Preliminary study of subfossil chironomid community at a lagoon of northern Patagonic steppe
by
Andrea Paula Rizzo
Lab. de Análisis por Activación Neutrónica, Centro Atómico Bariloche, Argentina
Coauthors: Serfio Ribeiro Guevara(LAAN- CAB, Bariloche, Argentina), S. Dutrús(NU.MA.DI.- CAB, Bariloche, Argentina), María Arribére(LAAN- CAB, Bariloche, Argentina) and J. Massaferro(CONICET, Ecotono - CRUB, Bariloche, Argentina)

Chironomids are widely used in paleolimnology as sensitive indicators of environmental changes. Their narrow ecological tolerances and habitat preferences are valuable tools for reconstructing past environmental fluctuations and disturbances.

El Toro is a steppe lagoon located in a small closed basin in NW Patagonia (40º14’S-70º22’O). Records of subfossil chironomid assemblages from short sediment cores, covering the last circa 200 yr, were studied in order to obtain a scenario of short-term changes occurred in the lagoon related to natural and anthropogenic events.

The 32 cm core activity profiles of 137Cs were measured to estimate the age. Chironomids taxa were sampled and identified. Organic matter (LOI) and biogenic silica were measured for each layer. Concentrations of rare earth elements (La, Ce, Yb, Lu, Sm, Eu, Tb) and others elements (Al, As, Ba, Br, Ca, Ce, Co, Cr, Cs, Fe, Hf, Mn, Na, Rb, Sb, Sr, Sc, Ta, Ti, Th, U and V) were measured in all layers.

The mean sedimentation rate estimated for the upper layers is 0.111 g.cm-2y-1 (0.27 cm.y-1). Chironomids sub-fossils are only abundant in the upper ten layers. Organic matter fluctuates between 9 and 21 % along the core, the lower values corresponding to layers with volcanic ashes.

Four taxa were identified corresponding to Subfam. Chironominae and Orthocladiinae and their relative composition were analyzed. Polypedilum sp. is the predominat taxon. The maximum abundance index (82 individuals/grams of sediment, ind.g-1 ) is coincident with the maximum values of biogenic silica (7.35 mg.g-1). There is a strong association between Br concentrations and organic matter. In the ten uppermost layers, chironomid abundance and biogenic silica are also associated with organic matter.

These results show a strong change in the environmental conditions in time period studied.

Date received: November 22, 2004

Copyright © 2004 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # caod-46.