Evaluation of cyanobacteria toxicity in tropical reservoirs using crude extracts bioassay with cladocerans

Abstract

During a cyanobacterial bloom in a eutrophic environment, particularly at the end when decomposition occurs, toxic compounds such as the cyanotoxins and the lipopolysaccharides can be released in high concentrations into the water column damaging aquatic organisms. In this work, the effects of this release of toxic compounds during a cyanobacterial bloom were investigated. The acute and chronic toxicity of cyanobacterial crude extracts from two natural blooms in the Barra Bonita and Ibitinga reservoirs (Middle Tietê River, São Paulo State, Brazil) and of a toxic strain cultured in the laboratory were tested. The cladocerans Daphnia similis, Ceriodaphnia dubia and Ceriodaphnia silvestrii were used as test organisms. In the chronic toxicity tests, only a native cladoceran found in Brazilian freshwaters, Ceriodaphnia silvestrii, was used. Microcystins were detected in all cyanobacterial samples. The acute toxicity tests showed that the crude bloom material extract from the Ibitinga Reservoir (48-h EC₅₀ values between 32.6 and 35.8 μg microcystin g⁻¹ of freeze-dried material) exhibited higher toxicity to cladoceran than did the crude bloom material extract from Barra Bonita Reservoir (48-h EC₅₀ values between 46.0 and 80.2 μg microcystin g⁻¹ of freeze-dried material). The chronic toxicity test data showed that the three extracts reduced the fecundity of C. silvestrii, and the crude extract of Barra Bonita Reservoir bloom material also affected the survival of this cladoceran. Both acute and chronic tests effectively prognosticated possible changes in the cladoceran population, and probably other components of the biota due to cyanobacterial blooms in natural aquatic ecosystems.