Microcystins bioaccumulate but do not biomagnify in an experimental aquatic food chain

Abstract

Microcystins—common hepatotoxins produced by cyanobacteria—have been detected in a wide range of organisms, though research examining the trophic transfer of microcystins and whether microcystins bioaccumulate or biomagnify in food webs has generated contradictory results. Here, we explored the trophic transfer of microcystins from the herbivorous water flea, Daphnia pulex, to the predatory larvae of a damselfly, Enallagma sp. We tested the hypotheses that microcystins transfer from the tissue of herbivorus zooplankton to that of predatory invertebrates and that these toxins biomagnify across trophic levels. We also assessed the relative contribution of toxin transfer from the gut and tissue of Daphnia pulex to Enallagma sp. We found that microcystins are effectively sequestered in the tissue of Daphnia pulex, and that these sequestered toxins are then transferred to the tissue of Enallagma sp. The contribution of gut contents to toxin transfer was negligible. Contrary to the pattern predicted by biomagnification, we found that the concentration of microcystins decreased with increasing trophic levels. Our results support the hypothesis that microcystins can be transferred trophically, but do not support the hypothesis that microcystins biomagnify from lower to higher trophic levels. Conversly, we observe biodilution in this system. These results have consequences for the impact of microcystins across trophic levels in a changing world with increasing intensity and duration of harmful algal blooms.