Manuela abelho

Abstract: A decomposition experiment using eucalyptus leaves was carried out in a Southeastern Brazilian mountain stream located at the transition between the Cerrado and the Atlantic Forest to test whether nutrient addition increases microbial and invertebrate colonisation and accelerates breakdown rates. The results show that none of the tested variables was significantly affected by nutrient addition, despite the average increase in ATP concentrations and invertebrate colonisation observed in the fertilised leaf bags. This could mean that breakdown in the stream was already at its maximum due to the relatively high water temperature and nutrient content, or that the breakdown rate of eucalyptus leaves was too fast to allow the detection of any effects of nutrient addition. Breakdown rates of eucalyptus leaves were much faster than the values reported in literature for most species in Brazilian Cerrado streams, suggesting that the replacement of the natural vegetation by eucalyptus may affect nutrient dynamics in the region.

Abstract: The feeding preferences and consumption rates of the exotic woodlouse Armadillidium vulgare were measured on litters from the autochthonous Rio Grande cottonwood and the exotic Russian olive. When offered a choice between five litter types (green and shed leaves of Russian olive and cottonwood, and fruits of Russian olive), the isopods consumed more of the exotic plant litter (green leaves and the fruits of Russian olive) than of the autochthonous cottonwood. When offered only one diet, consumption rates of green leaves and fruits of Russian olive were again the highest. Feeding was related with N and C:N ratios of the litter, with highest consumption of diets with low and high C:N ratios and lowest consumption of diets with intermediate C:N ratios. The N content was highest (and C:N ratios lowest) on isopods feeding shed leaves of Russian olive. Growth was not related to the amount or the C:N ratio of the food consumed, being highest for animals feeding on shed leaves of cottonwood. Our results show that the consumption patterns and the C and N body composition of the exotic woodlouse A. vulgare are altered when fed with Russian olive litter, suggesting that the replacement of the autochthonous cottonwood by the exotic Russian olive may negatively affect the maintenance of the population of these exotic detritivores in the area.

Abstract: Previous work in terrestrial and aquatic ecosystems has suggested that the relationship between breakdown rates of leaf litter and plant species richness may change unpredictability due to nonadditive effects mediated by the presence of key-species. By using single- and mixed-species leaf bags (7 possible combinations of three litter species differing in toughness; common alder [Alnus glutinosa], sweet chestnut [Castanea sativa], and Spanish oak [Quercus ilex ilex]), I tested whether leaf species diversity, measured as richness and composition, affects breakdown dynamics and macroinvertebrate colonization (abundance, richness and composition) during 90 days incubation in a stream. Decomposition rates were additive, i.e., observed decomposition rates were not different from expected ones. However, decomposition rates of individual leaf species were affected by the mixture, i.e., there were species-specific responses to mixing litter. The invertebrate communities colonizing the mixtures were not richer and more diverse in mixtures than in single-species leaf bags. On the opposite, mixing leaf species had a negative, non-additive effect on rates of shredder and taxa colonization and on macroinvertebrate diversity.

Abstract: Ergosterol and ATP concentrations, microbial respiration and sporulation rates of aquatic hyphomycetes associated with leaves of Castanea sativa decomposing in a 5th order stream were determined periodically over a period of 102 days in order to compare ergosterol and ATP as indicators of fungal biomass. ATP and ergosterol concentrations exhibited a significant positive correlation (F = 4.459, DF = 28, P < 0.001) during the first stages of leaf breakdown (until day 39), i.e., during periods of increasing fungal biomass. No correlation was found between ATP and ergosterol concentrations during later stages of decomposition (days 39 to 102). Respiration rates increased rapidly up to 0.525 mg O2 h–1 g–1 AFDM during the first month and remained high until the end of the experiment. Sporulation rates peaked at day 9 (1069 conidia day–1 mg–1 AFDM) and decreased during later stages of decomposition. ATP-to-biomass conversion factors were determined for both fungi (0.59 μmol ATP g–1 dry mass) and bacteria (1.30 μmol ATP g–1 dry mass) collected from the stream and grown in the laboratory. Estimates of fungal biomass based on ATP concentrations were similar to those calculated from ergosterol concentrations during the first 39 days of breakdown. The results here presented suggest that ATP is a reliable method to quantify microbial biomass in streams and that the relative importance of bacteria increases at later stages of decomposition.

Abstract: Dynamics of nutrient exchange between floodplains and rivers have been altered by changes in flow management and proliferation of nonnative plants. We tested the hypothesis that the nonnative, actinorhizal tree, Russian olive (Elaeagnus angustifolia), alters dynamics of leaf litter decomposition compared to native cottonwood (Populus deltoides ssp. wislizeni ) along the Rio Grande, a river with a modified flow regime, in central New Mexico (USA). Leaf litter was placed in the river channel and the surface and subsurface horizons of forest soil at seven riparian sites that differed in their hydrologic connection to the river. All sites had a cottonwood canopy with a Russian olive-dominated understory. Mass loss rates, nutrient content, fungal biomass, extracellular enzyme activities (EEA), and macroinvertebrate colonization were followed for three months in the river and one year in forests. Initial nitrogen (N) content of Russian olive litter (2.2%) was more than four times that of cottonwood (0.5%). Mass loss rates (k, in units of d1 ) were greatest in the river (Russian olive, k¼0.0249; cottonwood, k¼0.0226), intermediate in subsurface soil (Russian olive, k¼ 0.0072; cottonwood, k ¼ 0.0031), and slowest on the soil surface (Russian olive, k ¼ 0.0034; cottonwood, k ¼ 0.0012) in a ratio of about 10:2:1. Rates of mass loss in the river were indistinguishable between species and proportional to macroinvertebrate colonization. In the riparian forest, Russian olive decayed significantly faster than cottonwood in both soil horizons. Terrestrial decomposition rates were related positively to EEA, fungal biomass, and litter N, whereas differences among floodplain sites were related to hydrologic connectivity with the river. Because nutrient exchanges between riparian forests and the river have been constrained by flow management, Russian olive litter represents a significant annual input of N to riparian forests, which now retain a large portion of slowly decomposing cottonwood litter with a high potential for N immobilization. As a result, retention and mineralization of litter N within these forests is controlled by hydrologic connectivity to the river, which affects litter export and in situ decomposition.

Abstract: This study evaluated if there are differences in leaf breakdown and invertebrate colonization among tree species differing in quality (toughness), and which factors could influence these differences. Common alder leaves decomposed significantly faster then either sweet chestnut or Spanish oak (k values of –0.0332, –0.0108, and –0.0112, respectively) during the first 2 months. Shredder abundance was highest when leaf mass remaining was 50%, and the samples clustered in mixed groups of sampling dates and leaf species, suggesting that stage of decomposition was an important factor influencing shredder colonization. During the first two months of decomposition, the physicochemical characteristics of leaf litter and the interaction between leaf toughness and the occurrence of frequent spates seemed to be the main factors affecting leaf breakdown rates in the stream.

Abstract: This study assessed the effect of nutrient enrichment on rates of decomposition, ergosterol concentrations (as a measure of fungal biomass), and rates of fungal sporulation of sweet chestnut (Castanea sativa Miller) leaves in a 3rd order stream (Central Portugal), with medium to high background values of nutrients. Coarse and fine mesh leaf bags were attached to nutrient diffusing substrata containing NaNO3, KH2PO4, both nutrients, or no additions. Leaf breakdown rates were similar in the four treatments and in the two mesh sizes (k=−0.0155 to −0.0219 day−1). Phosphorus content of P or N + P enriched leaves was higher than in the other treatments after 28 days, but there were no differences in N concentrations. Ergosterol concentrations associated with decomposing leaves were similar among treatments. The peak sporulation rates of aquatic hyphomycetes were stimulated by the addition of N + P and N but not by P alone. Results from the experiment provide evidence that leaf breakdown in the study stream, as a model for streams with naturally medium to high level of nutrients, was not nutrient-limited, and that fungal reproductive activity was limited by dissolved N but not by dissolved P in stream water.

Abstract: The processing of leaves in temperate streams has been the subject of numerous studies but equivalent tropical ecosystems have received little attention.We investigated leaf breakdown of a tropical tree species (Hura crepitans, Euphorbiaceae), in a tropical stream using leaf bags (0.5 mm mesh) over a period of 24 days.We followed the loss of mass and the changes in adenosine triphosphate (ATP) concentrations and respiration rates associated with the decomposing leaves. The breakdown rate was fast (k = −0.0672/d, kd = −0.0031/degree-day), with 81 percent loss of the initial mass within 24 days. This high rate was probably related to the stable and high water temperature (22◦C) favoring strong biological activity. Respiration rates increased until day 16 (1.1 mg O2/h/g AFDM), but maximum ATP concentrations were attained at day 9 (725 nmol ATP/g AFDM) when leaf mass remaining was 52 percent. To determine the relative importance of fungi and bacteria during leaf decomposition, ATP concentrations, and respiration rates were determined in samples treated with antibiotics, after incubation in the stream. The results of the samples treated with the antifungal or the bacterial antibiotic suggest a higher contribution of the fungal community for total microbial biomass and a higher contribution of the bacterial community for microbial respiration rates, especially during the later stages of leaf decomposition. However, these results should be analyzed with caution since both antibacterial and antifungal agents did not totally eliminate microbial activity and biomass.

Abstract: This paper is a review of recent (≤10 years) information on litterfall, standing stock of benthic organic matter, breakdown rates, and fungal colonization of organic matter in streams. In some cases, recent research reinforces the findings of classic reference papers. In other cases, the additional knowledge provided by recent research introduces a higher variation in the processes analyzed. In many aspects, especially those concerning stream organic matter, the review is biased towards the temperate North American streams, reflecting the fact that most research was carried out there. However, during the 1990s European studies increased enormously, especially those related with instream processes, such as leaf litter decomposition. The first part of this review analyzes the origin of allochthonous organic matter to streams (litterfall, retention, and storage), and it provides data on the amounts estimated in different streams and on the methodology used in the studies. The second part analyzes the fate of detritus in streams: mechanisms of leaf breakdown, relative importance of fungi and bacteria, factors affecting the activity of microbial decomposers, and chemical changes of leaf litter during decomposition. A list of breakdown rates of several different leaf species is given, together with the methodology used, and the main characteristics of the incubation streams.

Abstract: A flow-through system to assess microbial respiration rates associated with decomposing leaves in streams was tested under two different conditions. We wanted to determine (1) the sensitivity of the system to microbial respiration rates, and (2) the effect of antibiotics on microbial respiration rates during the incubation period. The flow-through system was sensitive to the respiratory activity of microbial assemblages associated with decomposing leaves even during short incubation periods. Our results showed that microbial respiration rates did not change significantly during an incubation period of up to five hours. Due to the short incubation period and to the continuous stirring of the samples with 100% oxygenated water the method avoided much of the growth and death of cells that may be induced over longer incubation periods and changes in rates due to variations in oxygen concentration. The results of tests estimating respiration rates during incubation with antibiotics showed that (1) at least some prokaryotes were not retained by 0.2 um filters, and (2) oxygen uptake in the water column increased after the addition of antifungal antibiotics. This could be due to an inorganic effect of the antibiotics on oxygen chemistry, or to the enhancement of prokaryotic activity after the blocking of non-retained eukaryotes. However, the use of blanks allowed the removal of the expected and unexpected sources of variation.

Abstract: To evaluate the importance and fate of organic matter inputs in forested streams, we determined the litterfall inputs and the benthic coarse particulate organic matter (CPOM) in one headwater stream flowing through a mixed deciduous forest, during one year. Both vertical traps and the stream bottom were sampled monthly. The material collected was sorted into four main categories: leaves, fruits and flowers, twigs and debris. Litter production was 715 g m-2 y-1 and seasonal, with 73% of the annual total during October–December (autumn). Leaves comprised the largest litter component. Benthic organic matter was 1880 g m-2 y-1, and was also seasonal. Highest accumulation was attained in spring, and twigs and branches comprised the major component.

Abstract: There have been many studies of litter decomposition and the macroinvertebrates associated with litter. Nevertheless, the reasons why invertebrates colonize leaf bags are still a matter for diccussion. Although invertebrates use leaves as food, it has been shown that they may use the leaf bag a substrate. In this study, we investigate whether macroiivertebrates colonize leaf bags despite abundant natural leaf litter in the streams and whether they discriminate between leaves occurring naturally in the stream (Alnus glutinosa) and alien species in this case, the South American Hura crepitans). We found no differences in the relative abundance of shredders between Alnus and ura leaf bags. In relation to the relative abundance of shredders in the leaf bags and in the streambed, we found all the possible combinations. In two streams there were no differences between the benthic samples and the leaf bags. In another stream the relative abundance of shredders was higher in the benthic samples, and in a further stream, the relative abundance of shredders was higher in the leaf bags. It seems therefore, that when there is an abundant of high quality food in the streams there is no reason for the aggregation of shredders in the leaf bags.

Abstract: To test the hypothesis whether afforestation with Eucalyptus globulus affects litter dynamics in streams and the structure of macroinvertebrate aquatic communities, we compared streams flowing through eucalyptus and deciduous forests, paying attention to: (i) litterfall dynamics, (ii) accumulation of organic matter, (iii) processing rates of two dominant leaf species: eucalyptus and chestnut, and (iv) macroinvertebrate community structure. The amount of allochthonous inputs was similar in both vegetation types, but the seasonality of litter inputs differed between eucalyptus and natural deciduous forests. Eucalyptus forest streams accumulated more organic matter than deciduous forest streams. Decomposition of both eucalyptus and chestnut leaf litter was higher in streams flowing through deciduous forests. The eucalyptus forest soils were highly hydrophobic resulting in strong seasonal fluctuations in discharge. In autumn the communities of benthic macroinvertebrates of the two stream types were significantly different. Deciduous forest streams contained higher numbers of invertebrates and more taxa than eucalyptus forest streams. Mixed forest streams (streams flowing through eucalyptus forests but bordered by deciduous vegetation) were intermediate between the two other vegetation types in all studied characteristics (accumulation of benthic organic matter, density and diversity of aquatic invertebrates). These results suggest that monocultures of eucalyptus affect low order stream communities. However, the impact may be attenuated if riparian corridors of original vegetation are kept in plantation forestry.

Abstract: Structure and diversity of the benthic macroinvertebrate fauna were studied in two deciduous forest streams in Central Portugal. In the three sampling occasions, 120 taxa were collected from the two streams. Number of taxa per sampling occasion ranged from 53 to 60. Macroinvertebrate densities ranged from 1465 to 2365. Insects were the most abundant taxonomic group (at least 80%) in all samples. Detritivorous invertebrates were numerically dominant in both streams, representing 62 to 85% of the total macroinvertebrate community.