Spatio-temporal dynamics of cyanobacterial abundance and toxicity in a Mediterranean hypereutrophic lake

The Mediterranean basin is considered one of the most vulnerable areas in the world under the impacts of global warming and changes in precipitation patterns. Oubeira, a shallow and polymictic freshwater lake located in northeastern Algeria (36 degrees 50'N, 08 degrees 23'E), has recently experienced a significant proliferation of harmful cyanobacteria resulting in the generation of toxins. We carried out this study in order to understand the succession patterns of dominant cyanobacteria and cyanotoxin production and the factors driving this in Oubeira Lake. A total of 26 cyanobacterial genera were identified, and among them Microcystis and Planktothrix accounted for more than 60% of the overall cell abundance. The summer/fall period was dominated by Microcystis, Planktothrix, and in lesser extend by Cylindrospermum, Cylindrospermopsis. During the fall/winter transition, Dolichospermum, Pseudanabaena, and Aphanizomenon were the dominant genera. Statistically, the bloom-forming cyanobacteria showed significant differences between months but not between stations. Intracellular microcystins (MC-LR) was detected in all collected samples (0.62- and 19.14-mu g MC-LR equivalent/L), but appeared in high concentrations throughout the period of dominance of Microcystis and Planktothrix. Microcystis was more sensitive to nutrients than to temperature. Planktothrix was more dependent on temperature than on nutrients, which explains their coexistence during summer-fall period. However, both genera are positively correlated with MC-LR and would probably be the main producers of microcystins. Pseudanabaena, Dolichospermum, and Aphanizomenon co-occurred at the end of Planktothrix dominance period (December). Aphanizomenon and Pseudanabaena were correlated negatively with temperature and positively with water conductivity. Dolichospermum showed a strong positive correlation with MC-LR. Oubeira Lake, could serve as a model of how cyanobacteria blooms may develop in lakes within Mediterranean climates.