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

Abstract

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)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(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 antipredator 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 intermediatesized 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.

Relationship between cladoceran (Crustacea) functional diversity and lake trophic gradients

“…Functional differences between them arise from their body shape and habitat, since Leptodora is more elongated and Polyphemus tends to prey also in the littoral zone (Fl€ ossner 1972). These taxa have specific roles in the food web, consuming smaller invertebrates for food and being food items for fish and larger zooplankton, for example copepods, and their occurrence in the community provides information on structure of the food web (Herzig 1995;Packard 2001;Vogt et al 2013). Dynamics of predatory cladocerans have been used previously in examining long-term functioning of aquatic food webs with respect to, for example, eutrophication ) and biomanipulation (Nyk€ anen et al 2010), where the abundance and size structure of zooplankters changed significantly during these processes.…”

Size‐specific net avoidance behavior leads to considerable under‐estimation of the biomass of Leptodora kindtii during day time sampling

“…In Enonselkä, Secchi depth during the study was 2.5 m and the thickness of euphotic zone c. 5 m (Hertta database, ; Ketola et al, ). In a lake with a Secchi depth of 1.8 m, Vogt et al () reported a <2 m vertical migration amplitude of Leptodora . Chang and Hanazato () found <2 m vertical migration amplitudes for Leptodora in a eutrophic lake where Secchi depth varied between 0.5–3 m. At all study stations in Enonselkä, water depth exceeded 3 m, the average depth being 10.4 m. Thus, the nocturnal increase in the abundance of Leptodora at most of the stations in Enonselkä could not be explained by bottom‐dwelling behavior during the day.…”

“…Most zooplankton samples are taken during daylight. This affects the accuracy of density and biomass estimates, as several studies have shown that daytime estimates on the abundance of large pelagic invertebrates differ from those taken at night (Grossnickle & Morgan, ; Vogt, Matthews, Cobb, Graham, & Leavitt, ; Voss & Mumm, ). The reasons for the day‐night differences are not always clear however.…”

“…This is demonstrated for instance by observations on the large predatory cladoceran Leptodora kindtii . Vogt et al () found higher density and larger individual body size of Leptodora at night than during the day and concluded that this was due to size‐selective vertical migration. The whole water column was not however sampled.…”

“…studies have shown that daytime estimates on the abundance of large pelagic invertebrates differ from those taken at night (Grossnickle & Morgan, 1979;Vogt, Matthews, Cobb, Graham, & Leavitt, 2013;Voss & Mumm, 1999). The reasons for the day-night differences are not always clear however.…”