Sebastien Bertrand

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

Galletue Lake (38° 41S; 71° 17W,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 40m depth and sectioning it in 1 cm intervals. The core age was obtained with 210 Pb and 137 Cs 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.

Changes of sub-fossil chironomid assemblages associated with volcanic sediment deposition in an Andean lake (38°S), Chile

Se analizan las asociaciones de restos sub-fosiles de quironomidos y los parametros sedimentologicos de un nucleo sedimentario del lago Galletue (38°41 S, 71°17 O). Es evidente en este nucleo la presencia de un estrato de sedimentos volcanicos (tefra) de una composicion distinta en el contenido deµmateria organica, en el tamano de particula y en la mineralogia, respecto de los sedimentos del lago. De acuerdo a la geocronologia isotopica (210Pb y 137Cs), los sedimentos de esta tefra podrian corresponder a la erupcion del volcan Llaima de 1956-1957. Por otra parte la depositacion de esta tefra tambien provoco un cambio en las asociaciones de quironomidos, siendo evidente el aumento de Parakiefferiella y la disminucion de Ablabesmyia dentro del estrato de tefra. Si bien nuestros resultados, al igual que estudios previos, registran un cambio en las asociaciones de quironomidos debido a la entrada de sedimentos volcanicos, un aspecto diferente es la presencia de capsulas cefalicas dentro de la tefra. Una posible explicacion a tal diferencia podria ser la percolacion de los restos de quironomidos por los espacios intersticiales de la tefra. La recuperacion en la abundancia de algunos taxa luego de la depositacion de la tefra, sugiere que el lago restableceria las condiciones existentes previo a la caida de la tefra

Changes in sub-fossil chironomid assemblages in two Northern Patagonian lake systems associated with the occurrence of historical fires

Patagonia is commonly seen as an exceptionally pristine area because of its wildlife and practically unpolluted waters. However, during the twentieth century the burning of natural forests was one of the most important human activities in Northern Chilean Patagonia. Some estimations indicate that three million hectares were burned during the first three decades of the century. Hence the objective of this study was to evaluate the impacts of the historical fires in Lake Burgos (45º42′S) and Lake Thompson (45º38′S) in Chilean Patagonia. The impact was measured by evaluating chironomid assemblage since they are sensitive enough to be used as an indicator of aquatic ecosystem health. Fires have a direct and drastic effect on a lake watershed but also indirectly affect a lake ecosystem, changing sedimentation patterns or increasing nutrient inputs. In the studied lakes the periods with higher prevalence of fires were identified by charcoal analysis, while organic matter and magnetic susceptibility allowed the confirmation of pre-fire and post-fire periods. The chironomid composition was evaluated through a PCA and an analysis of similarity (ANOSIM) to test the significance among periods while a Detrended Correspondence Analysis was applied to the chironomid assemblage downcore to assess compositional structure and taxa turnover. In Lake Burgos the ANOSIM test indicated significant differences between the pre-fire and fire periods (pxa0<xa00.05), while in Lake Thompson differences were not significant. However, in Lake Thompson the PCA clearly separated the pre-fire from the fire period but not the fire from the post-fire periods. In both lakes chironomid composition changed in relation to the period of higher prevalence of fires, which in turn implies catchment changes, pollution, and other anthropogenic impacts. Particularly a marked change in mesotrophic/eutrophic taxa was detected, reflecting an increase in nutrient input due to deforestation. Our findings point out that the lacustrine ecosystems are still affected by the impact of fires and the subsequent increase in nutrient supply that occurred almost 50xa0years ago. No sign of reverting to pre-disturbance conditions was observed, which makes these lakes highly sensitive to current human-induced impacts.

CHEMICAL PROFILES IN LAKE SEDIMENTS IN LAGUNA CHICA DE SAN PEDRO (BIO-BIO REGION, CHILE)

Metal profiles in lake sediments could provide historical environmental information on impacts caused by human activities and natural events, with view to the formulation of effective environmental policies. This paper presents data on sedimentary metal profiles in Laguna Chica de San Pedro (LCSP) lake, located in the Bio Bio Region in Southern Chile, where important industrial activities are concentrated. Sediment properties (organic and inorganic matter, grain size, particle distribution, biogenic silica, Mn/Fe ratio), major (Al, Ca, K, Mg, Ti, Na), trace (V, As, Co, Cu, Zn, Ni, Cr, Pb, Sr), and mobile (Fe, Mn, P, S) elements, as well as mineral profiles (plagioclase, quartz, clays, amorphous material) are presented, up to 65 cm depth. In general, relatively constant concentration profiles are observed in the sedimentary core mid section, for most elements. Most changes in composition are seen at the top (recent industrial period) and bottom (before human intervention) sections. Variable redox conditions, generated by biological activity at the sediment-water interface are likely to account for composition profiles at the sediment-water interface. On the other hand, physical processes seem to be mostly responsible for concentration changes in Pre-industrial sediments. Mineral content profiles, such as plagioclase, clay and quartz, as well as total clay content remain fairly constant in most of the core, showing significant changes at its bottom part. High excess V, As, and S values, especially at the upper sediment, arise as a consequence of redox conditions in the lake. Certainly, such metal enrichment is mainly associated with natural sedimentary matter supply from the watershed