Many crustacean zooplankton, such as the Daphnia shown here, graze on small particles in the water column. To study how phytoplankton are grazed by these organisms, microspheres can be introduced into the water, and after a few minutes, you can remove the organism and view them under the microscope to see how many spheres they've eaten. From this the clearance rate, or milliliter of water filtered per hour, can be calculated.


In these photographs of the same Daphnia, you can see 4.8 and 9.4 um diameter spheres near the end of the digestive track.


Photo by Xavier Armengol, University of Valencia