CONICET | Buscador de Institutos y Recursos Humanos

Freshwater ostracods are a remarkable group of minute aquatic crustaceans with a bivalved carapace made of low magnesium-calcite. They are important members of the meiofauna and have a significant position in many food chains. Their valves readily fossilize, frequently providing numerous microfossils in cores from lake sediments. The ecology of freshwater ostracods is defined by water chemistry, water temperature and additional habitat parameters such as area, water depth, and water permanency and type. Most species are easily identifiable from their valves, and due to the sensitivity to environmental changes and the high durability of their remains in lacustrine sediments, they serve as good indicators for palaeoclimatic reconstructions. As with all other biological proxies, use of ostracods for paleoenvironmental reconstructions requires a deep knowledge of their taxonomy and ecology. One of the key questions of the PIPA Project is how do ostracod associations reflect the natural setting of modern aquatic environments in the Southern Patagonia, and if they can be used as modern analogues to interpret fossil records. Whereas fossil and modern ostracod fauna and their ecology in midlatitudes are relatively well known, there are only a few records concerning freshwater ostracods from Northern Patagonia and its knowledge is relatively limited, meanwhile studies from Southern Patagonia have been neglected for a long time. Still, knowledge about the ecology and biology of Patagonian freshwater ostracods needs improvement to apply modern analogues to fossil records and it use could be extended if more precise taxonomical and ecological information were available. Methods Samples from the upper layer of bottom sediments were analysed for ostracods. Surface sediment were preserved in 70% alcohol, washed under tap water on a 75 µm pore diameter sieve and dried by thermostatic stove at 40 ºC. Five grams of sediment from each sample were examined under stereomicroscope. Any ostracod found (living individuals as well as subfossil valves) were picked out with a brush, placed in a micropaleontological slide and finally counted and identified under a binocular microscope. Results Here, we present data from ten lakes situated on Southern Santa Cruz where ostracods were found, although only sporadic occurrences were registered in six of those samples. Abundance and diversity of assemblages are unexpectedly low; a total of eleven species were found (Table XX). Among the 11 recovered ostracod taxa, 9 taxa were identified down to the species level and one to the genus level, whereas one taxon comprises indeterminate juveniles of Candoninae. The ostracod assemblage consists partly of cosmopolitan or widely distributed species like Potamocypris smaradigma, Isocypris beauchampi and Penthesilenula incae. The second group of ostracods are typical species described previously from northern Patagonia (cf. Schwalb et al., 2002; Cusminsky et al., 2005) like Limnocythere patagonica, Limnocythere rionegroensis, Eucypris virgata, Eucypris cecryphalium, Eucypris fontana and Ilyocypris ramirezi. The ecological range for each species was defined according to the environmental parameters at the sampling site, when the species occurred. Anyway, these data do not reflect the species tolerance as the only a limited gradient is covered by the studied waters. L. patagonica was found in permanent lakes with Na-Ca-Mg, bicarbonate-dominated waters. In contrast, L. rionegroensis inhabits mainly ephemeral waterbodies with Cl/SO4-HCO3, sodium-dominated waters. In turn, E. virgata, E. cecryphalium and E. fontana characterize Na-Ca-Mg, bicarbonate-dominated waters; the latter species can also be found in Na-HCO3 waters. While E. virgata and E. fontana have been found both in permanent and ephemeral habitats, E. cecryphalium has been found so far only in permanent lakes and - with very low abundance - in a little stream (Schwalb et al., 2002; Cusminsky et al., 2005). I. ramirezi have been found in Na-Ca-Mg/HCO3 waters. This species had been proposed as an indicator of running waters. However, in this study it was found alive in permanent lakes with no creeks or seeps near the sampling site. Therefore, some considerations on the ecological preferences of I. ramirezi might need to be revisited. Finally, K. megapodus was found in Northern Patagonia in chlorurated and/or sulphated, sodium dominated waters. In this study K. megapodus was found in Na-Ca-Mg, bicarbonate dominated waters. This species might therefore be considered as (slightly) tolerant of saline waters, but characteristic of alkaline waters. A further dependence of species distribution on the water body type was not observed.