- Milan, Manuela, et al.
(author)
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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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In: Journal of Great Lakes research. - : Elsevier. - 0380-1330. ; 41:4, s. 982-994
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Journal article (peer-reviewed)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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| 2. |
- Salmaso, Nico, et al.
(author)
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Historical colonisation patterns of dolichospermum lemmermannii (Cyanobacteria) in a deep lake south of the alps
- 2015
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In: Advances in Oceanography and Limnology. - : PAGEPress Publications. - 1947-5721 .- 1947-573X. ; 6:1-2, s. 33-45
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Journal article (peer-reviewed)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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