Aquatic macrophytes are important resources for the maintenance of trophic chains and in biogeochemical processes, but they can also be deleterious for several uses if present in excess. Hydrilla verticillata was found in the Paraná River (Brazil) after 2005, which requires monitoring owing to the invasive potential of this species. In this study, we measured the growth of H. verticillata under controlled conditions and compared the growth dynamics for the two development strategies (branch and tuber). We show that this species has great potential to develop in tropical (Brazilian) aquatic ecosystems. The parameters from the modelling of the growth kinetics indicated a doubling time of 19.8 days for H. verticillata growing from stems; however, the growth from tubers were much faster, with doubling times ranging from 2.5 to 11 days. The delay for the tubers to sprout caused a decrease in the number of branches of H. verticillata stems. From the growth parameters obtained from the experiments under controlled conditions, we concluded that the high temperature and light availability in most South American reservoirs (including the Porto Primavera Reservoir where it was first recorded) are suitable for H. verticillata to compete and probably displace other native aquatic macrophytes, such as Egeria najas, Egeria densa, and Cerathophyllum demersum. This is a matter of concern because these and other submersed species are commonly found in several natural and man-made South-American aquatic ecosystems, where they are key for biodiversity maintenance.

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The temporal variation of stoichiometry between consumed oxygen and oxidized carbon was investigated for the aerobic mineralization of leachates from aquatic macrophytes. Seven species of aquatic plants, viz. Cabomba piauhyensis, Cyperus giganteus, Egeria najas, Eichhornia azurea, Salvinia auriculata; Scirpus cubensis and Utricularia breviscapa, were collected from Ó leo lagoon located in the floodplain of MogiGuac¸u river (Sa˜o Paulo State, Brazil). After being collected, the plants were washed, oven-dried and triturated. In order to obtain the leachate, the fragments were submitted to an aqueous extraction (cold). Mineralization chambers were incubated at 20°C containing leachates dissolved in water samples from Ó leo lagoon to a final concentration of ca. 200 mg l )1 on carbon basis. The chambers were maintained under aerobic conditions; the concentrations of the organic carbon (particulate and dissolved) and the dissolved oxygen were measured during approximately 80 days. Elemental analysis of the detritus and the concentrations of the remaining material (DOC and POC) were used to determine the amounts of mineralized organic carbon. The data were analyzed with first-order kinetics models, from which the daily rates of consumption (carbon and oxygen) and the stoichiometry (O/C) were determined. In the early phase of mineralization the O/C rates increased before reaching a maximum, after which they tended to decrease. For the mineralization of leachates from C. giganteus, S. auriculata and U. breviscapa, the decrease was relatively slow. For all substrata the initial values were smaller than 1, and ranged from 0.42 (S. cubensis) to 0.81 (C. piauhyensis). The maximum values were within the range from 0.58 (U. breviscapa) to 23.1 (E. najas) and at their highest 26th (C. piauhyensis) and 106th (C. giganteus) days. These variations are believed to be associated with the chemical composition of the leachates, with their transformations and alterations of metabolic pathways involved in the mineralization.

*. RESUMO Existem duas fontes de detritos para os ecossistemas aquáticos lóticos e lênticos: alóctone, composta principalmente por folhas provindas da vegetação ripária, e autóctone, onde se destacam as macrófitas aquáticas. Neste artigo faz-se uma revisão sobre o processo de decomposição, os fatores que o controlam, e os agentes biológicos que atuam em cada fase do processo: (i) lixiviação, (ii) condicionamento e (iii) fragmentação. Uma forma de sintetizar todo esse processo para que se permita a comparação dos dados é o cálculo do coeficiente de decaimento. Através da compilação e classificação de vários coeficientes de decaimentos de folhas da mata ripária e de macrófitas aquáticas obtidos em diversos estudos foi possível observar que: (i) o coeficiente de decomposição dos materiais autóctones foi superior ao dos materiais alóctones (t 1/2 = 56 e 35 dias, respectivamente) embora o decaimento dos dois tipos de material tenha sido classificada como rápido; (ii) o coeficiente de decaimento foi maior para as folhas decompostas em clima Tropical do que Temperado, com tempos de meia-vida respectivamente iguais a 25 e 44 dias; (iii) as macrófitas de hábito flutuantes foram as que apresentaram menores taxas de decomposição (lenta), seguidas pelas espécies emergentes (médio) e submersas (rápido), com tempos de meia-vida respectivamente iguais a 168, 112 e 33 dias. Palavras-chave: Decomposição; matéria orgânica alóctone; matéria orgânica autóctone; coeficientes de decaimento. ABSTRACT DECOMPOSITION OF ALLOCHTHONOUS AND AUTOCHTHONOUS ORGANIC MATTER IN AQUATIC ECOSYSTEMS. There are two sources of detritus for aquatic ecosystems: allochthonous, composed mainly by leaves from the riparian vegetation, and autochthonous, with emphasis on the aquatic macrophytes. This article presents a review of the decomposition process, the factors controlling it, and the biological agents acting at each stage of the process: (i) leaching, (ii) conditioning and (iii) fragmentation. A way to summarize the decomposition process allowing comparison of data is to calculate the decay coefficient. In compilation and classification of numerous decay coefficients from leaves of riparian vegetation and aquatic macrophytes obtained by several authors in studies of decomposition in aquatic ecosystems, we observed that: (i) the decomposition rate of autochthonous materials was higher than that of allochthonous material (t 1/2 = 56 and 35 days, respectively), despite the decay of both types of material has been classified as fast, (ii) the decay rate was higher for leaves decomposing in Tropical than in Temperate climate, with half-life respectively equal to 25 and 44 days, (iii) the floating macrophytes showed the lowest decomposition rates (slow), followed by emergent (middle) and submerged (fast) species, with half-life times, respectively, equal to 168 112 and 33 days.

Assays were carried out to evaluate the consumption of dissolved oxygen resulting from mineralisation processes in resources usually found in aquatic systems. They were also aimed at estimating the oxygen uptake rate of each investigated process. Experiments were conducted using substrates from 3 different places. A fixed amount of substrate was added to 5 litres of water from Lagoa do Infernão that was previously filtered with glass wool. After adding the substrates the bottles were aired and the amount of dissolved oxygen and the temperature were monitored for 55 days. The occurrence of anaerobic processes was avoided by reoxygenating the bottles. The experimental results were fitted to a first order kinetics model, from which the consumption of dissolved oxygen owing to mineralisation processes was obtained. The amount of oxygen uptake from the mineralisation processes appeared in the following decreasing order: Wolffia sp., Cabomba sp., Lemna sp., DOM (Dissolved Organic Matter), Salvinia sp., Scirpus cubensis, stem, Eichhornia azurea, sediment and humic compounds. The deoxygenation rates (day-1) were: 0.267 (humic compounds), 0.230 (Lemna sp.), 0.199 (E. azurea), 0.166 (S. cubensis), 0.132 (sediment), 0.126 (DOM), 0.093 (Cabomba sp.), 0.091 (stem), 0.079 (Salvinia sp. and Wolffia sp.). From these results, 2 groups of resources could be identified: the first one consists of detritus with higher amounts of labile (ready to use) compounds, which show a higher global oxygen uptake during the mineralisation process; the second one consists mainly of resources that show refracting characteristics. However, when the consumption rates are analysed it is noted that the mineralised parts of the refracting substrates can be easier to process than the labile fractions of the less refracting resources.

The dynamics of aquatic macrophytes in intermittent rivers is generally related to the characteristics of the resistance and resilience of plants to hydrologic disturbances of flood and drought. In the semi-arid region of Brazil, intermittent rivers and streams are affected by disturbances with variable intensity, frequency, and duration throughout their hydrologic cycles. The aim of the present study is to determine the occurrence and variation of biomass of aquatic macrophyte species in two intermittent rivers of distinct hydrologic regimes. Their dynamics were determined with respect to resistance and resilience responses of macrophytes to flood and drought events by estimating the variation of biomass and productivity throughout two hydrologic cycles. Twenty-one visits were undertaken in the rewetting, drying, and drought phases in a permanent puddle in the Avelós stream and two temporary puddles in the Taperoá river, state of Paraíba, Northeast Brazil. The sampling was carried out by using the square method. Floods of different magnitudes occurred during the present study in the river and in the stream. The results showed that floods and droughts are determining factors in the occurrence of macrophytes and in the structure of their aquatic communities. The species richness of the aquatic macrophyte communities was lower in the puddles of the river and stream subject to flood events, when compared to areas where the runoff water is retained. At the beginning of the recolonization process, the intensity of the floods was decisive in the productivity and biomass of the aquatic macrophytes in the Taperoá river and the Avelós stream. In intermediate levels of disturbance, the largest values of productivity and biomass and the shortest time for starting the recolonization process occurred.Keywords: stream, disturbance, flood, drought, aquatic macrophyte. RESUMO Ciclo hidrológico e dinâmica de macrófitas aquáticas em dois rios intermitentes da região semi-árida do BrasilA dinâmica de macrófitas aquáticas em rios intermitentes está relacionada com as características de resistência e resiliência das plantas as perturbações hidrológicas da cheia e da seca. A região semi-árida do Brasil apresenta rios e riachos intermitentes, com ocorrência de eventos de perturbação em diferentes graus de intensidade, freqüência e duração ao longo de um ciclo hidrológico. O objetivo deste estudo foi determinar a ocorrência e variação de biomassa de espécies de macrófitas aquáticas em dois rios intermitentes de regime hidrológico distintos. A dinâmica foi determinada pelas respostas de resistência e resiliência das plantas aos eventos de cheia e de seca, através da variação da biomassa e produtividade ao longo de dois ciclos hidrológicos. Foram realizadas 21 visitas de campo durante as fases de reinundação, 576 PEDRO, F., MALTCHIk, L. and BIANCHINI JR., I.

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The formation of reservoirs often affects water quality strongly, with the changes in the physicochemical properties being ascribed to decomposition of remaining organic matter arising from leaching and (biological and chemical) breakdown processes. In this study, experiments under laboratory conditions were performed to show that the nature of the course particulate organic matter (CPOM; i.e., leaves, branches, barks, and litter) determines the decomposition kinetics in new reservoirs. Effects on the water quality can be of short-, mid-, and long-term duration for all types of CPOM, as indicated in the mathematical modeling of the decomposition kinetics. Leaves and litter displayed the shortest half-life times (51 and 40 days, respectively) and the highest potential of leaching/oxidation of labile compounds (19 and 21%, respectively). On the other hand, decomposition of branches and barks generated the lowest oxygen consumption (74 and 44 mg oxygen/g dry mass (DM), respectively). During formation of the reservoir, the incorporation and decomposition of organic matter prevailed over material exportation. Therefore, in addition to a decrease in oxygen availability the concentration of biochemical oxygen demand (BOD) and nutrients increased. After the filling stage, there was significant loss of organic matter via oxidation, sedimentation, biological assimilation, and export, thus causing the BOD concentration and the fertility of the water to decrease.