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 to 300 μg L microcystins, which corresponds to values typically found in freshwaters during bloom season. Lethal concentrations in C. dubia (LC50 = 5.53 μg L) and D. magna (LC50 = 85.72 μg L) exposed to microcystins were among the lowest recorded to date, and reproductive effects were observed at concentrations as low as 2.5 μg L. Length of D. magna was significantly impacted in microcystin treatments great than 2.5 μg L. 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.