Resting cladoceran eggs and their contribution to zooplankton diversity in a lagoon of the Upper Paraná River floodplain
Zooplankton produce resting eggs and diapause stages as strategies to promote their survival during periods of adverse environmental conditions, thereby ensuring the community diversity. The goal of this study is to investigate the contribution of these eggs, which were present in the sediment of an isolated lagoon in the Upper Paraná River floodplain, to cladoceran diversity. The species hatching from the resting eggs in the sediment do not occur in the plankton. Sediment samples were collected at six sampling sites in the Osmar Lagoon banks, in September 2004 and June, July, August and September 2005. Zooplankton were sampled in the pelagic region, using a motorized pump (1000 L) and plankton net (68 µm). The pH, water temperature (°C), electrical conductivity (µS cm−1) and dissolved oxygen concentration (mg L−1) also were measured. The resting eggs were counted in the laboratory, being maintained under constant temperature and photoperiod until their analyses. The resting eggs were examined daily until hatching. A higher abundance of these eggs was observed in July, during a period of lower temperature and electrical conductivity, and higher pH and dissolved oxygen concentrations. ANOVA results indicated significant differences in resting egg abundance among the sampling months. Ceriodaphnia reticulata, Macrothrix spinosa, Ilyocryptus spinifer, Ephemeroporus tridentatus and Diaphanosoma sp. hatched from the resting eggs, although they were not observed in the plankton samples. C. reticulata was observed for the first time in this floodplain, with these study results corroborating the hypothesis that resting cladoceran eggs do contribute to zooplankton diversity in the lagoon.