Introduction

Invertebrate planktivores occupy an intriguing position in the pelagic food web. Like other zooplankton, they are vulnerable to planktivory by fishes, yet they can prey heavily on certain zooplankton (Dodson, 1972; Neill, 1981; Black & Hairston, 1988; Hanazato & Yasuno, 1989). In Paul, Peter, and Tuesday Lakes, the most important invertebrate planktivores are larvae of three species of Chaoborus, the phantom midge. Owing, in part, to behavioral responses and ontogenetic diet shifts of both fishes and Chaoborus, the interaction between the two and consequences for zooplankton communities can be difficult to predict (Luecke, 1986; Elser et al., 1987b; Neill, 1988; Hanazato & Yasuno, 1989). The trophic cascade hypothesis states that planktivory by invertebrates is inversely related to planktivory by fishes (Carpenter et al., 1985). Nevertheless, we anticipated little change in Chaoborus during our experiments, because of their cryptic morphology and pronounced diel vertical migration behavior (Carpenter & Kitchell, 1987). Here we examine how Chaoborus populations responded to the fish manipulations, and consider the consequences for the trophic cascade.

Chaoborus develop through four instars as planktonic larvae before pupating. Their body is relatively transparent, except for well-developed hydrostatic organs, hence the common name phantom midge (Fig. 8.1, Chapter 8). Nevertheless, they are susceptible to predation by fishes. In lakes with fish, third- and fourth-instar Chaoborus will spend the days in either the hypolimnion, metalimnion, or bottom sediments and come to the surface at night to feed (Roth, 1968; von Ende, 1979; Luecke, 1986).