The biomass and maximum depth of colonization (Z max ) of Egeria najas Planchon (submerged), and the biomass and area covered by a stand of Eichhornia crassipes (Mart.) Solms and Salvinia herzogii Raddi (floating species) were assessed to measure the effects of a 5 m drawdown in water level of the Itaipu Reservoir (Brazil-Paraguay), which lasted about 3 months. The frequency of occurrence of the two first species, and of Salvinia spp was also assessed in the main arms of the reservoir. A conspicuous decrease of E. najas biomass was observed and this attribute did not reach the previous values even 10 months after the water level had returned to normal. Rapid growth of free-floating species (surface t doub = 2.9 and 3.2 days for total biomass of S. herzogii and E. crassipes, respectively, and 2.3 days for surface area covered by both species) was recorded immediately after the water level returned to normal. This fast growth was related to phosphorus increases in water. A clear succession was observed over a period of 103 days, during which S. herzogii was slowly substituted by E. crassipes. The effects of water level drawdown were also observed on E. najas frequency, given that this species occurred in 38% of the stands investigated before the water drawdown, but in only 9 and 6% of stands following 1 and 10 months of water level recovery, respectively. Water drawdown did not affect the frequency of occurrence of the floating species, which remained approximately constant, and lower than 15% in several of the reservoir arms investigated.

Bacteria were identified from a large, seasonally flooded river (Paraná River, Brazil) and two floodplain habitats that were part of the same river system yet very different in nature: clearwater Garças Lagoon and the highly humic waters of Patos Lagoon. Bacterioplankton were collected during mid-summer (Jan. 2002) from water samples (2 l) filtered first through a 1.2-microm filter then a 0.2-microm membrane filter representing the particle-attached and free-living sub-communities, respectively. DNA was extracted from filters and purified and a 16S rRNA clone library established for each habitat. Over 300 clones were sequenced and checked for similarity to existing 16S sequences in GenBank using the BLAST algorithm with default parameters. Further classification of clones was done using a species "backbone" attachment followed by parsimony analysis. The majority (85%) of sequences, referred to here as operational taxonomic units (OTUs), were most similar to uncultured bacterium 16S sequences. OTUs from each Proteobacteria sub-phylum (alpha, beta, gamma, delta, epsilon) were present in the Upper Paraná River system, as well as members of the Bacteroidetes. The microbial assemblage from Patos Lagoon was least like other samples in that it had no Firmicutes present and was dominated by Actinobacteria. Verrucomicrobia OTUs were only found in the free-living assemblage. This study documents the presence of globally distributed phyla in Upper Paraná River and taxa unique to habitat and particle attachment.

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ABSTRACT

Leaf packs of the aquatic macrophyte Eichhornia azurea were used to experimentally evaluate changes in mass, carbon, nitrogen, and phosphorus concentrations, and C:N ratios during initial stages of decomposition in two different environments, the Paraná River and Garças Lake. Analysis of weight loss showed relatively slow decomposition rates in both environments (0.0047 d -1 and 0.0048 d -1 respectively). Over a 45-day period we observed significant changes (p < 0.05) in concentrations of carbon, nitrogen, and phosphorus, but only carbon differed between the environments (F = 10.479; p = 0.03). Therefore we concluded that detritus behaved similarly during decomposition, since intrinsic characteristics of the environments affected only the carbon concentrations.Key words: Eichhornia azurea, decomposition, floodplain, Paraná river. RESUMOPerda de peso e concentrações de carbono, nitrogênio e fósforo durante a decomposição de Eichhornia azurea na planície de inundação do Alto Rio Paraná, BrasilMaços de folhas da macrófita aquática Eichhornia azurea foram utilizados para avaliar experimentalmente as alterações na massa, concentrações de carbono, nitrogênio e fósforo, bem como a relação C:N, durante os estágios iniciais de decomposição em ambientes distintos (rio Paraná e lagoa das Garças). A perda de peso revelou uma taxa de decomposição relativamente lenta para ambos os ambientes (0,0047 d -1 e 0,0048 d -1 , respectivamente). Durante o período de 45 dias, observaram-se alterações significativas (p < 0,05) nas concentrações de carbono, nitrogênio e fósforo, sendo que apenas as concentrações de carbono diferiram entre os ambientes (F = 10,479; p = 0,03). Portanto, pode-se concluir que os detritos comportam-se de maneira semelhante durante o processo de decomposição, sendo que as características intrínsecas dos ambientes afetam apenas as concentrações de carbono.Palavras-chave: Eichhornia azurea, decomposição, planície de inundação, rio Paraná.

Foram empregadas técnicas de espectroscopia de fl uorescência e UV-VIS para investigar a principal fonte de matéria orgânica dissolvida (MOD) em diferentes ambientes da planície de inundação do alto rio Paraná. As relações de absorvância/carbono orgânico dissolvido (A/COD), intensidade de fl uorescência/carbono orgânico dissolvido (IF/COD), as intensidades de fl uorescência emitidas (FR) e o comprimento de onda de máxima intensidade (PW) foram utilizadas para distinguir a principal fonte da MOD. Os valores de FR, PW e A 285 / COD indicam que a predominância da fonte está relacionada com a infl uência do rio principal. Os espectros das amostras de água dos rios Baía, Ivinheima, lagoa dos Patos e lagoa Carão foram semelhantes aos do ácido fúlvico extraído do solo do entorno desses ambientes, indicando que a principal fonte de MO é pedogênica. Já nos ambientes com infl uência do rio Paraná, ocorre a predominância de MO autóctone. Palavras-chave: Fluorescência, UV-VIS, matéria orgânica dissolvida, planície de inundação. ABSTRACT SPECTROSCOPIC CHARACTERIZATION OF DISSOLVED ORGANIC MATTER IN THE UPPER PARANÁ RIVER FLOODPLAIN. In the present study fl uorescence and UV-VIS techniques were employed to determine the main source of dissolved organic matter (DOM) in different environments of the Upper Paraná River fl oodplain. The following relationships were analyzed: absorbance/dissolved organic carbon (A/DOC), fl uorescence intensity/dissolved organic carbon (FI/DOC), fl uorescence ratio (FR), and peak wavelength with the highest intensity (PW). The obtained FR, PW and A 285 /DOC values indicate that most DOM comes from the main river. The obtained spectra of water samples from the different analyzed sites were similar to that of fulvic acid (FA) extracted from soil from the surroundings of these same areas, thus indicating the source of DOM is chiefl y pedogenic. Regarding environments infl uenced by the Paraná River, most DOM proved to be autochthonous.

A matéria orgânica nos ambientes aquáticos exerce papel fundamental em diversos processos ecológicos sendo a decomposição uma das principais rotas de ciclagem da matéria. A contribuição das macrófi tas aquáticas se dá principalmente pela produção de detritos, já que seu carbono é de difícil assimilação direta pela maioria dos organismos. As análises espectroscópicas tem se constituído numa ferramenta importante no estudo da matéria orgânica, pois permitem uma distinção razoável entre materiais lábeis e refratários. A partir da comparação dos espectros de fl uorescência da matéria orgânica resultante de um experimento de decomposição de macrófi tas aquáticas e da matéria orgânica da água, do sedimento e do solo de ambientes da planície de inundação do alto rio Paraná, estimou-se a contribuição das macrófi tas para o reservatório de carbono nesses ambientes. Foi possível verifi car maior contribuição da decomposição nos ambientes oligotrófi cos conectados ao rio Paraná, enquanto que nos ambientes conectados aos rios Ivinheima e Baia predominam matéria orgânica alóctone, principalmente substâncias húmicas pedogênicas. Palavras-chave: Macrófi tas aquáticas, fl uorescência, decomposição, planície do rio Paraná ABSTRACT CONTRIBUTION TO DISSOLVED ORGANIC MATTER BY DECOMPOSITION OF AQUATIC MACROPHYTES, Eichhornia azurea. Organic matter plays a fundamental role in many ecological processes and decomposition is among the main cycling routes of organic matter. The decomposition of aquatic macrophytes generates much debris, since the carbon of these algae is not easily assimilated by other organisms. Spectroscopic analyses are important in organic matter studies, because it enables satisfactory distinction between labile and refractory materials can be obtained. Through comparing the fl uorescence spectrum of the organic debris generated by a decomposing macrophyte with those of water, sediment, and soil samples from the Upper Paraná River fl oodplain water bodies, we were able to estimate the contribution of decomposition to local carbon cycles. The results revealed a higher contribution of decomposition to the oligotrophic environments connected with the Paraná River, while some water bodies connected with the rivers Ivinheima and Baia showed a greater participation of allochthonous organic matter, mainly derived from humus.