Seasonal dynamics of cyanobacteria and cyanotoxins during 24 h diurnal cycle sampling in a eutrophic lake in Northeastern Brazil

Due to its semiarid climate the Ceara state (Northwestern Brazil) presents optimal conditions for cyanobacteria growth. Since a fatal incident caused by microcystin exposure (Caruaru Tragedy), studies on environmental mechanisms that promote cyanotoxin production have been carried out. This study was performed at Castanhao reservoir, the largest multi-use reservoir of Latin America. Samples were collected regularly each 6 hours (S1, S2, S3 and S4) in a 24 h diurnal cycle sampling during the dry season close to the dam (CTN-20), and a fish-rearing site (CTN-100). Cell counts, classifications, and Elisa tests were carried out. Previous results show cyanobacteria predominance, mainly Raphidiopsis raciborskii at S1 (9:30 am) with poor biological diversity at both points, although at CTN-100 cyanobacteria concentrations were around 24 times higher than at CTN-20, microcystins and saxitoxins were not observed at either point for S1. However, a greater cyanobacterial biodiversity was observed microcystins concentrations increased as well. Previous results imply that an increase in cyanobacteria diversity promotes cyanotoxins production, which suggests that allelopathy should be explored. Moreover, environmental patterns can potentially favor Raphidiopsis raciborskii displacement to deeper layers, in terms of water management; it means the best moment to pumping from intake point to the water treatment plants.