Abstract
Zooplankton exhibit several trends of variation in space and time, and these trends can be more evident in natural environments without anthropic perturbations. Examples of anthropic factors are climate change, eutrophication and construction of reservoirs. This study evaluated the influence of three factors – seasonality, type of environment and the presence of aquatic macrophytes – on various ecological attributes of rotifers in a river-lake system located in the Paraná River floodplain. Monthly samplings were conducted during 1993 and 1994. The mean species richness per sample was 60 species. The seasonality and the type of environment influenced the ecological attributes of rotifer assemblages, while the presence or absence of aquatic macrophytes did not. Species richness was highest in the lake system and during the months when water levels were low. Multivariate analysis indicates a small group of species associated with the low water-level phase. In contrast, many species were associated with high water levels or increasing water levels. The seasonal variation of hydrological cycle and the type of environment are the most important factors for rotifer structure in natural conditions.
Highlights
Floodplains include complex aquatic habitats and ecotones in the context of terrestrial environments (Junk and Welcomme 1990)
The presence or absence of aquatic macrophytes was not important in our study, contrasting with several other studies that pointed to the importance of macrophytes to provide food resources and refuge against zooplankton predators (Bonecker and Lansac-Tôha 1996, Lansac-Tôha et al 2004)
The analysis showed that there were clear differences in the species assemblages between low and high-water levels in the system rive-lake
Summary
Floodplains include complex aquatic habitats and ecotones in the context of terrestrial environments (Junk and Welcomme 1990). River-lake floodplain systems are highly productive due to the periodicity of nutrient input in the form of dissolved and particulate matter, derived from the lateral exchange between water and terrestrial habitats when the water is high (Junk and Welcomme 1990, Thomaz et al 2004). This seasonal characteristic results in temporal patterns according to hydrometric levels and fluviometric fluctuations. Species richness and abundance of aquatic communities are low when water levels are high, as a result of dilution, and higher when the water level is low (Casanova and Henry 2004)
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