Distribution patterns and predation risk of the coexisting cladocerans Bythotrephes longimanus and Leptodora kindtii in a large lake - Lake Constance

The distributions of the coexisting, planktonic, predatory cladocerans Byth otrephes longimanus and Leptodora kindtii were studied at four different st ations (Langenargen-Arbon, Fischbach-Uttwil, Bregenzer Bucht and Zeller See ) of a great lake, Lake Constance. The stations Langenargen-Arbon and Fisch bach-Uttwil are deep (> 200 m) with a high density of the coregonid fish Co regonus lavaretus, the stations Bregenzer Bucht and Zeller See are shallow (< 60 m) with lower densities of coregonid fish. B. longimanus was present in the pelagic zone from mid May to November at all stations. L. kindtii ge nerally appeared slightly later in the year than B. longimanus. Peak abunda nces of B. longimanus with more than 5000 ind. m(-2) (Langenargen-Arbon) ap peared in late spring. During summer, when predation pressure by fish is hi gh, abundance of B. longimanus decreased gradually. In contrast, L. kindtii displayed maximum abundances with more than 18 000 ind. m(-2) (Zeller See) in summer. During the summer months, B. longimanus lived in greater depths than L. kindtii. Differences of vertical distribution between both species were most pronounced at the deep stations (Langenargen-Arbon, Fischbach-Ut twil). Analysis of stomachs of the planktivorous fish Coregonus lavaretus s howed that B. longimanus was preferred by C. lavaretus over all other prey; Ivlev's selectivity index was 0.97-0.99. Due to this high positive selecti on for B. longimanus by coregonids, it is surprising that the cladoceran co uld maintain its existence since the first recordings in 1877. Spatial refu ge or indigestible resting eggs may be the underlying mechanism for this ph enomenon.