Seasonal dynamics of phytoplankton communities coupled with environmental factors in Joumine dam, Tunisia

The management of scarce water resources while preventing water quality deterioration is an important issue and constitutes a challenge for sustainable use. The Joumine reservoir, situated in the northwestern part of Tunisia, is part of the main master plan for water transfer between watersheds in order to enhance stock capacities. It is mainly used for drinking and irrigation purposes. This reservoir is affected by eutrophication. The proliferation of phytoplankton depends on many physical, chemical and biological factors. Aiming to present an assessment framework of the temporal water quality variability and the influence of environmental parameters over the phytoplankton’s dynamic, we used multivariate statistical methods such as Pearson’s correlation, Cluster analysis (CA), principal component analysis (PCA) and redundancy analysis (RDA). Water measurement and sampling were conducted at 8 stations, during May, June, August, and October 2021 and March and June 2022, in order to cover the surface of the lake and the temporal variability of water quality parameters. These measurements included physico-chemical parameters, concentrations of nutrients and of Chlorophyll a (Chla), as well as the quantification and the identification of Phytoplankton species. PCA identified three factors that explained 77% of the total variance of the data set. This suggests that the variations in water nutrient concentrations are mainly related to point source contamination (domestic and industrial wastewater), non-point sources contamination (agriculture activities), as well as natural processes (soil weathering). For the biological part, a total of 25 species of phytoplankton belonging to 7 algal classes were identified, predominately Chlorophyceae (44%) followed by Cyanobacteria (24%), Diatoms and Euglenophyceae representing respectively 12% and 8% of all identified taxa. Other taxa as Dinoflagellates (Dinophyceae) appear very irregularly represent 4%. The phytoplankton biovolume averaged 3.34 µm3/L. RDA was used to analyze the relationships between the abundance of phytoplankton species and the environmental parameters. The results showed that NO3-, NH4+, dissolved oxygen, pH and water temperature were the primary influencing factors impacting the phytoplankton community distribution. These preliminary results, will allow us to model the dynamic of cyanobacteria in the Joumine reservoir and better anticipate cyanobacteria blooms.