Filtering efficiency and feeding mechanisms of Daphnia pulex on Microcystis aeruginosa and Nannochloropsis

The filtering and feeding rate of Daphnia pulex from the Old Durham Reservoir, Durham, NH, were measured to determine the feeding efficiency on different concentrations of non-toxic Nannochloropsis (Class Eustigmatophyceae) and toxigenic cyanobacteria Microcystis aeruginosa. Direct observations of thoracic appendage beats and post-abdominal rejections were also measured at 5*105 and 106 cells mL-1 concentrations of Nannochloropsis and M. aeruginosa to examine the feeding mechanisms of the D. pulex. In the presence of increasing Nannochloropsis concentrations, the filtering rates decreased and the feeding rates increased. When exposed to M. aeruginosa, both the filtering and feeding rates decreased. The thoracic beats decreased and the post-abdominal rejections increased from 5*105 and 106 cells mL-1 of both food types. Similarly, the thoracic beats decreased and post-abdominal rejections increased from food suspensions of Nannochloropsis to M. aeruginosa. These results indicate that when compared to Nannochloropsis, M. aeruginosa had a negative effect on the feeding rates over a wide range of food concentrations of the Daphnia. The lowered feeding rates are in part due to a reduction in collection rates (i.e. lower thoracic appendage beats) and an increase in the food rejection as evidenced by the higher post abdominal rejection rate