Factors controlling phytoplankton dynamics in an arid reservoir in Tunisia (case of Sidi Saad dam).

Temporal variation of the phytoplankton community and environmental factors were investigated over 8 months: from January 2018 to November 2018 in the Sidi Saad reservoir (central Tunisia): May, June (spring season), July, August (summer season) September, October and November (autumn season), and January (winter season). The relationships between phytoplankton and environmental factors were explored using Canonical Correspondence Analysis (CCA). General linear models (GLMs) were used to predict the phytoplankton abundance. Analysis of variance (ANOVA) was used to test the hypothesis that the abundance of each group of taxa differed between sampling months. The results of chemical analysis of the reservoir showed that the environment was enriched in nitrates, nitrites, ammonium, and orthophosphate, especially in May. Carlson's trophic index using average chemical variables showed that Sidi Saad has a mesotrophic statute. There is temporal phytoplankton succession in the Sidi Saad reservoir. Cyanophyceae dominated over the year except in winter and late autumn (November and January). Chlorophyceae was the dominant group in winter month. The CCA results showed that six environmental factors, orthophosphates (PO), total phosphates (TP), Secchi disc depth (SD), total nitrogen (TN), temperature (T), and nitrite (NO) had significant influences on the changes in phytoplankton. GLM showed that PO, TP, TN, SD, and T were the significant predictors of phytoplankton abundance. Phytoplankton composition is largely dominated by the species Microcystis aeruginosa which formed a bloom with excessive abundance (up to 89.76 billion cell l in spring). We recommend banning the fishing and their consumption during the period of Microcystis bloom and installing a system of biomonitoring of the levels of toxins in the water.