Ecology and distribution of ostracods in a polluted Mediterranean river

The present survey examines possible trends in the ecology of ostracod asse mblages in a small Mediterranean river affected heavily by sewage and indus trial effluents. Between January and July 1995, ten sampling stations situa ted along a 50 km stretch of the main channel of the River Magre (Valencia, eastern Iberian Peninsula) were sampled monthly (physico-chemical variable s being measured simultaneously). Although the entire sire fauna was used t o calculate diversity and biotic indexes, only Ostracoda were studied in de pth, as little is known of the ecological preferences of this group. Ostrac od assemblages encompassed 10 species, the most abundant genera being Herpe tocypris and Heterocypris. Most species peaked in abundance towards the end of the study period (May-June) when water temperatures were rising. Ostrac ods were completely absent from the most disturbed sites, i.e. those with t he lowest water quality index values. Different ostracod assemblages occurr ed along a gradient from high pollution towards the final 'recovery' zone. Combined classification (TWINSPAN, MDA) and ordination (DCA) techniques usi ng species and site variable data showed that different groups of samples, related to the abundance of a particular ostracod assemblage, were linked t o particular habitat features. Eucypris virens appeared in samples near to the source in winter, and in the early months of spring. During this period these sites were characterised by cool waters and relatively high alkalini ty/Cl- ratios. Furthermore two major groups of samples were related to the relative abundance of either Heterocypris incongruens and Herpetocypris int ermedia, or Herpetocypris brevicaudata. Moderately polluted, clean waters a re linked to these two groups, respectively. Additional division of both gr oups of samples was based on the total abundance of individual animals, and may be related to water temperature and season. These results may be used to assess possible changes in the trophic status of a palaeoenvironment whe n the species taken into account are recorded from a given sedimentary sequ ence. (C) 1999 Elsevier Science B.V. All rights reserved.