Comparison of benthic diatoms from Mediterranean and Atlantic Spanish streams: Community changes in relation to environmental factors

Abstract

Water and benthic diatom samples were collected in different climatic and geological areas of Spain. Seventy-two sites were sampled in Atlantic Galicia (NW mainland) and 47 sites in the Mediterranean Balearic Islands (NE) in spring season during 2004 and 2006 to identify the most important environmental factors influencing water composition. Furthermore, spring samples were explored to assess differences among diatom assemblages. Streams were selected to cover a wide range of environmental variability. Overall, the Atlantic streams had higher discharge and the Mediterranean streams had higher conductivity. In second instance, in both areas water chemistry was most importantly influenced by diffuse agriculture and local point source organic inputs, leading to high contents of nitrate, ammonium and phosphate. Two-way indicator species analysis (TWINSPAN) produced five diatom groups with different species composition in each study area. The unpolluted streams in the Balearic Islands were characterized by the presence of Cymbella microchepala and Cymbella vulgata, while in Galicia Eunotia subarcuatoides, E. intermedia and Surirella roba characterized minimally disturbed streams. Achnanthidium minutissimum sensu lato appeared in high abundance in both studied areas. Taxa inhabiting organic polluted Mediterranean streams were Cocconeis euglypta, Navicula veneta, Nitzschia inconspicua and Planothidium frequentissimum, while organic loading led to a dominance of Cocconeis euglypta and Eolimna minima in Atlantic streams. The first two CCA axes explained 82 and 69% of total variance in diatom distribution in Galicia and the Balearic Islands, respectively. In spite of the presence of different diatom communities across Mediterranean–Atlantic streams in undisturbed conditions predictable changes in diatom assemblages do occur in response to organic and nutrient loading gradients.