Seasonal Occurrence of Cyanobacteria and First Detection of Microcystin-LR in Water Column of Foum-Gleita Reservoir, Mauritania

Abstract

This work was carried out to study the seasonal occurrence of cyanobacteria and their microcystin-LR in the water column of Foum-Gleita reservoir (Mauritania). Limnological and biological factors were investigated at three depths (surface, − 3, and − 6 m) in this reservoir during a full year. Nutrients were analyzed by Spectrophotometry, phytoplankton was analyzed by Inverted Microscopy, microsystins were analyzed by High Performance Liquid Chromatography-tandem Mass Spectrometry, and environmental factor relationships were analyzed by Pearson’s correlation and Multiple Linear Regression. Physicochemical analyses have shown that this reservoir is hypertrophic with dissolved inorganic nitrogen and total phosphorus concentrations relatively high, varying from 1.39 to 6.53 and 0.21 to 0.57 mg/L, respectively. The annual surface water temperature was exceptionally high (27.8 ± 3.6 °C), characterizing Sahelian climatic conditions. Phytoplankton analyses have shown dominance of two potentially toxic cyanobacteria species, Microcystis aeruginosa and Dolichospermum flos-aquae, during the warm season (May-September). Microcystin analysis revealed only the presence of the most toxic variant, microcystin-LR. Microcystin-LR concentration in the surface water samples, during cyanobacterial blooms, was consistently high (5.638 µg/L), exceeding 5-times the World Health Organization drinking water limit (1 µg/L); however, it was much lower (0.83 µg/L) at depth (− 6 m). Analysis of environmental factor relationships showed that the most influential factors on abundance of Microcystis aeruginosa and Dolichospermum flos-aquae and variability of microcystin-LR concentrations were total phosphorus, dissolved inorganic nitrogen, iron, temperature and pH. Finally, the study clearly demonstrated the need for regular monitoring of cyanobacteria and cyanotoxins in the reservoir.