Population growth capacities and regulatory factors in monospecific cultures of the cladocerans

The cladocerans Moina micrura and Diaphanosoma excisum and the copepod Thermocyclops decipiens were studied in microcosms (0.8 m3) under semi-controlled experimental conditions at 25–29 °C for 32 days, by daily sampling after an initial monospecific inoculation. For each species, the time series began with an exponential population growth phase. M. micrura showed a higher daily population growth rate (mean = 1.19) than D. excisum (0.78) and T. decipiens (0.45). The growth phase of M. micrura coincided with bacterial and phytoplanktonic peaks while the growth phase of the two other species followed these peaks. After this phase, M. micrura quickly disappeared (day 10), while D. excisum biomass decreased but showed a second increase, followed by a stabilization sequence. T. decipiens biomass had a slower increase and stabilized after day 17. The passage to sexual reproduction in relation to crowding was the main regulating factor for M. micrura, whereas food limitation was important for D. excisum. For T. decipiens, population growth was limited by decreased recruitment to copepodite stages which could have resulted from cannibalism exerted by older stages.