Ecological Quality Assessment of Mediterranean Wadis in the Bizerte Lagoon Catchment (Northern Tunisia) Using Benthic Diatoms

The study presents an integrated assessment of water quality in wadis of the Bizerte Lagoon catchment (Northern Tunisia), using benthic diatom assemblages. Seasonal sampling was conducted between winter 2023 and winter 2024 in three wadis exposed to varying anthropogenic pressures. In each wadi, upstream and downstream stations were surveyed to assess spatial variability. Diatom assemblages showed clear responses to both environmental gradient and seasonal variation. Pollution-tolerant taxa, particularly Navicula veneta, Gomphonema parvulum, and Nitzschia inconspicua, were mainly recorded during winter in downstream stations of wadis most impacted by nutrient enrichment and organic pollution. halophilic and marine-related taxa, including Gyrosigma kuetzingii, Navicula salinarum, Navicula simulata, Halamphora coffeaeformis and Haslea ostrearia, dominated during warmer seasons in the high-conductivity wadi. Pollution-sensitive taxa, such as Encyonema minutum, Cymbella affinis and Gomphonema affine, were restricted to the least disturbed wadi. Diatom-based indices confirmed the predominance of hypertrophic conditions in all wadis and revealed a clear gradient of environmental degradation, with ecological status ranging from moderate to poor and bad quality. These findings highlight the strong potential of benthic diatoms and their ecological indices as valuable tools for monitoring eutrophication and salinization gradients in Mediterranean coastal wadis, although their adaptation and calibration to Tunisian freshwater ecosystems still require further validation.