Cyanobacterial equilibrium phases in a small tropical impoundment

The cyanobacterium Limnothrix has been reported especially in shallow, nutrient-rich lakes and lowland rivers of temperate regions. However, the first bloom of a member of this genus in Brazil was reported in Manacás Lake. Phytoplankton seasonal succession was studied weekly for 2 years, with the aim of identifying steady-state periods of Limnothrix dominance. During a 2-year period, L. bicudoi comprised 87% of the total phytoplankton biomass. Two (13- and 7-week period) steady-state phases of L. bicudoi dominance were observed during two cold dry seasons, although cyanobacterial blooms are more expected during summer months, even in the tropics. Compared with temperate systems, L. bicudoi may represent an example of steady-state formation at higher temperatures. Low disturbance during cold dry seasons, especially related to temperatures below 20°C, long mixing periods, nutrient availability and high turbidity have favored these long steady-state phases. Although competitive exclusion is the primary mechanism that selects species towards equilibrium, high adaptability to these conditions seems to be the factor related to the occurrence of these steady-state phases, with the unusual dominance of Limnothrix in a tropical impoundment.