Changes in diatom, pollen, and chironomid assemblages in response to a recent volcanic event in Lake Galletue (Chilean Andes)

in Limnologica (2007), 37(1), 49-62

Several lakes in Chile are near important volcanic areas where eruption impacts can limit the quality of lacustrine sediments for reconstructing past environmental changes. In this study, we report ... [more ▼]

Several lakes in Chile are near important volcanic areas where eruption impacts can limit the quality of lacustrine sediments for reconstructing past environmental changes. In this study, we report changes in diatoms, pollen, and chironomids assemblages after a tephra deposition in Lake Galletue (Chilean Andes). A sediment core obtained from Lake Galletue (40 m water depth) was sliced in I cm intervals and subsamples were taken to analyze each proxy. Pb-210 and 137 Cs activities were measured to obtain the geochronology and mineralogical analyses were performed to determine the mineral composition of the tephra. Diatom species composition and productivity were modified when the lake received the tephra; Aulacoseira granulata decreased and was later replaced by Cyclotella af. glomerata. After the tephra input, Aulacoseira granulata abundance increased to pre-disturbance levels and Cyclotella af. glomerata decreased. These changes seem to suggest a momentary increase in lake nutrient levels after the tephra deposition. Chironomid assemblages also decreased in head capsules just after the tephra deposition, but the most important change was the replacement of Ablabesmyia by Parakiefferiella, probably due to the sedimentological changes produced by the input of coarse tephra grains. Furthermore, unlike other studies, chironomid assemblages in Lake Galletue did not show a decrease drastically in diversity within the tephra layer. The pollen analysis indicated that, prior to the volcanic event, the vegetal community was dominated by Nothofagus sp., Araucaria araucana, and Blechnum sp.-type. After the tephra deposition, the same taxa are dominant, indicating that the volcanic event seems not produce changes in the vegetation. Nevertheless, within the tephra layer it is possible to see an increase in Poaceae, which represent - due to the percolation process - the effect of eruption on the vegetation. According to our results, diatoms were the most sensitive proxy for describing the changes produced by tephra deposition into the aquatic ecosystem and, despite the noticeable changes in its sedimentological properties; the lake seems to have a high resilience capacity, allowing it to return to pre-tephra input conditions. (c) 2006 Elsevier GmbH. All rights reserved. [less ▲]

Changes in diatom assemblages in an Andean lake in response to a recent volcanic event

in Archiv für Hydrobiologie (2006), 165(1), 23-35

Galletue Lake (38 degrees 41'S; 71 degrees 17'W, IX Region-Chile) is located in an area that is highly influenced by eruptive activity of the Lonquimay and Llaima volcanoes. As a consequence, during the ... [more ▼]

Galletue Lake (38 degrees 41'S; 71 degrees 17'W, IX Region-Chile) is located in an area that is highly influenced by eruptive activity of the Lonquimay and Llaima volcanoes. As a consequence, during the last century tephra was deposited into the lake. Sedimentary diatom assemblages were reconstructed in order to establish the tephra effect on their composition and productivity. This was done by taking a sediment core from Galletue Lake at 40 m depth and sectioning it in 1 cm intervals. The core age was obtained with Pb-210 and Cs-137 activities and the mineral composition of the tephra was determined with mineralogical analyses. Furthermore, sedimentological analyses were performed and total organic matter was determined in the profile. The diatom assemblages were analysed using light and electronic microscopy. The results showed that the diatom stratigraphy can be separated in three zones recognizing the periods before and after tephra input. Aulacoseira granulata decreased during the tephra influx and was replaced by Cyclotella aff. glomerata. When the tephra disappeared from the sedimentary record, A. granulata returned to pre-volcanic event levels. At the same time, Cyclotella aff. glomerata decreased within the assemblages. Asterionella formosa and Urosolenia eriensis increased after the tephra input, with values surpassing earlier ones. These results indicate that, after the tephra input, the lake recovered its previous nutrient levels, which agrees with the organic matter content in the sedimentary matrix. Finally, the results reveal that the volcanic disturbance changed the structure and productivity of diatom assemblages in Galletue Lake. [less ▲]