Benthic diatoms as indicators of biological quality and physical disturbance in Mediterranean watercourses: a case study of the Rio Mannu di Porto Torres basin, northwestern Sardinia, Italy

This study focused on expanding existing knowledge about benthic diatoms and their use as indicators of biological quality and physical disturbance in six Mediterranean watercourses within the Rio Mannu di Porto Torres basin in northwestern Sardinia, Italy. Benthic diatoms were collected from 11 stations during a 1-year cycle from summer 2010 to summer 2011. The Eutrophication/Pollution Index – Diatombased (EPI-D) was used to evaluate biological quality, and the Navicula Nitzschia Surirella Index (NNS’) was used to estimate the degree of physical disturbance at each station. In addition, physico-chemical variables and Escherichia coli were measured and analyzed each month. In total, 193 diatom taxa were identified, of which 157 belonging to 57 genera were present in the counts and were used to calculate the diatom indices. Five taxa were recorded for the first time for Sardinian watercourses; of which, Amphora paraveneta Lange-Bertalot, Cavacini, Tagliaventi&Alfinito, Fallacia pygmaea ssp. subpygmaea Lange-Bertalot, Cavacini, Tagliaventi&Alfinito and Diploneis spec. (aff.?) modica Hustedt are biogeographically relevant, while Navicula kotschyi Grunow and Reimeria uniseriata Sala, Guerrero & Ferrario are of particular interest as exotic species with relatively restricted distribution. The EPI-D revealed mediocre and bad water quality for all the investigated stations, in agreement with the environmental variables and the heavy anthropogenic use of the study area. The EPI-D was effective at inferring the biological quality of the studied watercourses, although some of the species that were found (7% of the total) are not included in the method. We suggest including these species in the list to improve the performance of the index in Mediterranean rivers. A comparison between the results obtained by the EPI-D and seven other European diatom indices was made. Significant statistical correlations between the EPI-D and four of these indices were observed. The NNS’ was applied for the first time to Sardinian watercourses and it indicated a significant degree of physical disturbance at nearly all the stations, due to seasonal fluctuations in water flow, which are typical of the Mediterranean climate, combined with the effects of human activities.