Community structure of many systems changes across space in many different ways (e.g., gradual, random or clumpiness). Accessing patterns of species spatial variation in ecosystems characterized by strong environmental gradients, such as estuaries, is essential to provide information on how species respond to them and for identification of potential underlying mechanisms. We investigated how environmental filters (i.e., strong environmental gradients that can include or exclude species in local communities), spatial predictors (i.e., geographical distance between communities) and temporal variations (e.g., different sampling periods) influence benthic macroinfaunal metacommunity structure along salinity gradients in tropical estuaries. We expected environmental filters to explain the highest proportion of total variation due to strong salinity and sediment gradients, and the main structure indicating species displaying individualistic response that yield a continuum of gradually changing composition (i.e., Gleasonian structure). First we identified benthic community structures in three estuaries at Todos os Santos Bay in Bahia, Brazil. Then we used variation partitioning to quantify the influences of environmental, spatial and temporal predictors on the structures identified. More frequently, the benthic metacommunity fitted a quasi-nested pattern with total variation explained by the shared influence of environmental and spatial predictors, probably because of ecological gradients (i.e., salinity decreases from sea to river). Estuarine benthic assemblages were quasi-nested likely for two reasons: first, nested subsets are common in communities subjected to disturbances such as one of our estuarine systems; second, because most of the estuarine species were of marine origin, and consequently sites closer to the sea would be richer while those more distant from the sea would be poorer subsets. Understanding how community structure of many systems changes across space and how mechanisms, driven mostly by dispersal and environmental filters, determine species distribution patterns in local communities is a central question in community ecology 1-3. Testing how community assembly mechanisms determine species distribution has also become important in metacommunity ecology, an offshoot of community ecology, which has emerged to describe processes occurring at local and regional scales 1,4. A metacommunity can be defined as a set of local communities potentially, but not necessarily, linked by the dispersal of multiple, likely interacting, species 5,6. Assessing processes that affect metacommunity composition particularly in ecosystems characterized by strong environmental gradients is important to provide useful information on species responses to environmental changes across ecological gradients. To understand patterns of spatial variation in species composition, two different and complementary metacommunity approaches have been proposed 7 : one focusing on patterns 7,8 and another focusing on mechanisms 1,9. The patt...

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The aim of this study was to evaluate the effects of increasing concentrations of lead (Pb) in the nutrient solution on growth and macronutrient and organic solute contents of Helianthus annuus plants. The experimental design was completely randomized with four replications. The dry mass yield was not affected by the treatments up to a Pb concentration of 0.6 mM, indicating that H. annuus was tolerant to this Pb concentration in the growth medium. The reductions in leaf, stem and root dry masses at a Pb concentration of 0.8 mM were 74, 84 and 85%, respectively. Lead stress did not reduce the levels of nitrogen (N), phosphorus (P) and potassium (K), indicating that the growth reduction observed at 0.8 mM Pb could not be attributed to deficiencies of these nutrients. On the other hand, Pb significantly reduced calcium (Ca) and magnesium (Mg) contents in leaves, stems and roots, which might, at least in part, explain the Pb-induced growth reduction in the H. annuus plants. Pb increased soluble carbohydrate, free amino acid and proline contents in leaves, and soluble protein and proline contents in roots, showing stress-induced changes in cell metabolism. The data also suggest that Ca and Mg concentrations may be used as nutritional indicators and the proline content may be used as a biochemical indicator of Pb toxicity in H. annuus.

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The objective of the present study was to evaluate the tolerance to cadmium (Cd) of sunflower genotypes grown in greenhouse conditions, and the effectiveness of using photosynthetic parameters as physiological indicators of this tolerance. Seeds of two sunflower genotypes previously identified as tolerant (H358) and Cd-sensitive (AG960) to Cd were used. The seeds were germinated in plastic cups containing plant substrate; after 9 days, the seedlings were transplanted to plastic basins containing a nutrient solution with 0 or 10 µM of Cd, where they remained for 16 days. Samples of the plants were harvested every 5 days. The experiment was carried out in a randomized complete design, using a 4×2×2 factorial arrangement (4 days of grown in a nutrient solution with Cd, 2 sunflower genotypes, and 2 Cd levels) with four replications. Cd stress decreased CO 2 net assimilation, stomatal conductance, carboxylation efficiency, photosynthetic pigment contents, potential quantum yield (F v /F m ) , and effective quantum yield of plants of the two evaluated genotypes. The decrease in photosynthetic rates of these plants was caused by both stomatal and non-stomatal limitations. Plants of the AG960 genotype showed more pronounced deleterious effects due to Cd stress than those of the H358 genotype. Thus, CO 2 net assimilation rate, stomatal conductance, and chloroplast pigment content are good physiological indicators of sunflower tolerance to Cd and can at least in part, explain the greater tolerance of the H358 genotype to Cd stress when compared to the AG960 genotype. EFICÁCIA DOS PARÂMETROS FOTOSSINTÉTICOS COMO INDICADORES FISIOLÓGICOS DA TOLERÂNCIA AO CÁDMIO EM GIRASSOLRESUMO -O presente estudo foi conduzido em casa de vegetação objetivando-se demonstrar a tolerância ao cádmio (Cd) em genótipos de girassol e avaliar a eficácia dos parâmetros fotossintéticos como indicadores fisiológicos dessa tolerância. Para isto, foram utilizadas sementes de dois genótipos de girassol previamente identificados como tolerante (H358) e sensível (AG960) ao Cd.As sementes foram germinadas em copos plásticos contendo substrato vegetal e, após 9 dias, as plântulas foram transferidas para bacias plásticas com solução nutritiva contendo 0 ou 10 µM de Cd , onde permaneceram por 16 dias. As plantas foram coletadas a cada 5 dias. O delineamento experimental foi o inteiramente casualizado, em um arranjo fatorial 4 (dias de exposição) × 2 (genótipos) × 2 (níveis de Cd), com quatro repetições. A exposição ao Cd reduziu a taxa de assimilação líquida do CO 2 , a condutância estomática e a eficiência de carboxilação, bem como os teores de todos os pigmentos fotossintéticos, a razão F v /F m e o Y II das plantas de ambos os genótipos estudados. Demonstrando que a redução na taxa fotossintética dessas plantas foi ocasionada tanto por limitações estomáticas como não-estomáticas. Os efeitos deletérios do Cd foram mais pronunciados no genótipo AG960 em comparação com o H358. Assim, as variáveis A, gs e os teores de pigmentos mostraram-se bons indicadores fis...