The reduction in water volume favors filamentous cyanobacteria and heterocyst production in semiarid tropical reservoirs without the influence of the N:P ratio.

Predictions of more frequent and intense toxic cyanobacterial blooms in the future arise due to the synergistic effects of eutrophication combined with climate change. Thus, the intensification of dry conditions in arid and semiarid areas promotes a reduction in the water level of aquatic ecosystems and favors the growth of cyanobacteria. Among these conditions, there is a lack of consensus on the role of nitrogen (N) or phosphorus (P) limitation, particularly when cyanobacterial communities are dominated by N-fixing taxa. The present study evaluated the effects of water volume reduction on cyanobacterial abundance and heterocyst production in semiarid tropical reservoirs during a period of prolonged drought. Physicochemical variables and the phytoplankton community were analyzed, and a structural equation model was built to identify the variables that explain the heterocystous cyanobacteria and heterocysts. The reduction in water volume increased the cyanobacterial biomass due to the increase in the biomass of heterocystous filamentous cyanobacteria associated with higher production of heterocysts. Heterocysts were correlated with an increase in the trophic status, low light availability and high salinity maintained under the lower volume conditions. In addition, contrary to expectations, heterocysts were not correlated with the N:P ratio, which suggests that in the tropics, N fixation does not necessarily occur when the N:P ratio is low, and N is limiting. These results indicate the potential for dominance of heterocystous filamentous cyanobacteria in future climate change scenarios, and therefore, we recommend that bloom mitigation strategies prioritize the reduction of N and P associated with the control of variables that favor heterocystous filamentous (i.e., light and salinity). We also recommend including cyanotoxin analysis in the relationship with lowering the water level to determine whether cell-bound toxin production and the associated risk to aquatic biota and humans are increased in the dry period.