Ostracods are microcrustaceans that produce resting eggs under adverse conditions. In this study we evaluated the spatial variation of ostracod resting eggs in different regions of temporary lakes in a Brazilian flood plain. Based on the homogenisation effect of flood pulses on aquatic communities in flood plains, we hypothesised that the composition and abundance of ostracod eggs in the centre of temporary lakes would be similar to those in edge regions. Samples were collected from the centre and edge regions of five temporary lakes. Sediment was oven dried, rehydrated and hatching was monitored in germinating chambers. Twelve ostracod species hatched from the egg banks during our experiments. The abundance and species composition were similar between the two regions of the lakes. Flood events may be responsible for the homogenisation of the egg banks as a result of the connection of lakes with principal river channels. During flooding, water masses powerfully enter lakes and can redistribute sediments. This study shows that egg banks have the potential to contribute to the maintenance of local biodiversity and the resilience of biodiversity of temporary lake ecosystems.

Most changes in water bodies are result of human activities that have the potential to undermine the environmental integrity of aquatic ecosystems. Changes in genetic material can be evidenced by the frequency of nuclear abnormalities in fish blood cells, in response to genotoxic agents even at low concentrations. Thus, we aimed at comparing the frequencies of nuclear abnormalities of fish kept in acclimation, with fish collected in Timbó river (Santa Cruz do Timbó, Santa Catarina State), under low anthropogenic interference, and fish collected in Iguaçu river (União da Vitória, Paraná State), a polluted river. The highest frequency of changes in nuclear morphology of fish erythrocytes was found in the urbanized area around the Iguaçu river, while, the fish collected in areas with preserved riparian forest surrounding the Timbó river and acclimated fish showed no morphological changes. Our results suggest that genotoxic compounds in the Iguaçu river are acting as stressors to aquatic communities, especially fish. In addition, the absence of nuclear abnormalities in fish from the Timbó river suggests that rivers without significant environmental changes can serve as reference point for comparative studies of genetic modifications for the species studied.

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Freshwater ostracods are commonly found in temporary ponds and lakes, surviving drought periods by producing resting eggs. We investigated the hatching phenology of ostracod resting eggs from the sediments of temporary floodplain lakes, considering both the taxonomic distance between species and their functional traits, such as carapace size (length and height) and shape, valve ornamentation, and reproductive mode. In addition, we tested the hypotheses: (1) that the hatching time is more similar between congeneric ostracod species than between noncongeneric species; (2) that differences in hatching time between congeneric species are often related to differences in functional traits; (3) that both species composition and functional traits composition of hatchlings change over the incubation time, but with a reduction in the variability over time. The experiment was conducted for 98 days in the laboratory, and the microcosms were monitored weekly. The first hatching of an ostracod was recorded during the second week of incubation after hydration of the sediments. A total of 12 ostracod species hatched, belonging to the families Cyprididae and Candonidae. The above three hypotheses were corroborated. The time between inundation and first hatching was mostly similar for congeneric species in the genus Chlamydotheca, but was different between some species of Strandesia, which might be owing to differences in functional traits. The species composition and functional trait composition of the hatched ostracods were significantly different over the 14 weeks of incubation due to the different hatching phenology of different species. Furthermore, our results show that both taxonomic distance and functional traits can influence the hatching time of ostracod resting eggs from temporary floodplain lakes. Thus, future studies addressing the hatching phenology of ostracod resting eggs (and resting stages from other invertebrates) should also emphasize the use of functional traits.

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