Some technical problems of feeding rate measurements in rotifers with fluorescently labelled algae.

Grazing of metazooplankton on phytoplankton and protozooplankton has been widely studied using the radiotracer technique. An interesting alternative to radioactive label is the use of fluorescent dies. Indeed, the use of fluorescently labelled algae (FLA) or protozoans does not imply particular security measures or equipment; moreover they allow the visualisation of the labelled prey in the predator sometimes providing useful informations on the variability between individuals and on the mechanism of feeding processes.
Prior to the use of this method in ingestion rate measurements with Brachionus calyciflorus discrimination experiments were performed. The FLAs were Chlorella-like algae (Dictyosphaerium ehrenbergianum) labelled with DTAF (5-(4,6-dichlorotriazin-2-yl) aminofluorescein). The ingestion rate of the rotifer on the FLAs was 3 to 15 times lower than the ingestion rate on non labelled. Evidence was gathered that the staining procedure (heat-killing of the algae) was mainly responsible for the discrimination of the rotifer rather than the dye itself. Other technical considerations will be discussed like the occurrence of crashed algae in the rotifers gut and the inter-individual variability.