No abstract

1. Detecting the impacts of invading Bythotrephes cederstrœmi (Crustacea, Onychopoda, Cercopagididae) on zooplankton in North American lakes has been hampered by the brevity of pre‐invasion data, and by the difficulty of distinguishing the effects of the invader from other stresses. The data from Harp Lake in Ontario, Canada, circumvent these difficulties. Bythotrephes appeared in the lake in 1993. There is a 15‐year pre‐invasion data set, and no significant complicating concurrent stresses. 2. The species composition and the size structure of the crustacean zooplankton community of Harp Lake changed after the invasion. Several small species either declined dramatically in abundance (e.g. Bosmina longirostris, Tropocyclops extensus) or disappeared (Chydorus sphaericus, Diaphanosoma birgei, Bosmina (Neobosmina) tubicen). In contrast the abundance of the larger cladocerans Holopedium gibberum and Daphnia galeata mendotae and the hypolimnetic copepod Leptodiaptomus sicilis increased. Several univariate and all multivariate summarizations of zooplankton abundance, biomass and size structure highlighted the uniqueness of the post‐invasion community. 3. The alterations in the zooplankton community could not be attributed to changes in lake acidity, thermal regimes, penetration by ultraviolet light, nutrient status, fish stocking or the abundances of native invertebrate predators, but they were correlated with Bythotrephes abundance, both within and among years. Hence, we hypothesize that the invasion by Bythotrephes has significantly altered the crustacean zooplankton community of Harp Lake.

We reared Daphnia pulex in a fully defined medium at seven calcium (Ca) concentrations (0.1, 0.5, 1, 1.5, 2, 5, and 10 mg Ca L 21 ), two mixed algal densities (high food: 30 mg chlorophyll a L 21 and low food: 3 mg Chl a L 21 ), and four temperatures (20uC, 24uC, 28uC, and 32uC) in a fully factorial design. The minimum Ca concentration required for the survival and reproduction of D. pulex was between 0.1 and 0.5 mg Ca L 21 . Daphniids reared at 0.1 mg Ca L 21 lived no longer than 10 d and did not reproduce. Although offspring were produced at 0.5 mg Ca L 21 and above, reproduction was reduced below 1.5 mg Ca L 21 due to delays in maturity and reductions in both the brood size and number of broods produced within the 15 d of the experiment. As a result, the intrinsic rate of natural increase, r, decreased dramatically between 1.5 and 1 mg Ca L 21 , and was undefined at 0.1 mg Ca L 21 . Higher temperatures and reduced algal biomass enhanced the susceptibility of D. pulex to low Ca concentrations by raising the reproductive threshold to 1.5 mg Ca L 21 . Thermal stress at 32uC was so great that daphniids lived no longer than 5 d and did not reproduce. Hence, r was undefined at 32uC. In order to sustain D. pulex populations and potentially other Ca-sensitive daphniids, fresh waters must maintain a Ca concentration of at least 0.5 mg Ca L 21 , although concentrations as high as 1.5 mg Ca L 21 may be required for daphniids to withstand the concurrent reductions in algal biomass and rising water temperatures that are now commonplace on the south-central Canadian Shield.

No abstract

The crustacean zooplankton community of Harp Lake, Ontario, Canada, has changed appreciably since the invasion by the spiny water flea, Bythotrephes. Crustacean species richness has declined, large-bodied Cladocera have replaced small-bodied ones, and there has been a downward trend in the total abundance of zooplankton because copepod abundance has remained stable while Cladoceran abundance has declined. Although the zooplankton community has now been stable for 4 years (1995–1998), the biology of the invader has changed dramatically. In particular, there have been 10-fold differences in the mean annual abundance of Bythotrephes in this 5-year period and substantial changes in the timing of population maxima. We attribute these changes to two factors: (i) transition from a summer to a fall switch from parthenogenesis to gametogenesis and (ii) interannual differences in the thickness of a warm, dark stratum in the lake. We hypothesize that this stratum provides a refuge for Bythotrephes from predation by lake herring, Coregonus artedii.