Leaf decomposition rates of nine types of litter, ranging in lignin content from 1.04 to 25.71%, were measured at three wetlands with different inundation regimes and related to internal and external factors. In these sites (located in the alluvial floodplain along at transect perpendicular to the main channel of the Parana ´River), 180 litter bags were incubated under water. To test the importance of initial nitrogen, lignin, phosphorus, or L:N content of the litter in predicting decay rates, we analyzed the relationship between leaf quality and dry weight remaining (at 30 days and at the end of the incubation) within each wetland and under uniform environmental conditions. To elucidate the effects of environmental factors (especially hydroperiod) on decomposition processes, we compared the decay coefficient of one of these species (Eichhornia crassipes) across the three wetlands. Our results strongly suggest that under uniform environmental conditions, decomposition rates were best explained by a single internal factor related to litter quality. We found two different situations. 1) In the marsh with riverine forest, where decomposition was rapid and the half-life of the litter ranged from 9.6 to 63 days, the initial nitrogen content of leaves was a good predictor of decay rates both at 30 days and at the end of the incubation. 2) In the oxbow lake and palm swamp forest, where decomposition was lower and the half-life varied between 31.5 and 219 days, the L:N explains the variability on dry weight remaining at the end of the incubation. The decay coefficient of E. crassipes across sites was significantly related to NO 3 Ϫ concentration of water. We concluded that both environmental variables (i.e., the fluvial quotient of connectivity and the number of days in high water) and leaf litter quality (i.e., nitrogen content and L:N) are important factors regulating leaf litter decomposition in Parana Ŕiver floodplain wetlands.

Litterfall and leaf decomposition rates were measured in Choui Island, 45 km downstream from the confluence of the Parana and Paraguay rivers. The material was collected biweekly from April 1985 through September 1986. Decomposition was measured in situ by the litter bag technique.Annual litterfall of Tessaria integriflia gallery forest measured in the period April 1985 to March 1986 was 8.15 t ha-'. Leaflitterfall was seasonal, i.e. significantly less leaflitter was shed during the high water phase than during the low water phase. The 'half life' of the T. integrzzlia litter over 38 days of decomposition was 20 days. At the beginning of the experiment, 15 and 38 days subsamples of remaining detritus were analyzed in order to determine changes in the nutrient content. After 38 days of incubation, the order of nutrient disappearance was Ca > K > N > Mg > Na > P.The number of invertebrates per g remaining litter of Tessaria integriflia increased between incubations days 7 and 3 1. Collector-gatherers were more abundant after 38 days incubation; there were no shredders colonizing the leaf litter bags.

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In this paper, we analyze the changes in composition and abundance of fish assemblages in seven vegetated floodplain wetlands with different connectivity across different hydrologic conditions: after a prolonged connection of the floodplain with the main channel, during receding water, and after a prolonged isolation. We also investigated the size and abundance of large-sized migratory species found in these wetlands and the food resources exploited by the dominant fish. Fishes were captured by diurnal seining (8.0 m x 1.50 m, 5 mm mesh) along macrophyte banks. Despite the high total number of species registered (100), sample species richness varied between 7 and 31, depending on the sampling site and the sampling date. Cluster analysis indicated low similarity between sites during both the isolation and the prolonged connection. Species turnover decreased from high water (β = 40.33) to low water (β = 33.83), with the minimum value of beta diversity index obtained during the isolation of the floodplain wetlands (β = 26.83). Our results indicated that different dominant populations of fish occur in different hydrological conditions, even though high water and isolation phases occur in the same season of different years. The ordination (NMDS) indicated the importance of hydrologic conditions in structuring fish assemblages in the studied floodplain. Small-sized characids, typically associated with macrophytes, dominated the fish assemblages, whereas the younger stages of large sized migratory species were found in low abundance. The maximum standard length of the fish captured was 28 cm and for large migratory fish, standard length varied between 1.6 and 25.0 cm. The dominant fish used several food resources, but littoral macrophytes-associated organisms had a high frequency of occurrence in the three hydrologic conditions. The high species richness of fish in the small, vegetated lakes was related to the high spatial heterogeneity during different hydrological conditions. Disturbances in the hydrological pulses could reduce the biodiversity by modifying the connectivity of the floodplain with the river channel. Conservation of these vegetated wetlands requires maintenance of actual width range of connectivity that provide diverse habitat along the time.Neste artigo analisamos as mudanças na composição e abundância das assembléias de peixes de sete áreas úmidas de planície de inundação com vegetação e com diferentes conectividades e em diferentes condições hidrológicas: depois de uma conexão prolongada da planície de inundação com o canal principal, durante o recuo das águas e após um isolamento prolongado. Nós também investigamos o tamanho e abundância das espécies migratórias de grande tamanho encontradas nestas áreas úmidas e os recursos alimentares explorados pelas espécies dominantes. Os peixes foram capturados com rede durante o dia (8.0 m x 1.50 m, malha de 5 mm) nos bancos de macrófitas. Apesar do alto número de espécies registradas (100), a riqueza de espécies nas amostras variou entre 7 e 31, de acordo c...

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