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| Titel |
Diatom responses to precipitation and anthropogenic disturbances in an azorean lake during the last seven centuries |
| VerfasserIn |
David Vázquez-Loureiro, Roberto Bao, Vitor Gonçalves, Maria J. Rubio-Inglés, Alberto Sáez, Armand Hernández, Pedro M. Raposeiro, Juan J. Pueyo, Pere Masque, Ricardo Trigo, Santiago Giralt |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250111844
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| Publikation (Nr.) |
 EGU/EGU2015-11990.pdf |
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| Zusammenfassung |
| Sedimentary lacustrine records provide an important source of knowledge of past
environmental changes at regional and local scales. Here we perform a diatom-based
paleoenvironmental reconstruction, complemented with geochemical proxies, of the recent
history of Lake Azul (37°52’ 21” N – 25°46’ 26” W), located in the crater caldera of Sete
Cidades volcano, São Miguel Island.
A 132 cm long sediment core from the offshore and deepest part of the lake was selected,
from a total of fourteen cores extracted in 2011, and dated with21Pb and137Cs, as
well as AMS14C. Two main litological intervals were described: at the lower part
there are hard volcanic rocks interbedding silty lacustrine episodes, whereas the
upper part is made up by silty mud lacustrine sediments. Eight terrestrial-nearshore
levels with high values of TOC/TN ratio, low values of δ13C and high abundances
of aerophilic diatoms, plus nine volcanic levels, were removed from any further
analysis in order to assess changes in in-lake processes. Main environmental gradients
driving the composition of the diatom assemblages were explored by a Principal
Component Analysis (PCA) on the diatom relative abundance data, and four statistically
significant Diatom Assemblage Zones (DAZs) were defined with a cluster analysis
(CONISS).
The first axis of PCA (PC1), explaining 47.3% of total variance, confronts benthic vs
tychoplanktonic and euplanktonic taxa, suggesting a relationship with water depth. The
second axis (PC2) explained 23.1% of total variance, showing high positive values for
eutrophic taxa of the genus Aulacoseira, and negative values for oligotrophic taxa and
diatoms of a broad trophic spectrum. This axis is very likely related to a trophic
gradient.
DAZ-1 (c. 1280 – 1400 cal yr AD), dominated by benthic taxa, is characterized by
positive values of PC1 and PC2, indicating relative shallow lake conditions and a high trophic
level, respectively. A transition from a benthic to a facultatively planktonic community (lower
PC1 values) occurs in DAZ-2 (c. 1400 – 1800 cal yr AD), suggesting a rise in water level,
probably triggered by an increase in precipitation. Negative values of PC2 in this zone and
DAZ-3 (c. 1800 – 1960 cal yr AD) might be explained by the dilution of nutrients in a larger
water volume. The sharp transition to high positive values of PC2 in DAZ-4 (c. 1960 cal yr
AD) coincides with the start in the use of nitrate- and phosphate-rich fertilizers in the
surrounding farming area.
Long-term changes in diatoms assemblages since the late XIII century in Lake
Azul are driven by two factors of natural and anthropogenic origin, respectively;
the strong inter-annual variability of the precipitation (largely controlled by the
North Atlantic Oscillation, NAO), and the eutrophication of lakes in recent decades. |
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