Ecology and distribution of ostracods in a polluted Mediterranean river

The present survey examines possible trends in the ecology of ostracod assemblages in a small Mediterranean river affected heavily by sewage and industrial effluents. Between January and July 1995, ten sampling stations situated along a 50 km stretch of the main channel of the River Magre (València, eastern Iberian Peninsula) were sampled monthly (physico-chemical variables being measured simultaneously). Although the entire site fauna was used to calculate diversity and biotic indexes, only Ostracoda were studied in depth, as little is known of the ecological preferences of this group. Ostracod assemblages encompassed 10 species, the most abundant genera being Herpetocypris and Heterocypris. Most species peaked in abundance towards the end of the study period (May–June) when water temperatures were rising. Ostracods were completely absent from the most disturbed sites, i.e. those with the lowest water quality index values. Different ostracod assemblages occurred along a gradient from high pollution towards the final `recovery' zone. Combined classification (TWINSPAN, MDA) and ordination (DCA) techniques using species and site variable data showed that different groups of samples, related to the abundance of a particular ostracod assemblage, were linked to 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 alkalinity/Cl − ratios. Furthermore two major groups of samples were related to the relative abundance of either Heterocypris incongruens and Herpetocypris intermedia, or Herpetocypris brevicaudata. Moderately polluted, clean waters are linked to these two groups, respectively. Additional division of both groups 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 when the species taken into account are recorded from a given sedimentary sequence.