Bosmina

Abstract: Because phytoplankton succession involves pulses of short duration and a degree of unpredictability, most workers have argued against resource specialization by zooplankters. Yet, in Lake Mitchell, Vermont, two genera of cladocerans (Bosmina and Daphnia) coexist under nearly equilibrium conditions that involve an intriguing conflict. Daphnia can depress Bosmina, but apparently cannot exclude it, even though both share substantial portions of available resources. This coexistence appears mediated by the specialized foraging mode of Bosmina, which deviates markedly from filter feeding in a way that permits efficiency removal of highly edible flagellates, particularly when these items occur at low density. Evaluation of the flagellate—Bosmina association and its relationship to Bosmina—Daphnia interactions involved: (1) analysis of population responses in large—scale enclosures, (2) in situ grazing experiments with natural and introduced resources resources, and (3) laboratory preference tests. See full-text...

Abstract: The effects of fish predation and food availability on population densities and demography of zooplankton were investigated in Dynamite Lake, Illinois, USA, a lake with a high density of size-selective planktivorous fish and low food levels. Fish predators (bluegill sunfish) and food levels (phytoplankton) were manipulated in replicated, factorial- design field experiments during two summers (1980 and 1981). Overall, population densities of zooplankton were affected much more by manipulations of food availability than by manipulations of fish predation. The cladocerans Bosmina longirostris, Ceriodaphnia lacustris, and Diaphanosoma birgei were greatly increased in density by elevated phytoplankton levels in both years, in the presence and absence of fish. Demographic analysis in 1981 revealed that increased densities in response to elevated food levels resulted from both an increase in birth rates (Bosmina, Diaphanosoma) and a decrease in mortality rates (Ceriodaphnia, Diaphanosoma). The rotifers Lecane and Mon- ostyla also increased dramatically in response to elevated phytoplankton densities. Co- pepods were less responsive to manipulations of food levels, but several taxa exhibited increases in density in response to increased phytoplankton abundance. Few species were reduced in density by fish predation. Ceriodaphnia density was reduced by fish more than any other species in the entire community, and the density of even this species was much more affected by food availability. In terms of percent change relative to controls, increased food availability had much more of an effect than fish predation on the density of most zooplankton species and on total zooplankton abundance. Fish predation had several effects on the size structure and life history traits of the cladocerans. All three species attained larger sizes when fish were excluded than when fish were present. Cladoceran individuals also initiated reproduction at a smaller size and produced smaller offspring in the presence of fish. The reductions in mean body size, size at maturity, and offspring size in the presence of fish were most pronounced in Diapha- nosoma and Ceriodaphnia, the two largest species. Smaller body size and size at maturity apparently allow the cladocerans to reproduce before reaching a size at which they become vulnerable to size-selective fish predators. Larger size at first reproduction and larger off- spring size in the absence of fish may be a response to invertebrate predators, which assume more importance in the absence of fish and prey most heavily on smaller size classes. Flexibility in these life-history traits allows the cladocerans to withstand what appears to be intense size-selective predation by planktivorous fish.

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.

Abstract: SUMMARY. The food of 0+ perch, Perca fluviatilis L. has been investigated in Windermere, the largest lake in the English Lake District. Whole guts and stomachs from 800 fish were investigated during four 24-h periods between June and September in 1975 and in 1976. Ciliates, algae, rotifers and cyclopoid nauplii constituted the first food of larval perch in Windermere. Vertical samples were taken from the zooplankton, at the same time and in the same location where the fish were sampled, and the concentration of the different food organisms studied. Electivity of the different food organisms was studied from lvlev's index, which showed positive electivity, or selection for some organisms, e.g. Bosmina obtusirostris (Sars.) and copepods (both cyclopoid and calanoid), and negative for others such as Daphnia hyalina var. galeata (Sars.), Leptodora kindti (Focke) and Bythotrephes longimanus (Leydig). In studying predatorprey relationships, Daphnia hyalina was chosen as the ‘standard prey’, being the most abundant of all food organisms in the diet of young perch, also because its maximum body-depth (carapace width (mm)) was among the largest of the cladocera and copepods in Windermere. Comparison of the maximum carapace width of Daphnia eaten with that of the plankton revealed that smaller specimens were taken by juvenile perch in July, but they tended to select larger organisms during August and September.