Distribution of diatoms and mollusks in shallow lakes from the semiarid Pampa region

Abstract

The aim of this study was to assess the potential of diatoms and mollusks as paleoenvironmental indicators in semiarid lakes of the Argentinean Pampas. At each lake, physical and chemical variables were quantified, and diatoms and mollusks collected. The relationships between environmental variables and biological assemblages were explored through ordination techniques. Lakes were highly alkaline (pH 9.5-10.5), and varied significantly in conductivity (0.2-15.7 mS/cm) and hardness (120-1874 mg/l CaCO3). A total of 79 diatom and two mollusk species were identified. The highest percentage variance in diatom data was explained by hardness, conductivity, and mud percentage. Mollusk variability was significantly related to conductivity, hardness, and vegetation cover. Diatom-based ordination of sites accurately reflected the main environmental gradients characteristic of the region, while the resolution provided by mollusk-based ordinations was lower, mainly as consequence of the wide ranges of tolerance to hardness and conductivity exhibited by the two species represented. Hence, while the use of mollusks as paleoenvironmental indicators is only valid at a large, regional scale, diatoms allow discriminating local conditions and subtle environmental differences. As a consequence, the results emphasize the use of mollusks as qualitative and semi-quantitative indicators, while the strong statistical relationship between diatoms and climate-driven environmental parameters highlights their potential use as quantitative indicators of Quaternary climatic changes in the semiarid Pampas.