Factors regulating cladoceran dynamics in a Venezuelan floodplain lake

Five cladoceran zooplankton species in a Venezuelan floodplain lake, Laguna la Orsinera, were analyzed to identify factors controlling their dynamics over a period encompassing isolation, inundation by the Orinoco River and drainage. The abundance of each species increased rapidly to a maximum associated with flooding of the lake by the Orinoco River. We calculated birth, death and population growth rates, and developed a method for estimating uncertainty in these parameters, in order to interpret observed cladoceran abundance patterns. Rapid increases in population sizes resulted both from high birth rates associated with decreasing turbidity following inundation and from the hatching of resting eggs. Birth rates remained high throughout the study, but high death rates restricted each population to a brief maximum and then maintained small population sizes throughout the remainder of the study. Our demographic analyses suggest that high mortality resulted from intense predation by fish and by Chaoborus larvae, rather than from resource limitation, and that this mortality was the major factor regulating cladoceran abundance during inundation and drainage in this tropical floodplain lake.