Experimental models of microcystin accumulation in Daphnia magna grazing on Planktothrix rubescens: implications for water management

In this study, we investigated the kinetic aspects of the microcystin (MC) transfer from Planktothrix rubescens to Daphnia magna by carrying out exposure experiments in small simple mesocosms. We hypothesized that higher fractions of toxic cyanobacteria in the diet of grazers would shift the balance towards a greater than linear, i.e. non-linear accumulation of MC in D. magna. This hypothesis was tested by exposing D. magna to varying initial densities of MC-producing P. rubescens. The evolving models of MC accumulation differed largely as a result of the duration of exposure and initial MC concentrations used. Within the first 24 h of exposure, MC accumulation in D. magna was linear, irrespective of the initial densities of toxic P. rubescens and thus MC concentrations. After 48 h of exposure, MC accumulation in D. magna showed an exponential pattern, possibly due to a delayed digestion of P. rubescens and/or decreased MC detoxification capabilities when compared with higher ambient concentrations of MC. After 72 h toxin concentrations in Daphnia drop in all experiments as a consequence of the reduced cyanobacterial cells in the medium and the detoxification of MC within Daphnia. The results obtained suggest that in lakes with higher MC content and longer cyanobacterial bloom period MC accumulation in D. magna should be more pronounced than in mesotrophic lakes with lower MC content. The latter interpretation, however, should be verified investigating accumulation of MC both in larger mesocosms and in situ, in lakes of different trophic status