Study of subfossil chironomid assemblages of the last 900 yr recorded in a sediment sequence from a high altitude lake of Northern Patagonia (Argentina)

RIZZO, A, ; RIBEIRO GUEVARA, S.; ARRIBÉRE, M.A.; MASSAFERRO, J.

Barcelona

Congreso; 4th International Limnogeology Congress; 2007

International Association of Limnogeology

Chironomids are widely used in paleolimnology as sensitive indicators of environmental changes. Their narrow ecological optima are valuable tools for reconstructing past environmental fluctuations and disturbances. Records of subfossil chironomid assemblages were studied in a short sedimentary sequence sampled from Lake Tonček (41º 12? S - 71º 29? W), a high altitude water body (1700 above see level) located in Nahuel Huapi National Park, Northern Patagonia, in order to obtain a scenario of recent environmental changes associated to natural and anthropogenic events. Head capsules were removed manually from each sediment layer. They were identified using current taxonomic guides, determining the relative abundance profile of each taxon. The sequence was dated by 210Pb and 137Cs techniques; the sedimentation rate determined in upper layers, corresponding to 1900 to present, was 26.3 mg.cm-2.y-1 (0.105 cm.y-1). Twelve taxa were identified, corresponding to Subfam. Orthocladiinae, Tanypodinae, Podonominae and Chironominae (Tribu Chironomini). The dominant taxon of the chironomid communities along the sequence, that represents 900 yr of sediment accumulation according to the extrapolation of the sedimentation rate, was the rheophilous ?Pseudosmittia Goetghebuer. Several changes in the chironomid assemblages were observed. Reductions of the abundance and specific richness occur associated to tephra layers, followed by a swift recovery of the communities to the conditions previous to the volcanic event. The changes of taxa composition and relative abundance allowed the identification of two sections in the sedimentary sequence. In the older section, corresponding to the period including 11th to 17th centuries, the more abundant taxa were, together with ?Pseudosmittia Goetghebuer, cold-stenothermic Podonominae, Macropelopiini ind.1 and ?Paratrichocladius Santos Abreu. In the upper section, including 18th century to present, the abundance and diversity indices increased; Podonominae decreases while Tanypodinae increases progressively to present. In upper layers of this section (1945 to present) this trend is sharper and other taxa emerge, namely ?Microtendipes Kieffer and several Orthocladiinae taxa. It should be noted that Tanypodinae and ?Microtendipes Kieffer are taxa adapted to temperate and warm water environments. The increase of the abundance of warm-adapted carnivore Tanypodinae could be associated with an increase in the shallow flooding area of the lake, which results in the development of warmer and more heterogeneous habitats. This hypothesis is consistent with the emerging of Orthocladiinae taxa in this period, taxa typical of the littoral zone.