The differing crustacean zooplankton communities of Canadian Shield lakes with and without the nonindigenous zooplanktivore Bythotrephes longimanus Stephanie A. Boudreau and Norman D. Yan Abstract: The nonindigenous predatory cladoceran Bythotrephes longimanus is spreading rapidly among Canadian Shield lakes, but only one case study of its impacts exists. In Harp Lake, the abundances of several cladoceran and one cyclopoid species fell after the invasion, and far fewer species benefited. To determine if Harp Lake provides typical results, we compared the summer crustacean zooplankton communities of 17 invaded and 13 noninvaded (reference) lakes in Ontario. The communities of the two lake groups differed. Average species richness was 30% higher in the reference (15.3 species) vs. the invaded lakes (11.8 species). Total zooplankton biomass was significantly lower in the invaded lakes, mainly because of lower abundances of all common epilimnetic cladoceran species. As these results were quite similar to those of Harp Lake, it is apparent that current summer zooplankton communities of Canadian Shield lakes with Bythotrephes differ substantially from noninvaded lakes. Résumé : Bien que le cladocère prédateur non indigène Bythotrephes longimanus se répande rapidement dans les lacs du Bouclier canadien, il n’existe qu’une seule évaluation de son impact. Dans le lac Harp, les abondances de plusieurs espèces de cladocères et d’une espèce de cyclopoïdes ont décliné depuis l’invasion, alors qu’un petit nombre d’espèces en a tiré bénéfice. Pour voir si la situation au lac Harp est caractéristique, nous avons comparé les communautés de crustacés zooplanctoniques d’été dans 17 lacs colonisés et 13 lacs non colonisés (témoins) de l’Ontario. Les commu- nautés des deux groupes de lacs sont différentes. La richesse moyenne en espèces est de 30 % plus élevée dans les lacs témoins (15,3 espèces) que dans les lacs affectés (11,8 espèces). La biomasse totale du zooplancton est significati- vement plus basse dans les lacs envahis, principalement à cause des densités réduites de toutes les espèces communes de cladocères épilimnétiques. Ces résultats sont très semblables à ceux obtenus au lac Harp et il est donc clair que les communautés actuelles de zooplancton en été des lacs envahis par Bythotrephes diffèrent considérablement de celles des lacs non affectés. [Traduit par la Rédaction] Boudreau and Yan 1313 Introduction Bythotrephes longimanus Leydig (Crustacea, Branchio- poda, Onychopoda), hereafter simply called Bythotrephes, is a large, predatory cladoceran species native to much of Eur- asia (Grigorovich et al. 1998; Therriault et al. 2002). First discovered in the Laurentian Great Lakes in the early 1980s, Bythotrephes almost certainly arrived in the ballast water of transoceanic ships (Mills et al. 1993). It was first reported in inland lakes in Ontario in 1989 (Yan et al. 1992), and since then it has spread to close to 50 Canadian Shield lakes in Ontario (MacIsaac et al. 2000; Therriault et al. 2002). We have only one thorough case study of the impacts of the in- vader on zooplankton of Shield lakes. In 1993, Bythotrephes was recorded in Harp Lake, a lake with a long record of pre- invasion monitoring. Following the invasion, zooplankton species richness declined significantly, as did average abun- dances of many small-bodied cladoceran species and Meso- cylops edax. A smaller number of larger species (Daphnia mendotae, Holopedium glacialis) became more abundant (Yan and Pawson 1997; Yan et al. 2001, 2002). Although there are no other Shield lake case studies, Bythotrephes has also been implicated in zooplankton community changes in Lake Michigan (Lehman and Cácares 1993; Makarewicz et al. 1995) and in Long Lake, Mich., U.S.A. (Hoffman et al. 2001). Bythotrephes is found in a wide range of lakes in Europe (Grigorovich et al. 1998; Therriault et al. 2002); however, it most commonly occurs in large, deep, oligotrophic lakes (MacIsaac et al. 2000). There are many such lakes in tem- perate North America, particularly on the Canadian Shield. Thus, if the Harp Lake results prove to be typical, there may Can. J. Fish. Aquat. Sci. 60: 1307–1313 (2003) doi: 10.1139/F03-111 © 2003 NRC Canada 1307 Received 1 January 2003. Accepted 31 August 2003. Published on the NRC Research Press Web site at http://cjfas.nrc.ca on 17 November 2003. J17259 S.A. Boudreau 1 and N.D. Yan. 2 Department of Biology, York University, 4700 Keele Street, Toronto, ON M3J 1P3, Canada. 1 Present address: 55 Brittany Avenue, Kingston, NS B0P 1R0, Canada. 2 Corresponding author (e-mail: nyan@yorku.ca).