Impacts of phosphorus loads on the water quality and the proliferation of harmful cyanobacteria in Foum-Gleita Reservoir (Mauritania)

Excess phosphorus and nitrogen inputs into freshwater ecosystems is one of the main causes of expansion of harmful cyanobacterial blooms worldwide. This work was conducted to study the impacts of phosphorus leaching from the exploitation of phosphate mines present in the watershed of Foum-Gleita Reservoir (Mauritania) on the water quality and its major contribution to the proliferation of harmful cyanobacteria throughout the year. The physicochemical parameters, the occurrence and abundance of phytoplankton (including cyanobacteria), and the microcystins concentration were monitored monthly from September 2017 to August 2018. The relationships between limnological and biological variables were explored by using Pearson's correlation and forward stepwise multiple linear regression (MLR) analysis. Microcystins were detected by high performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS). Our results showed that this reservoir can be classified as hypereutrophic throughout the year and that Microcystis aeruginosa (M. aeruginosa) and Dolichospermum flos-aquae (D. flos-aquae) were dominant species during the rainy season (July-September), and only the congener microcystin-LR (MC-LR) was detected with a peak at 3.55 µg L−1. Pearson's correlation and MLR analysis showed that water temperature pH, phosphorus, nitrogen, and iron concentrations were the most important variables accounting for M. aeruginosa and D. flos-aquae abundance and MC-LR concentration. Our study provides new insight into the effects of moderate nitrogen concentrations on phytoplankton community composition with dominance of the cyanobacteria phylum in phosphorus-rich freshwater ecosystems.