Responses of a tropical micro-crustacean, Daphnia lumholtzi, upon exposures to dissolved toxins and living cells of cyanobacteria

The mass development and expansion of cyanobacteria release cyanotoxins in the aquatic environment and cause serious problems for grazers such as micro-zooplankton. In contrast to aquatic ecosystems in temperate regions, impacts of cyanobacteria and their toxins on tropical micro-crustaceans are relatively understudied. In this study, acute and chronic effects of pure microcystin-LR (MC-LR), a crude extract of water bloom sample with a dominance of Microcystis aeruginosa containing microcystins (MCE) and living cells of Cylindrospermopsis curvispora were tested on a tropical micro-crustacean, Daphnia lumholtzi. The 24 h- and 48 h-LC₅₀ values for MC-LR ranged from 247–299, and 331–409 μg MCE L⁻¹, respectively. Exposures to 1–25 μg MCE L⁻¹ decreased survival, fecundity, and reproduction of D. lumholtzi. The impacts of C. curvispora cells on life-history traits of D. lumholtzi were density-dependent and more severe than the impacts of dissolved microcystins. It could be that the adverse effects of C. curvispora on D. lumholtzi are linked to a combination of potential toxic metabolites, mal-nutrients, feeding and swimming interference. Daphnia lumholtzi used in this study is more sensitive to microcystins compared to Daphnia species from temperate regions. Therefore, we highly suggest using D. lumholtzi as a model species for toxicity testing and monitoring on water quality, particularly in tropical countries.