Predicting diet composition from body length in the zooplankton predator Leptodora kindti

Leptodora kindti and many other species of zooplankton predator range widely throughout temperate-zone lakes, yet these species are rarely included in size-based studies of food webs. This oversight derives mainly from a lack of predictive functions that quantify their effects on prey communities. With laboratory experiments I measured the handling time and phosphorus ingestion of L. kindti fed three cladoceran species radiolabeled with P-33 as a tracer of dry weight (DW). In general, laboratory results show that predator body length exerts strong influence on maximum prey length, on prey handling time (and indirectly predation rate), and on prey profitability (expressed as mu g DW ingested time(-1)) in systematic ways that may be useful in predicting predatory effects of L. kindti on natural prey communities. The prey-capturing structure of L. kindti, called the trap basket, constrains the maximum length of prey that is consumable, and handling times on prey increase exponentially as prey length approaches the maximum dimensions of the trap basket. The empirical relationship is given by the following equation: Handling time = 1/[0.067(Trap basket length/Prey length - 1)(0.436)]. I use handling time and body length data to evaluate the predictive nature of two previously published regression models that relate L. kindti trap basket length to body length. This note offers a length-based approach to analysis of diet composition in L. kindti that will hopefully encourage increased consideration of this species in future studies of food webs.