Factors affecting the distribution of recent lacustrine ostracoda from the Caicedo de Yuso-Arreo Lake (Western Ebro Basin, Spain)

Recent Ostracoda assemblages of the Caicedo de Yuso Lake have been described as representative of palustrine/lacustrine environments in Iberian temperate lakes. This study considers the changes in assemblages at a fixed station during two consecutive annual cycles, as well as their distribution in summer and winter profiles of the lake. Total sample-assemblages have been quantified as species diversities, measured by the Shannon-Wiener index H (S). The trends of this index are compared to physical parameters (temperature, conductivity, dissolved oxygen, CO2) of the bottom waters of this lake. The Ostracoda assemblages (8100 adults and juvenile living specimens, belonging to 20 species) are dominated by Cypria ophtalmica, Cypridopsis vidua and Pseudocandona marchica. Biocenotic assemblages are well developed in the western, flat shelf area of the lake (0 to 9 m in depth), where a major substrate of charophytes permits assemblages with moderate diversity levels to be maintained. Waters below 10 m are practically barren of living individuals, and only few specimens of the generalist species Cypria ophtalmica have been found alive at that depth. Differential responses of most abundant species to changes in the environmental parameters of the bottom of the lake have been interpreted in terms of the ecological requirements of those species. Most of these responses are clearly influenced by shifts in the parameters mentioned, and decreases in oxygen and increases in CO2 produce particularly marked falls in assemblage diversity levels. Conversely, when the oxygen increases and the CO2 falls, the diversity levels increase. The time delay in responding to these changes is estimated at less than one month. This general behaviour of the ostracod assemblages of Caicedo Lake is not followed by C. ophtalmica, which is considered a particular species that can stand hypoxic waters, with relatively high CO2 concentrations. This ability of C. ophtalmica allows specimens of this species to occupy deep and environmentally unfavourable niches in the lake, that no other ostracod species of Caicedo Lake does. The knowledge of areal and temporal distribution of Ostracoda species in a temperate lake, such as Caicedo de Yuso, exhibits a good potential for applying these results to the interpretation of past lacustrine/palustrine environments of Neogene lakes in similar palaeogeographical contexts. © 2005 Elsevier B.V. All rights reserved.Peer Reviewe