Diet and trophic position of zooplankton, Chaoborus, and yellow perch as determined by stable isotopes

Abstract

Zooplankton are prey for 2 very different types of predators. Large or "macro" invertebrates such as Chaoborus, Mysis, and Bythotrephes are tactile, gape-limited predators, while planktivorous fish are visual hunters. These 2 types ofpredators may compete for zooplankton prey in an Intra-Guild Predation pattem (RAMCHARAN et al. 2001, HART 2002, HYAIT et al. 2005) or they may partition the prey resource creating a trophic cascade pattem (CARPENTER & KITCHELL 1988). These pathways of energy flow may have another important component; we now know that many "planktivorous" and even "piscivorous" fishes rely heavily on benthic invertebrates. All the above pattems of energy flow have been studied in lakes that have complete food webs. Lakes that have suffered industria! damage offer the advantage of simplified food webs that are lacking or depauperate in key components. Lakes in the area of Sudbury, Ontario, have suffered decades of metal and acid deposition (GUNN & SANDOY 2003), and although they have largely recovered in terms o f water chemistry (KELLER & PITBLADO 1986, KELLER et al. 1992), many stilllack top piscivores (KELLER et al. 2007) and are depauperate in benthos (GRIFFITHS & KELLER 1992). Here, we focus on 2 questions: (l) Have the pattems of energy flow been altered in these damaged food webs? and (2) What is the role ofthe macroinvertebrate predator, Chaoborus? We compare our results to those for other camivorous macroinvertebrates such as Mysis (BRANSTRATOR et al. 2000, JoHANNSSON et al. 2001) and Bythotrephes (PERGA & GERDEAUX 2006) which, like Chaoborus, typically occupy a trophic position intermediate between zooplankton and planktivorous fish. From 3 lakes in the area of Sudbury, Ontario, samples of zooplankton (including Cladocera, cyclopoids, and calanoids >150 llJil in body size), Chaoborus spp., and yellow perch (Perca jlavescens) were collected. Stable isotopes of o13C and o15N were analyzed by isotope-ratio mass-spectrometry (IRMS) to determine diets and trophic positions ofthe 3 different consurners. Our analyses suggest that Chaoborus is the top pelagic predator in these lakes, while perch is the top predator ofbenthos.