Assessing the toxicity of cell-bound microcystins on freshwater pelagic and benthic invertebrates

Cyanobacterial harmful algal blooms dominated by Microcystis frequently produce microcystins, a family of toxins capable of inflicting harm to pelagic and benthic freshwater invertebrates. Research on the effect of microcystins on invertebrates is inconclusive; from one perspective, studies suggest invertebrates can coexist in toxic blooms; however, studies have also measured negative food-associated effects from microcystins. To test the latter perspective, we examined the reproduction, growth, and survival of laboratory-cultured Ceriodaphnia dubia, Daphnia magna, and Hexagenia spp. exposed to cell-bound microcystins through a series of life-cycle bioassays. Test organisms were exposed to a concentration gradient ranging from 0.5 μg L⁻¹ to 300 μg L⁻¹ microcystins, which corresponds to values typically found in freshwaters during bloom season. Lethal concentrations in C. dubia (LC50 = 5.53 μg L⁻¹) and D. magna (LC50 = 85.72 μg L⁻¹) exposed to microcystins were among the lowest recorded to date, and reproductive effects were observed at concentrations as low as 2.5 μg L⁻¹. Length of D. magna was significantly impacted in microcystin treatments great than 2.5 μg L⁻¹. No lethality or growth impairments were observed in Hexagenia. This information will improve our understanding of the risks posed by microcystins to food webs in freshwaters.