Assessment of the short-term salinity effect on algal biofilm through field transfer in the Drâa river (Southeastern...

As a result of human activities and climate change, salinity levels have risen considerably in many of the world's rivers, particularly in arid and semi-arid areas. This freshwater salinization primarily affects microalgal biofilms, the primary producers in aquatic ecosystems. This study is aimed at assessing short-term salinity effects on benthic algal communities in the Drâa river, Morocco, using biofilm field-transfer experiments. Artificial substrates were initially positioned in three sites of the Drâa river with different salinity levels. After 4 weeks, the biofilm-colonized substrates were transferred from one site to another in both directions. After a further 4 weeks, the algal biofilms were sampled to assess their community composition, alpha and beta diversity, and biomass in response to salinity changes using molecular and morphological analyses. Transferring biofilms from low-salt to saline sites significantly reduced biomass but increased it in the reverse transfer. Eliminating certain sensitive microalgae taxa decreased alpha diversity in all biofilm transfers from low-salt sites to the most saline one. Biofilm diversity increased significantly in the opposite transfer. Significant changes were observed in biofilm composition when transferred into saltwater showing an increase in halophilic and euryhaline diatom taxa. In contrast, transfers from saltwater sites to low-salt sites increased oligohaline diatom species. As a result, increasing salinity reduced algal biomass and diversity, while decreasing salinity caused the opposite effect. These results can help predict the salinity variations effects on benthic algae, highlight the potential dangers of increased river salinization, and promote salinity reduction in aquatic ecosystems subject to secondary salinization.