Response of biotic communities to salinity changes in a Mediterranean hypersaline stream

BACKGROUND This study investigates the relationship between salinity and biotic communities (primary producers and macroinvertebrates) in Rambla Salada, a Mediterranean hypersaline stream in SE Spain. Since the 1980's, the mean salinity of the stream has fallen from about 100 g L(-1) to 35.5 g L(-1), due to intensive irrigated agriculture in the watershed. Furthermore, large dilutions occur occasionally when the water irrigation channel suffers cracks. RESULTS Along the salinity gradient studied (3.5-76.4 g L(-1)) Cladophora glomerata and Ruppia maritima biomass decreased with increasing salinity, while the biomass of epipelic algae increased. Diptera and Coleoptera species dominated the community both in disturbed as in re-established conditions. Most macroinvertebrates species found in Rambla Salada stream are euryhaline species with a broad range of salinity tolerance. Eight of them were recorded in natural hypersaline conditions (approximately 100 g L(-1)) prior to important change in land use of the watershed: Ephydra flavipes, Stratyomis longicornis, Nebrioporus ceresyi, N. baeticus, Berosus hispanicus, Enochrus falcarius, Ochthebius cuprescens and Sigara selecta. However, other species recorded in the past, such as Ochthebius glaber, O. notabilis and Enochrus politus, were restricted to a hypersaline source or absent from Rambla Salada. The dilution of salinity to 3.5-6.8 g L(-1) allowed the colonization of species with low salininty tolerance, such as Melanopsis praemorsa, Anax sp., Simulidae, Ceratopogonidae and Tanypodinae. The abundance of Ephydra flavipes and Ochthebius corrugatus showed a positive significant response to salinity, while Anax sp., Simulidae, S. selecta, N. ceresyi, N. baeticus, and B. hispanicus showed significant negative correlations. The number of total macroinvertebrate taxa, Diptera and Coleoptera species, number of families, Margalef's index and Shannon's diversity index decreased with increasing salinity. However, the rest of community parameters, such as the abundance of individuals, evenness and Simpson's index, showed no significant response to changes in salinity. Classification and ordination analysis revealed major differences in macroinvertebrate community structure between hypersaline conditions (76.4 g L(-1)) and the rest of the communities observed at the lower salinity levels, and revealed that below approximately 75 g L(-1), dissimilarities in the communities were greater between the two habitats studied (runs and pools) than between salinity levels. CONCLUSION Salinity was the first factor determining community composition and structure in Rambla Salada stream followed by the type of habitat.