A cyanobacterial bloom prevents fish trophic cascades

1. We experimentally compared the impacts of visually feeding zooplanktivorous fish and filter‐feeding omnivorous fish in shallow tropical Dakar Bango reservoir, Senegal. We provoked a cyanobacterial Anabaena bloom under mesotrophic to eutrophic N‐limited conditions in 18 enclosures assigned to six Nile tilapia life‐stage treatments, at typical biomasses: fishless control (C), zooplanktivorous fry (Z), omnivorous juveniles (O), herbivorous fingerlings (H) and two combinations (OZ, OH). 2. All fish grew well, but as prevalent inedible phytoplankton dampened fish effects, community‐level trophic cascades did not occur. Planktivore types acted independently and affected differentially the biomasses of total zooplankton, cyclopoids, nauplii, cladocerans, invertebrate carnivores, large herbivores, colonial cyanobacteria and Chlorophyta. They neither influenced the total biomass of phytoplankton, nor most water chemistry characteristics. Responses were apparently not fish‐biomass related. The bloom collapsed synchronously in all enclosures, coinciding with enrichment ending, with a return to clear water within 12 days. 3. Our results support the hypothesis that excess nutrients and prevalent inedible cyanobacteria inhibit the cascading effects of natural biomass levels of both visually feeding zooplanktivores and filter‐feeding omnivores. In N‐limited meso‐eutrophic shallow tropical lakes with predominantly small herbivorous zooplankton, neither the type nor the biomass of planktivorous fish present seems likely to prevent the transient outburst of cyanobacterial blooms. Such fragile ecosystems may thus not sustain a trophic state suitable for drinking water production, unless human impacts are restricted. The generality of restoration approaches based on ecological engineering should be further explored.     

GLOBE students, teachers, and scientists demonstrate variable differences between urban and rural leaf phenology

The urban heat island effect, classically associated with high impervious surface area (ISA), low vegetation fractional cover (Fr), and high land surface temperature (LST), has been linked to changing patterns of vegetation phenology, especially spring growth. In this study, a collaboration with the Global Learning and Observations to Benefit the Environment (GLOBE) program, we investigated the effect of the urban environment on the timing of leaf budburst of native deciduous trees in seven cities: Asia (Tokyo, Japan; Bangkok and Korat, Thailand), Europe (Jyväskylä, Finland; Bishkek, Kyrgyzstan), Africa (Dakar, Senegal), and North America (Fairbanks, Alaska). The cities differed not only in population size but also in climate and vegetation type. Using Landsat satellite imagery from each city, we calculated LST, Fr, and ISA, and classified sites within each study area as rural or urban. The timing of leaf flushing, measured by students using GLOBE budburst protocols, was statistically different within all cities, with absolute differences ranging from 1 to 23 days. We assessed the classic urban phenology paradigm, which proposes higher LST, lower Fr, and earlier budburst in urban areas of temperate cities. Of the four temperate cities, Tokyo followed the classic paradigm, but no other city demonstrated consistent support. Urban budburst was advanced in three of the four temperate cities, but in only one of the three tropical cities. Results suggest that while vegetation phenology is consistently different between urban and rural areas, a uniform paradigm based on the explanatory variables in this study did not emerge. Although not testable here, it is likely that alterations to chilling requirements in temperate climates and humidity in tropical climates may also influence observed budburst differences.     

Water, temperature, and vegetation regulation of methyl chloride and methyl bromide fluxes from a shortgrass steppe ecosystem

Temperate grasslands are considered to be a significant sink for CH₃Br, although large uncertainties exist about the magnitude of this sink because of a paucity of field measurements. Here, we report the results of a combined field and laboratory study that investigated the effects of water, temperature, and plant community composition on CH₃Cl and CH₃Br fluxes in a semiarid temperate grassland. A novel stable isotope tracer technique was also employed to deconvolute simultaneous production and oxidation of CH₃Cl and CH₃Br. Net and gross fluxes were measured from different landforms (ridges, floodplains) and cover types (grass‐dominated, shrub‐dominated) to capture a representative range of hydrologic regimes, temperatures, and plant communities. In field experiments, net CH₃Cl and CH₃Br uptake was observed at all grass‐dominated sites (?400±77 nmol CH₃Cl m^?2 day^?1 and ?3.4±0.9 nmol CH₃Br m^?2 day^?1), while net CH₃Cl emission (439±58 nmol CH₃Cl m^?2 day^?1) was observed at sites dominated by the shrub Atriplex canescens, indicating that this plant is a strong CH₃Cl producer. Gross CH₃Cl and CH₃Br oxidation were comparable with estimates from other dryland ecosystems (507±115 nmol CH₃Cl m^?2 day^?1 and 9.1±2.2 nmol CH₃Br m^?2 day^?1), although CH₃Br oxidation rates were at least five times lower than those observed in more mesic temperate grasslands. We suggest that estimates of the temperate grassland CH₃Br sink should be reduced by ≥19% (≥1.8 Gg yr^?1) to account for the weaker sink strength of semiarid environments. Identification of A. canescens as a ‘new’ CH₃Cl source may have important ramifications for the global atmospheric budget of CH₃Cl, given the global distribution of this plant and its congeners and their widespread presence in many dryland ecosystems. Laboratory experiments revealed that soil water was the chief regulator of CH₃Cl and CH₃Br oxidation, while temperature had no observed effect between 14 and 26 °C. Oxidation rates rose most rapidly between 0.4% and 5% volumetric water content, suggesting that methyl halide‐oxidizing bacteria respond strongly to small inputs of water under the very driest conditions. Soil drying and rewetting experiments did not appear to affect the oxidation of CH₃Cl and CH₃Br by soil microorganisms, which are presumably adapted to frequent wet/dry cycles.     

Biogeography in the Death Valley region: evidence from springsnails (Hydrobiidae: Tryonia)

Allozyme and mitochondrial DNA variation were analysed to examine evolution of the nine species of springsnails (genus Tryonia) living in the Death Valley system (Owens and Amargosa basins) of southeastern California and southwestern Nevada. Both allozyme and mtDNA evidence indicate that this highly endemic fauna is non-monophyletic. Species from the upper Amargosa basin comprise a clade most closely related to snails living in the Colorado basin. Snails from the lower Amargosa basin (Death Valley trough) reflect a complex evolutionary history and two of these species are more closely related to an estuarine species from western California than to other snails of the region. These results indicate a commonality of pattern with the well-studied Death Valley pupfishes (Cyprinodon), which also are non-monophyletic and include species that are most closely related to Colorado basin congeners. These biogeographic patterns are interpreted within the context of a recently proposed model for the early history of the lower Colorado River.     

Above- and below-ground responses of C3–C4 species mixtures to elevated CO2 and soil water availability

We evaluated the influences of CO₂[Control, ～ 370 µ mol mol ^{? 1}; 200 µ mol mol ^{? 1} above ambient applied by free‐air CO₂ enrichment (FACE)] and soil water (Wet, Dry) on above‐ and below‐ground responses of C₃ (cotton, Gossypium hirsutum) and C₄ (sorghum, Sorghum bicolor) plants in monocultures and two density mixtures. In monocultures, CO₂ enrichment increased height, leaf area, above‐ground biomass and reproductive output of cotton, but not sorghum, and was independent of soil water treatment. In mixtures, cotton, but not sorghum, above‐ground biomass and height were generally reduced compared to monocultures, across both CO₂ and soil water treatments. Density did not affect individual plant responses of either cotton or sorghum across the other treatments. Total (cotton + sorghum) leaf area and above‐ground biomass in low‐density mixtures were similar between CO₂ treatments, but increased by 17–21% with FACE in high‐density mixtures, due to a 121% enhancement of cotton leaf area and a 276% increase in biomass under the FACE treatment. Total root biomass in the upper 1.2 m of the soil was not influenced by CO₂ or by soil water in monoculture or mixtures; however, under dry conditions we observed significantly more roots at lower soil depths ( > 45 cm). Sorghum roots comprised 81–85% of the total roots in the low‐density mixture and 58–73% in the high‐density mixture. CO₂‐enrichment partly offset negative effects of interspecific competition on cotton in both low‐ and high‐density mixtures by increasing above‐ground biomass, with a greater relative increase in the high‐density mixture. As a consequence, CO₂‐enrichment increased total above‐ground yield of the mixture at high density. Individual plant responses to CO₂ enrichment in global change models that evaluate mixed plant communities should be adjusted to incorporate feedbacks for interspecific competition. Future field studies in natural ecosystems should address the role that a CO₂‐mediated increase in C₃ growth may have on subsequent vegetation change.     

Protein Synthesis During Rehydration, Germination and Seedling Growth of Naturally and Precociously Matured Soybean Seeds (Glycine max)

Soybean seeds [Glycine max (L.) Merr.] synthesize de novo andaccumulate several non-storage, soluble polypeptides duringnatural and precocious seed maturation. These polypeptides havepreviously been coined ‘maturation polypeptides’.The objective of this study was to determine the fate of maturationpolypeptides in naturally and precociously matured soybean seedsduring rehydration, germination, and seedling growth. Developingsoybean seeds harvested 35 d after flowering (mid-development)were precociously matured through controlled dehydration, whereasnaturally matured soybean seeds were harvested directly fromthe plant. Seeds were rehydrated with water for various timesbetween 5 and 120 h. Total soluble proteins and proteins radio-labelledin vivo were extracted from the cotyledons and embryonic axesof precociously and naturally matured and rehydrated seed tissuesand analyzed by one-dimensional PAGE and fluorography. The resultsindicated that three of the maturation polypeptides (21, 31and 128 kDa) that had accumulated in the maturing seeds (maturationpolypeptides) continued to be synthesized during early stagesof seed rehydration and germination (5–30 h after imbibition).However, the progression from seed germination into seedlinggrowth (between 30 and 72 h after imbibition) was marked bythe cessation of synthesis of the maturation polypeptides followedby the hydrolysis of storage polypeptides that had been synthesizedand accumulated during seed development. This implied a drasticredirection in seed metabolism for the precociously maturedseeds as these seeds, if not matured early, would have continuedto synthesize storage protein reserves. Glycine max (L.) Merr, soybean, cotyledons, maturation, germination/seedling growth     

Overwintering of Microcystis in Lake Mendota

SUMMARY. Microcystis , an important bloom-forming blue-green alga in Lake Mendota, Wisconsin, was found to accumulate at the lake bottom and overwinter there. Microcystis collected from winter sediments were photosynthetically active, showing temperature and light optima similar to summer and autumn populations. Sediments incubated under in situ conditions in the laboratory maintained photosynthetic activity for at least four months. Such a maintenance ability may be part of an overall strategy by Microcystis populations to ensure a suitable inoculum for the following growth season.     

Correlations of stomatal conductance with hydraulic and chemical factors in several deciduous tree species in a natural habitat

Recent research in whole-plant stomatal physiology, conducted largely with potted plants in controlled environments, suggests that stomatal conductance ( g _s) might be more closely linked to plant chemical variables than to hydraulic variables. To test this in a field situation, seasonal g _s was examined in relation to a number of plant and environmental variables in 11 temperate, deciduous forest tree species. Stomatal conductance was generally better correlated with environmental variables (air temperature, vapor pressure deficit, PPFD) than with plant variables, and slightly better correlated with plant hydraulic variables (shoot water and osmotic potentials) than with plant chemical variables (xylem sap ABA concentration, xylem sap pH). We examined a model, developed previously for maize, which describes regulation of g _s by xylem sap ABA concentration with leaf water status acting to modify stomatal sensitivity to the ABA signal. This model explained slightly more variation in seasonal g _s in the forest trees than did single plant variables but not more variation than most single environmental variables. Response surface models, especially those incorporating environmental variables, were more consistently successful at explaining g _s across species.     

On the Mechanism of Action of <Emphasis Type="Italic">lac</Emphasis> Repressor

Chen et al. have proved conclusively that lac repressor and RNA polymerase bind independently to wild type lac DNA in vitro. To explain the lacp ^s mutation, which causes competitive binding between repressor and polymerase, they suggest that a new promoter site has been created near the lac operator.     

The ionic relations of Porphyra purpurea(Roth) C.Ag. (Rhodophyta, Bangiales)

Abstract Radioisotope equilibration techniques have been used to determine the intracellular concentration of K⁺, Na⁺ and Cl_?, together with the unidirectional ion fluxes across the plasmalemma of Porphyra purpurea. Influx and efflux of ⁴²K⁺, ²⁴Na⁺ and ³⁶C1^? are biphasic, the rapid, initial uptake and loss of tracer from individual thalli being attributable to desorption from extracellular regions. Cellular fluxes are slower and monophasic, cells discriminating in favour of K⁺ and Cl^? and against Na⁺. A comparison between the equilibrium potential of individual ion species and the measured membrane potential demonstrates that there is an active component of K⁺ and Cl^? influx and Na⁺ efflux. ‘Active’ uptake and ‘passive’ loss of K⁺ and Cl^? are reduced when plants are kept in darkness, suggesting that a fraction of the transport of K⁺ and Cl^? may be due to ‘exchange diffusion’ (K⁺/K⁺ and Cl^?/Cl^?antiport).