Body size and food size in freshwater zooplankton

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Abstract
Double-label liquid scintillation techniques were used to measure the efficiencies with which 8 different-sized zooplankton species ingested 4 cell types relative to a standard cell type (Chlamydomonas). Efficiency ratios (ER: clearance rate on cell type X .div. clearance rate on Chlamydomonas) on the 3 ultraplankton (< 5 .mu.m in diameter) cells (a coccoid bacterium and the algae Synechococcus and Nannochloris) varied greatly among zooplankton species but were not correlated with zooplankton body length. Variation in ER on a much larger (17 .times. 14 .mu.m) algal cell (Cryptomonas) was only partly explained by zooplankton body length. The 8 zooplankton species were classified into 3 functional groups: species having moderate to high ER on all ultraplankton (0.4 < ER < 1.6) and ER on Cryptomonas proportional to their body lengths (Conochilus, Diaphanosoma, and probably Keratella cochlearis and Ceriodaphnia); species having extremely low ER on bacteria (mean ER < 0.05), higher but still low ER on ultraphytoplankton (ER generally < 0.04), and ER on Cryptomonas proportional to their body lengths (Bosmina, Diaptomus copepodites and adults); species having extreme low Er on all ultraplankton (mean ER < 0.05) and ER on Cryptomonas much higher than expected given their body lengths (K. crassa, Polyarthra and Diaptomus nauplii). These functional groups follow neither taxonomic nor body-length groupings. Zooplankton body length may influence the maximal particle size that a species can ingest but has little influence on the ingestion of smaller particles. Two frequently used models relating zooplankton body size and food size are unrealistic.