Silvina Stutz

Holocene changes in trophic states of shallow lakes from the Pampa plain of Argentina

Changes in trophic status of shallow lakes from the southeastern Pampa plain of Argentina are evaluated based on the relative presence of submerged macrophytes and phytoplankton. The evolution of lake Lonkoy is reconstructed for the last 5000 years, using pollen, non-pollen palynomorphs (NPPs) and macrofossil remain analyses, revealing three main lake stages. A macrophyte-dominated clear lake prevailed until 670 cal yr BP. Afterwards, the lake changed to a turbid phytoplankton-dominated state. Recently, the lake has turned to a turbid phase, caused by resuspended sediment. These results are compared with two other paleolimnological records from the region, lakes Hinojales and Nahuel Rucá, revealing a similar late-Holocene evolution, with a shift from a macrophyte- to phytoplankton-dominated lake, occurring at broadly the same time. The shift of states may be attributed to a shift in climate after 1000 cal. yr BP towards wetter conditions during the Medieval Climate Anomaly.

Modern pollen–vegetation relationship of plant communities in the Uruguayan Atlantic coast

Modern pollen–vegetation relationships are the basis for any paleo-study and are especially needed to understand fossil pollen assemblages and their ecological inferences. Some authors have proposed that lakes and ponds represent the surrounding vegetation through pollen grains captured by the lake. The aim of this work was to establish the modern pollen–vegetation relationships of coastal plant communities and pollen spectra represented in the surface sediments of a coastal shallow lake, and to evaluate whether spatial heterogeneity was captured by the Lake Chaparral in Perla de Rocha, Rocha, Uruguay. Pollen grains of five surface sediment samples were analyzed and related with a stratified sampling of vegetation communities surrounding the lake. Correspondence analysis and t test were used to determine plant communities and analyze the diversity of pollen assemblages. Several plant species relevant to conservation were registered, and plant communities were differentiated: coastal forest, dunes, prairies, and wetlands. However, due to limitations in taxonomic identification level for the grains of graminoids (Poaceae, Cyperaceae) in pollen assemblages, herbaceous communities highly represented by these botanical families were inferred as open environments. Pollen spectra registered at Chaparral Lake represented local and nearby vegetation well, according to both old and new theoretical models of lakes as a catchment pollen system. Several pollen grains were good indicators of the coastal forest (Myrsine sp., Ephedra tweediana Fisch. & C. A. Mey., Lithraea sp. and Tripodanthus acutifolius (Ruiz & Pav.) Tiegh.), and dunes (Chenopodiaceae, and Ambrosia sp. and Senecio sp.). The modern pollen–vegetation relationship established in this work is similar to those registered for the southern coast of Brazil, due to a shared forest component in both vegetation and pollen spectra. These results constitute primary data for the area, and we think this system is very appropriate for the historic reconstruction of coastal vegetation, particularly the coastal forest.