Use of fluorescently labelled algae to measure the clearance rate of the rotifer Keratella cochlearis

SUMMARY 1. Fluorescently labelled algae (FLA) were used to measure clearance rates of the rotifer Keratella cochlearis. The freshwater algae Chlorella vulgaris and Stichococcus bacillaris were labelled with a fluorescent dye, 5‐(4,6‐dichlorotriazin‐2‐yl) aminofluorescein (DTAF), following a modified staining procedure. 2. Keratella cochlearis ingested both algal species. Clearance rates on tracer foods varied between 2.4 and 6.9 μl ind−1 h−1, which are comparable with those determined using other techniques. 3. The main drawback of the FLA technique was that only a little more than one‐third of the total amount of algal cells of both C, vulgaris and S. bacillaris were well stained with the dye (DTAF), despite the use of a higher concentration of dye and a longer staining period than recommended in the literature. 4. The FLA method can be successfully applied in grazing studies involving size selection and competition for food among zooplankton. The method complements existing techniques for measuring the clearance and ingestion rates of filter‐feeders.