Environmental factors influencing Cyanobacteria community structure in Sidi Salem reservoir (North of Tunisia)

Abstract

The reservoir of Sidi Salem (North of Tunisia) suffered from eutrophication during the second part of the 20th century, like many large freshwater ecosystems in Tunisia. In the present study, we analyze a dataset comprised of field measurements of physicochemical and biological variables in Sidi Salem reservoir covering the period from February 2009 to January 2011, in order to understand the variability of cyanobacteria community structure during recent years and the main drivers of these changes. Monthly sampling of cyanobacteria was carried out from the deepest station of the reservoir according to the water column (surface 0m, 5m,10m,15m,20m and 25m). In this way, a total of 15 cyanobacterial species, spread over 4 orders, were identified. This study investigates the vertical and temporal dynamics of phytoplankton and cyanobacterial communities in Sidi Salem Reservoir. Phytoplankton biovolume exhibited strong temporal variability, ranging from near 0 to 60 × 10⁶ mm³ L⁻¹, with mean values generally below 5 × 10⁶ mm³ L⁻¹ for most months. Maximum biovolumes were recorded in autumn and winter, particularly at 20 m depth, reflecting bloom events. Cyanobacteria dominated the phytoplankton community, with Planktothrix accounting for 70–90% of relative abundance in deeper layers (≥10 m) and Oscillatoria showing seasonal peaks in surface layers (0–5 m) during summer. Subdominant genera, including Microcystis, Raphidiopsis, and Pseudanabaena, were sporadically distributed. Principal component analysis (PCA) revealed that vertical distribution patterns were strongly associated with nutrient gradients, light availability, and physicochemical conditions, with surface layers acting as hotspots of productivity and bloom formation, and deeper layers functioning as nutrient reservoirs and zones of biogeochemical transformation