Past vegetation changes in the Brazilian Pantanal arboreal–grassy savanna ecotone by using carbon isotopes in the soil organic matter

Measurements of the organic carbon inventory, its stable isotopic composition and radiocarbon content were used to deduce vegetation history from two soil profiles in arboreal and grassy savanna ecotones in the Brazilian Pantanal. The Pantanal is a large floodplain area with grass-dominated lowlands subject to seasonal flooding, and arboreal savanna uplands which are only rarely flooded. Organic carbon inventories were lower in the grassy savanna site than in the upland arboreal savanna site, with carbon decreasing exponentially with depth from the surface in both profiles. Changes in ¹³C of soil organic matter (SOM) with depth differed markedly between the two sites. Differences in surface SOM ¹³C values reflect the change from C₃ to C₄ plants between the sites, as confirmed by measurements of ¹³C of vegetation and the soil surface along a transect between the upland closed-canopy forest and lowland grassy savanna. Changes of ¹³C in SOM with depth at both sites are larger than the 3–4 per mil increases expected from fractionation associated with organic matter decomposition. We interpret these as recording past changes in the relative abundance of C₃ and C₄ plants at these sites. Mass balances with ¹⁴C and ¹³C suggest that past vegetational changes from C₃ to C₄ plants in the grassy savanna, and in the deeper part of the arboreal savanna, occurred between 4600 and 11 400 BP, when major climatic changes were also observed in several places of the South American Continent. The change from C₄ to C₃, observed only in the upper part of the arboreal savanna, was much more recent (1400 BP), and was probably caused by a local change in the flooding regime.     

Phlebotomine sandflies associated with a focus of cutaneous leishmaniasis in Valle del Cauca, Colombia

Abstract. A survey was made of the phlebotomine sandfly fauna of La Guaira, a village with coffee plantations near Cali, Colombia, from which cases of American cutaneous leishmaniasis had been reported due to Leishmania (Viannia) panamensis and Le. ( V. ) braziliensis. Among six species of sandfly collected on human bait, Lutzomyia youngi was most important in terms of biting nuisance. Lu. columbiana, Lu. lichyi and Lu. scorzai as well as Lu. youngi adults occurred throughout the year. Sandfly man-biting activity occurred throughout the night and was highest within 2 h of sunset. Despite its abundance in nocturnal samples, Lu. youngi was rarely taken in diurnal resting site collections. In contrast, Lu. lichyi was collected on tree-trunks during the day in large numbers and was the only species biting in daylight. The implications of these and other findings for leishmaniasis control measures in La Guaira are considered.     

Alternative states in the phytoplankton of Lake Kinneret, Israel (Sea of Galilee)

1. Through analyses of a 34‐year record of phytoplankton, zooplankton and physicochemical parameters from Lake Kinneret, Israel, we show that distinct and persistent phytoplankton assemblage states occurred from winter to summer. 2. The most obvious characteristic of these states was the presence or absence of a spring bloom of the dinoflagellate, Peridinium gatunense. 3. Analyses of the data within the framework of the alternative states model revealed a possible complex triggering mechanism, and system hysteresis. 4. A change in zooplankton biomass and body size coincident with changes in predation pressure associated with the collapse of the Kinneret Bleak, Acanthobrama terraesanctae, fishery appeared to be the ‘slow changing’ variable in the context of the alternative states model. Alternative phytoplankton states were only possible after this variable crossed a threshold in 1993–94, following the collapse of the fishery. 5. When alternative states were possible, some physicochemical parameters and the structure of the zooplankton assemblage appeared to control which phytoplankton state emerged in a given year. In years without a P. gatunense bloom, important physicochemical parameters in winter included low NO₃ loading, high water temperature, high water level, a deeper thermocline, low transparency, high concentrations of NO₃ and Cl in the epilimnion, and low concentration of epilimnetic total phosphorus. In addition, the cladoceran Chydorus sphaericus and adults of the copepod Mesocyclops ogunnus were observed in winter in years without a bloom. 6. Zooplankton biomass and body size of some taxa have recovered since the 1993–94 collapse of the fishery, yet incidence of both phytoplankton states in Lake Kinneret was still possible. Within the framework of the alternative states model, this suggests that the slow changing variable threshold where alternative states became possible is different from the threshold where alternative states will no longer be possible. In other words, the system is characterised by a hysteresis.     

Phenotypic plasticity of Brevicoryne brassicae in responses to nutritional quality of two related host plants

Abstract 1. The host species used by a herbivorous insect may impose different selective pressures promoting host race formation, yet the presence of plasticity can potentially constrain host race formation. 2. The goal of this study is to determine if there is phenotypic plasticity in life history traits of Brevicoryne brassicae in response to host and nutritional quality of two host species, Brassica oleraceae and Brassica campestris, and to what extent there are genetic differences among genotypes in plasticity. 3. Plants of B. oleraceae and B. campestris were fertilised with three different nitrogen doses (with nutritive solutions of 50, 200 and 400 ppm of soluble nitrogen) to produce plants with different nutritional qualities. Eight clones of B. brassicae were reared on those plants, and days to reproduction (DTR), number of nymphs, and fitness (r_m) were recorded. 4. A significant genotype × host interaction was detected in days to reproduction. Genotype × nitrogen interaction (plasticity) was detected in the number of nymphs when aphids were raised on B. campestris. Aphids showed plasticity in DTR and marginal plasticity in r_m in reaction to the varying nitrogen content of B. oleraceae. 5. The phenotypic plasticity to fine‐scale variation of host (nutritional quality) documented here may be an important source of phenotypic variation and may potentially constrain host race formation.     

Warming increases the proportion of primary production emitted as methane from freshwater mesocosms

Methane (CH₄) and carbon dioxide (CO₂) are the dominant gaseous end products of the remineralization of organic carbon and also the two largest contributors to the anthropogenic greenhouse effect. We investigated whether warming altered the balance of CH₄ efflux relative to gross primary production (GPP) and ecosystem respiration (ER) in a freshwater mesocosm experiment. Whole ecosystem CH₄ efflux was strongly related to temperature with an apparent activation energy of 0.85 eV. Furthermore, CH₄ efflux increased faster than ER or GPP with temperature, with all three processes having sequentially lower activation energies. Warming of 4 °C increased the fraction of GPP effluxing as CH₄ by 20% and the fraction of ER as CH₄ by 9%, in line with the offset in their respective activation energies. Because CH₄ is 21 times more potent as a greenhouse gas, relative to CO₂, these results suggest freshwater ecosystems could drive a previously unknown positive feedback between warming and the carbon cycle.     

Warming alters the size spectrum and shifts the distribution of biomass in freshwater ecosystems

Organism size is one of the key determinants of community structure, and its relationship with abundance can describe how biomass is partitioned among the biota within an ecosystem. An outdoor freshwater mesocosm experiment was used to determine how warming of～4 °C would affect the size, biomass and taxonomic structure of planktonic communities. Warming increased the steepness of the community size spectrum by increasing the prevalence of small organisms, primarily within the phytoplankton assemblage and it also reduced the mean and maximum size of phytoplankton by approximately one order of magnitude. The observed shifts in phytoplankton size structure were reflected in changes in phytoplankton community composition, though zooplankton taxonomic composition was unaffected by warming. Furthermore, warming reduced community biomass and total phytoplankton biomass, although zooplankton biomass was unaffected. This resulted in an increase in the zooplankton to phytoplankton biomass ratio in the warmed mesocosms, which could be explained by faster turnover within the phytoplankton assemblages. Overall, warming shifted the distribution of phytoplankton size towards smaller individuals with rapid turnover and low standing biomass, resulting in a reorganization of the biomass structure of the food webs. These results indicate future environmental warming may have profound effects on the structure and functioning of aquatic communities and ecosystems.     

Production of Plant Growth Regulators by Rhizosphere Phosphate-solubilizing Bacteria

Culture supernatant fluids of 50 phosphate-dissolving bacteria isolated from rhizospheres of crop plants were examined for IAA, gibberellins and cytokinins. These bacteria possessed phytase activity and 27 could dissolve rock phosphate. Twenty bacteria synthesized all 3 types of plant hormones, 43 produced IAA, 29 formed gibberellins and 45 cultures produced cytokinin-like substances. Of the 50 bacteria tested 28 decomposed IAA. Plant growth inhibitors were detected in cultures of some isolates. The ecological significance of these rhizosphere bacteria and their mode of action when used as inoculants is considered.     

Inactivation of some third-generation cephalosporins by β-lactamases of non-fermentative Gram-negative bacteria isolated from freshwater of a remote environment

Inactivation of cefotaxime, cefoperazone and ceftazìdime by β-lactamases from strains of Pseudomonas sp. and Acinetobacter calcoaceticus isolated from freshwater of a remote environment is demonstrated. Inactivation rates showed some relation-ship to susceptibility of the strains as a whole, but this relationship was absent for individual strains, probably because of some other resistance mechanisms. Inac-tivation was also observed with low inocula if incubation was prolonged. These β-lactamases seem to be coded by chromosomal genes, since no plasmids were found and they exhibited relatively high isoelectric points.