Food web consequences of size‐based predation and vertical migration of an invertebrate predator ( Leptodora kindtii )

In an intensive study of the vertical and horizontal distribution of zooplankton in a eutrophic lake (Katepwa Lake), we found that only adult Leptodora kindtii (> 5 mm in body size) exhibited diel vertical migration (DVM), whereas juvenile Leptodora (< 5 mm) and other zooplankton species did not. Even though a longitudinal survey of five lakes (1994–2004) indicated that variation in Leptodora density was correlated with several indicators of habitat use (e.g., water temperature) and resource availability (e.g., zooplankton abundance), feeding experiments performed in both lit and unlit conditions demonstrated that a vertebrate predator (perch) strongly reduced Leptodora abundances under all conditions and always preferentially selected large‐bodied individuals. Collectively, this evidence suggests that the migratory behavior of large Leptodora is consistent with an anti‐predator defense strategy. To estimate the ecological significance of Leptodora DVM behavior, we modeled how predation rates on different zooplankton taxa differed between day and night in Katepwa Lake. We found that Leptodora had as much as five‐fold higher prey‐specific predation rates at night, particularly for intermediate‐sized prey. We conclude that ignoring the habitat use, size‐structure, and vertical migration behavior of Leptodora could considerably underestimate the significance of invertebrate predation in lake food webs, particularly in eutrophic lakes where Leptodora can coexist at high densities with planktivorous fish.