Tree size frequency distributions, plant density, age and community disturbance

We show that explicit mathematical and biological relationships exist among the scaling exponents and the allometric constants (α and β, respectively) of log–log linear tree‐community size frequency distributions, plant density N_T, and minimum, maximum and average stem diameters (D_min, D_max, and , respectively). As individuals grow in size and D_max increases, N_T is predicted to decrease as reflected by a decrease in the numerical value of α and an increase in the value of β. Our derivations further show that N_T decreases as increases even if D_min or D_max remain unchanged. Because D_max and the age of the largest individuals in a community are correlated, albeit weakly, we argue that the interdependent relationships among the numerical values of α, β, N_T, and shed light on the extent to which communities have experienced recent global disturbance. These predicted relationships receive strong statistical support using two large datasets spanning a broad spectrum of tree‐dominated communities.     

Effects of moderate ammonium enrichment on three submersed macrophytes under contrasting light availability

1. Increased ammonium concentrations and decreased light availability in a water column have been reported to adversely affect submersed vegetation in eutrophic waters worldwide. 2. We studied the chronic effects of moderate enrichment (NH₄–N: 0.16–0.25 mg L^?1) on the growth and carbon and nitrogen metabolism of three macrophytes (Ceratophyllum demersum, Myriophyllum spicatum and Vallisneria natans) under contrasting light availability in a 2‐month experiment. 3. The enrichment greatly increased the contents of free amino acids and nitrogen in the shoot / leaf of the macrophytes. This indicates that was the dominant N source for the macrophytes. 4. Soluble carbohydrate contents remained relatively stable in the shoot / leaf of the macrophytes irrespective of the treatments. Under ambient light, the starch contents in the shoot / leaf of C. demersum and M. spicatum increased with enrichment, whereas V. natans did not exhibit any change. The starch contents decreased in C. demersum, increased in M. spicatum and remained unchanged in V. natans after the combined treatment of enrichment and reduced light. 5. The enrichment did not affect the growth of the three macrophytes under the ambient light. However, it did suppress the growth of C. demersum and M. spicatum under the reduced light. The results indicate that a moderate enrichment was not directly toxic to the macrophytes although it might change their viability in eutrophic lakes in terms of the carbon and nitrogen metabolism.     

INTERACTIONS BETWEEN NITRATE UPTAKE AND NITROGEN FIXATION IN CONTINUOUS CULTURES OF THE MARINE DIAZOTROPH TRICHODESMIUM (CYANOBACTERIA)1

Diazotrophic cyanobacteria can take up combined nitrogen (nitrate, ammonium, amino acids, dissolved organic nitrogen) from solution, but the interaction between N₂ fixation and uptake of combined nitrogen is not well understood. We studied the effects of combined nitrogen ) additions on N₂ fixation rates in the cyanobacterium Trichodesmium erythraeum (IMS‐101) maintained in continuous culture in an N‐free medium (YBCII) and a 12:12‐h light:dark cycle. We measured acetylene reduction rates, nutrient concentrations, and biomass throughout the 12 h of illumination after the addition of nitrate (0.5–20 μM) at the start of the light period. Compared with unamended controls, Trichodesmium showed strong inhibition of acetylene reduction (up to 70%) in the presence of , with apparent saturation of the inhibition effect at an initial concentration of approximately 10 μM. The inhibition of acetylene reduction persisted through much of the light period as concentration in the culture vessel decreased. Recovery of N₂ fixation was observed late in the light period in cultures amended with low concentrations of (<5 μM) when ambient concentrations had decreased to 0.3–0.4 μM in the culture vessel. Nitrate uptake accounted for as much as 86% of total N uptake and, at the higher treatment concentrations, more than made up for the observed decrease in N₂ fixation rates. We conclude that Trichodesmium can obtain significant quantities of N through uptake of nitrate and does so in preference to N₂ fixation when sufficient is available.     

Quantifying photosynthetic capacity and its relationship to leaf nitrogen content for global-scale terrestrial biosphere models

Photosynthetic capacity and its relationship to leaf nitrogen content are two of the most sensitive parameters of terrestrial biosphere models (TBM) whose representation in global‐scale simulations has been severely hampered by a lack of systematic analyses using a sufficiently broad database. Here, we use data of qualitative traits, climate and soil to subdivide the terrestrial vegetation into functional types (PFT), and then assimilate observations of carboxylation capacity, V_max (723 data points), and maximum photosynthesis rates, A_max (776 data points), into the C₃ photosynthesis model proposed by Farquhar et al. to constrain the relationship of (V_max normalised to 25 °C) to leaf nitrogen content per unit leaf area for each PFT. In a second step, the resulting functions are used to predict per PFT from easily measurable values of leaf nitrogen content in natural vegetation (1966 data points). Mean values of thus obtained are implemented into a TBM (BETHY within the coupled climate–vegetation model ECHAM5/JSBACH) and modelled gross primary production (GPP) is compared with independent observations on stand scale. Apart from providing parameter ranges per PFT constrained from much more comprehensive data, the results of this analysis enable several major improvements on previous parameterisations. (1) The range of mean between PFTs is dominated by differences of photosynthetic nitrogen use efficiency (NUE, defined as divided by leaf nitrogen content), while within each PFT, the scatter of values is dominated by the high variability of leaf nitrogen content. (2) We find a systematic depression of NUE on certain tropical soils that are known to be deficient in phosphorous. (3) of tropical trees derived by this study is substantially lower than earlier estimates currently used in TBMs, with an obvious effect on modelled GPP and surface temperature. (4) The root‐mean‐squared difference between modelled and observed GPP is substantially reduced.     

Modelling evolutionary processes in small populations: not as ideal as you think

Evolutionary processes are routinely modelled using ‘ideal’ Wright–Fisher populations of constant size N in which each individual has an equal expectation of reproductive success. In a hypothetical ideal population, variance in reproductive success (V_k) is binomial and effective population size (N_e) = N. However, in any actual implementation of the Wright–Fisher model (e.g., in a computer), V_k is a random variable and its realized value in any given replicate generation () only rarely equals the binomial variance. Realized effective size () thus also varies randomly in modelled ideal populations, and the consequences of this have not been adequately explored in the literature. Analytical and numerical results show that random variation in  and  can seriously distort analyses that evaluate precision or otherwise depend on the assumption that  is constant. We derive analytical expressions for Var(V_k) [4(2N – 1)(N – 1)/N³] and Var(N_e) [N(N – 1)/(2N – 1) ≈ N/2] in modelled ideal populations and show that, for a genetic metric G = f(N_e), Var(?) has two components: Var_Gene (due to variance across replicate samples of genes, given a specific ) and Var_Demo (due to variance in ). Var(?) is higher than it would be with constant N_e = N, as implicitly assumed by many standard models. We illustrate this with empirical examples based on F (standardized variance of allele frequency) and r² (a measure of linkage disequilibrium). Results demonstrate that in computer models that track multilocus genotypes, methods of replication and data analysis can strongly affect consequences of variation in . These effects are more important when sampling error is small (large numbers of individuals, loci and alleles) and with relatively small populations (frequently modelled by those interested in conservation).     

Apparent Survival Estimates for Five Species of Tropical Birds in an Endangered Forest Habitat in Western Ecuador1

Estimates of annual survival are essential for addressing topics in evolutionary and conservation ecology. However, most demographic studies of land birds are based on north temperate species, and few robust estimates of survival based on mark–recapture statistics are available for continental South American birds. We used time‐since‐marking models to estimate apparent survival of adult birds from 7 yr of mist netting data in the Colonche Hills. This site is one of few remaining large tracts of premontane forest in southwestern Ecuador, and an area of high priority for avian conservation. Species with sufficient data for analysis included three hummingbirds (Adelomyia melanogenys, Heliodoxa jacula, Phaethornis baroni), a cotinga (Schiffornis turdinus), and a wren (Henicorhina leucophrys). Our parameter estimates had reduced precision because the number of recaptures was small. Probability of recapture was low in three species , and moderate in two others . Adelomyia and Phaethornis had moderate apparent survival (; probability that a bird neither died nor emigrated from our survey area in a given year). Adults of Adelomyia moved seasonally, and it is possible that permanent emigration from our survey area contributed to low estimates of apparent survival. The other three species had relatively high estimates of adult apparent survival ranging from in Heliodoxa and Schiffornis to a high of in Henicorhina.     

Phosphorus limitation of the freshwater benthic diatom Didymosphenia geminata determined by the frequency of dividing cells

1. Unlike other nuisance algal species, the freshwater benthic diatom Didymosphenia geminata typically forms blooms in low‐nutrient rivers. The negative association between D. geminata blooming behaviour and nutrient levels appears at both catchment and smaller scales. We conducted a series of trials in streamside experimental channels colonised with D. geminata using water from the D. geminata‐affected, oligotrophic Waitaki River, South Island, New Zealand to determine how elevated nitrate and phosphate concentrations affected D. geminata cell division. Because D. geminata blooms are typically most pronounced in unshaded waters, we also investigated the growth response to shading. In all experiments, we used the frequency of dividing cells (FDC) as a metric of cell division. 2. Concentrations of nitrate and dissolved reactive phosphorus (DRP) in the Waitaki River were very low (4 mg m^?3‐N and <1 mg m^?3 DRP). In pilot trials, substrata colonised by D. geminata were subjected to enrichment by either switching the water source toN‐ and P‐rich spring water or by adding a stock solution. Both trials resulted in periods of rapid cell division lasting at least 8 days. 3. Experimental addition of alone triggered an initial cell division which was not sustained. However, addition of alone or together with resulted in prolonged elevation in cell division indicating that the cell division rate was P‐limited. 4. Reduced light levels resulted in decreased FDC in D. geminata in both ambient and N, N + P and P‐enriched river water. 5. Stimulation of D. geminata division rate by addition of above ambient levels confirms that, while blooming behaviour is often associated with oligotrophic rivers, the cells divide faster with greater levels of phosphorus enrichment.     

Relative influence of bioturbation and predation on organic matter processing in river sediments: a microcosm experiment

1. Our objective was to measure the effects of bioturbation and predation on the physical characteristics and biogeochemical processes in river sediments. 2. We investigated the impacts of tubificid worms tested separately and together with an omnivore (Gammarus pulex), which does feed on tubificids, on sediment distribution, water flux, sediment organic carbon, biofilm biomass and microbial activities, and the concentrations of dissolved oxygen, dissolved organic carbon, PO, NO, NO and NH in slow filtration sand–gravel columns. We hypothesised that gammarids, which exploit the top 2–3 cm of the sediment, would modify the impact of worms at the sediment surface. 3. In experiments both with and without gammarids, bioturbation by the tubificids modified both the distribution of surface particles in the sediment column and water flux. In addition, microbial aerobic (oxygen consumption) and anaerobic (denitrification and fermentative decomposition of organic matter) processes in the sediment were stimulated in the presence of tubificid worms. However, G. pulex did not affect either the density or bioturbation activity of the tubificid worms. 4. Bioturbation by the benthos can be a major process in river habitats, contributing to the retention of organic matter in sediment dynamics. The presence of at least one predator had no effect on bioturbation in sediments. In such systems, physical heterogeneity may be sufficient for tubificids to escape from generalist predators, though more specialised ones might have more effect.     

Microbial activities in the burrow environment of the potamal mayfly Ephoron virgo

1. The impact of burrowing larvae of Ephoron virgo (Ephemeroptera, Polymitarcidae) on sediment microbiology has not been previously investigated because of difficulties in sampling the sediment of large rivers under in situ conditions. Therefore, we conducted experiments in the on‐ship Ecological Rhine Station of the University of Cologne (Germany), in which ambient conditions of the River Rhine can be closely mimicked. 2. In two consecutive seasons, experimental flow channels were stocked with Ephoron larvae and continuously supplied with water taken directly from the River Rhine. Sediment from the immediate vicinity of Ephoron burrows (i.e. U‐shaped cavities reaching 10–80 mm deep into the sediment) and bulk sediment samples were analysed for (i) particulate organic matter content, (ii) microscale in situ distribution of O₂, NO, and NH, and (iii) potential activities of exoenzymes. 3. Sediment surrounding the Ephoron burrows had markedly higher organic matter contents and exoenzyme activities compared with the bulk sediment. Microsensor measurements demonstrated that local O₂ and NO penetration into the sediment were greatly enhanced by larval ventilation behaviour. Volumetric O₂ and NO turnover rates that were calculated from steady state concentration profiles measured directly in the burrow lining were considerably higher than at the sediment surface. 4. In the sediment of the fast flowing River Rhine Ephoron burrows are preferential sites of organic matter accumulation and dissolved oxidant penetration. Our data suggest that the burrows are surrounded by a highly active microbial community that responds to the inputs from the water column with elevated O₂ and NO turnover, and release of exoenzymes into the sediment pore water. Especially during periods of mass occurrence, the larvae of E. virgo may thus significantly contribute (i) to the ecological connection between the water column and the sediment and (ii) to biogeochemical processing of organic matter in the riverbed.     

Distribution of benthic diatoms in U.S. rivers in relation to conductivity and ionic composition

• 1 We quantified the relationships between diatom relative abundance and water conductivity and ionic composition, using a dataset of 3239 benthic diatom samples collected from 1109 river sites throughout the U.S.A. [U.S. Geological Survey National Water‐Quality Assessment (NAWQA) Program dataset]. This dataset provided a unique opportunity to explore the autecology of freshwater diatoms over a broad range of environmental conditions.
• 2 Conductivity ranged from 10 to 14 500 μS cm^?1, but most of the rivers had moderate conductivity (interquartile range 180–618 μS cm^?1). Calcium and bicarbonate were the dominant ions. Ionic composition, however, varied greatly because of the influence of natural and anthropogenic factors.
• 3 Canonical correspondence analysis (CCA) and Monte Carlo permutation tests showed that conductivity and abundances of major ions (HCO + CO, Cl^?, SO, Ca²⁺, Mg²⁺, Na⁺, K⁺) all explained a statistically significant amount of the variation in assemblage composition of benthic diatoms. Concentrations of HCO + CO and Ca²⁺ were the most significant sources of environmental variance.
• 4 The CCA showed that the gradient of ionic composition explaining most variation in diatom assemblage structure ranged from waters dominated by Ca²⁺ and HCO + CO to waters with higher proportions of Na⁺, K⁺, and Cl^?. The CCA also revealed that the distributions of some diatoms correlated strongly with proportions of individual cations and anions, and with the ratio of monovalent to divalent cations.
• 5 We present species indicator values (optima) for conductivity, major ions and proportions of those ions. We also identify diatom taxa characteristic of specific major‐ion chemistries. These species optima may be useful in future interpretations of diatom ecology and as indicator values in water‐quality assessment.

     

Estimating parameters in a land-surface model by applying nonlinear inversion to eddy covariance flux measurements from eight FLUXNET sites

Flux measurements from eight global FLUXNET sites were used to estimate parameters in a process‐based, land‐surface model (CSIRO Biosphere Model (CBM), using nonlinear parameter estimation techniques. The parameters examined were the maximum photosynthetic carboxylation rate () the potential photosynthetic electron transport rate (j_{max, 25}) of the leaf at the top of the canopy, and basal soil respiration (r_{s, 25}), all at a reference temperature of 25°C. Eddy covariance measurements used in the analysis were from four evergreen forests, three deciduous forests and an oak‐grass savanna. Optimal estimates of model parameters were obtained by minimizing the weighted differences between the observed and predicted flux densities of latent heat, sensible heat and net ecosystem CO₂ exchange for each year. Values of maximum carboxylation rates obtained from the flux measurements were in good agreement with independent estimates from leaf gas exchange measurements at all evergreen forest sites. A seasonally varying and j_{max, 25} in CBM yielded better predictions of net ecosystem CO₂ exchange than a constant and j_{max, 25} for all three deciduous forests and one savanna site. Differences in the seasonal variation of and j_{max, 25} among the three deciduous forests are related to leaf phenology. At the tree‐grass savanna site, seasonal variation of and j_{max, 25} was affected by interactions between soil water and temperature, resulting in and j_{max, 25} reaching maximal values before the onset of summer drought at canopy scale. Optimizing the photosynthetic parameters in the model allowed CBM to predict quite well the fluxes of water vapor and CO₂ but sensible heat fluxes were systematically underestimated by up to 75 W m⁻².     

Methane production by microbial mats under low sulphate concentrations

Cyanobacterial mats collected in hypersaline salterns were incubated in a greenhouse under low sulphate concentrations ([]) and examined for their primary productivity and emissions of methane and other major carbon species. Atmospheric greenhouse warming by gases such as carbon dioxide and methane must have been greater during the Archean than today in order to account for a record of moderate to warm palaeoclimates, despite a less luminous early sun. It has been suggested that decreased levels of oxygen and sulphate in Archean oceans could have significantly stimulated microbial methanogenesis relative to present marine rates, with a resultant increase in the relative importance of methane in maintaining the early greenhouse. We maintained modern microbial mats, models of ancient coastal marine communities, in artificial brine mixtures containing both modern [] (c. 70 mm ) and ‘Archean’[] (<0.2 mm ). At low [], primary production in the mats was essentially unaffected, while rates of sulphate reduction decreased by a factor of three, and methane fluxes increased by up to 10‐fold. However, remineralization by methanogenesis still amounted to less than 0.4% of the total carbon released by the mats. The relatively low efficiency of conversion of photosynthate to methane is suggested to reflect the particular geometry and chemical microenvironment of hypersaline cyanobacterial mats. Therefore, such mats were probably relatively weak net sources of methane throughout their 3.5 Ga history, even during periods of low environmental levels oxygen and sulphate.     

Organic and inorganic nutrient effects on growth rate–irradiance relationships in the Texas brown‐tide alga Aureoumbra lagunensis (Pelagophyceae)1

Pelagophyte species in the genera Aureococcus and Auroumbra form brown tides in coastal bays that cause food‐web disruption and extensive shading of benthic primary producers. Organic nutrients have been suggested as key factors in the origination and persistence of the East Coast (USA) brown‐tide alga Aureococcus anophagefferens Hargraves et Sieburth. To evaluate this finding for the Texas brown‐tide alga Aureoumbra lagunensis D. A. Stockw., DeYoe, Hargraves et P. W. Johnson, we grew strain TBA‐2 with dissolved inorganic nitrogen (DIN; or ) or dissolved organic nitrogen (DON; urea or glutamate) as the nitrogen (N) source under eight light intensities. Maximum growth rates decreased with N source from (1.0 div · d^?1) to (0.48 div · d^?1). Neither growth rate efficiency (α) nor I_k varied significantly between N treatments. Both inorganic phosphorus (P) and β‐glycerophosphate supported growth. Aureoumbra lagunensis can utilize at least some forms of organic N and P and can use them to persist or grow when inorganic forms become limiting. We found no evidence to support the hypothesis that organic utilization enhances or supplements growth at low light levels.     

Planktonic phosphorus pool sizes and cycling efficiency in coastal and interior British Columbia lakes

1. Limnologists have long acknowledged the importance of phosphorus (P) in determining the organism biomass and productivity of lake ecosystems. Despite a relatively large number of studies that have examined P cycling in lake ecosystems, there remain several substantial methodological issues that have impeded our understanding of P cycling in limnetic plankton communities. Two critical issues confronting ecologists are (1) a lack of precise measurements of the dissolved inorganic phosphorus (PO) and (2) accurate or complete measurements of dissolved P regeneration rates by plankton communities. 2. Here, we examine patterns of epilimnetic planktonic P pool sizes and turnover rates in eight lakes in British Columbia, Canada over a 2‐year period. We determine the concentrations and turnover times of P in various planktonic compartments (dissolved and various planktonic size fractions), using recently developed methods for estimating phosphate concentration and planktonic regeneration rates. 3. The pico‐ and nanoplankton size fraction (0.2–20 μm) played a central role in planktonic P cycling in lakes examined by this study. On average across lakes, pico‐ and nanoplankton contained >60% of the planktonic P, accounted for >90% PO uptake, and contributed 50% of the plankton community dissolved P regeneration rate. 4. PO concentrations determined by steady state bioassays (ssPO) were extremely low (87–611 pmol L⁻¹) and were 2–3 orders of magnitude less than simultaneously measured colorimetric soluble reactive phosphorus estimates. Lake ssPO concentrations increased linearly with total phosphorus (TP), and the slope of this relationship was approximately 1, indicating that PO remained a consistent proportion of the TP pool across a range of TP concentrations. 5. Turnover rates of the total planktonic P pool and the <20 μm pool became more rapid with increasing lake TP, indicating that, according to this metric, planktonic P cycling efficiency increased with TP concentrations. We also detected a significant relationship between particulate phosphorus (PP) <20 μm turnover time and seston N : P ratios, with PP <20 μm turnover times becoming slower with increasing seston N : P. These findings suggest that long‐standing conceptual models of nutrient cycling that predict slower cycling rates and decreasing cycling efficiency with increasing TP concentrations require further empirical examination. We postulate that patterns in lake P turnover and cycling efficiency are a result of complex interactions between plankton biomass and composition, and the ratios of multiple nutrients (C, N, P), rather than solely a function of the TP pool.     

Nitrogen cycling and dynamics in a macrophyte‐rich stream as determined by a release

1. A tracer release study was conducted in a macrophyte‐rich stream, the River Lilleaa in Denmark. The objectives of the study were to compare uptake rates per unit area of by primary producers and consumers in macrophyte and non‐macrophyte habitats, estimate whole‐stream uptake rates of and compare this to other stream types, and identify the pathways and estimate the rate at which enters the food web in macrophyte and non‐macrophyte habitats. 2. Macrophyte habitats had four times higher primary uptake rates and an equal uptake rate by primary consumers per unit habitat area as compared to non‐macrophyte habitats. These rates represent the lower limit of potential macrophyte effects because the rates will be highly dependent on macrophyte bed height and mean bed height in the River Lilleaa was low compared to typical bed heights in many lowland streams. Epiphytes accounted for 30% of primary uptake in macrophyte habitats, illustrating a strong indirect effect of macrophytes as habitat for epiphytes. N flux per unit habitat area from primary uptake compartments to primary consumers was four times lower in macrophyte habitats compared to non‐macrophyte habitats, reflecting much greater biomass accrual in macrophyte habitats. Thus, we did not find higher N flux from macrophyte habitats to primary consumers compared to non‐macrophyte habitats. 3. Whole‐stream uptake rate was 447 mgN m^?2 day^?1. On a habitat‐weighted basis, fine benthic organic matter (FBOM) accounted for 72% of the whole‐stream uptake rate, and macrophytes and epiphytes accounted for 19 and 8%, respectively. 4. We had expected a priori relatively high whole‐stream N uptake in our study stream compared to other stream types mainly due to generally high biomass and the macrophyte’s role as habitat for autotrophic and heterotrophic organisms, but our results did not confirm this. In comparison with other release study streams, we conclude that nutrient concentration is the overall controlling factor for N uptake rates across streams, mostly as a result of high biomass of primary uptake compartments in streams with high nutrient concentrations in general and not in macrophyte streams in particular. 5. Our results indicate that macrophytes play an important role in the longer‐term retention of N and thus a decrease in net downstream transport during the growing season compared to streams without macrophytes, through direct and indirect effects on the stream reach. Direct effects are high uptake efficiency, low turnover rate (partly due to no direct feeding on macrophytes) and high longevity. An indirect effect is increased sedimentation of FBOM in macrophytes compared to non‐macrophyte habitats and streams which possibly also increase denitrification. Increased retention with macrophyte presence would decrease downstream transport during the growing season and thus the N loading on downstream ecosystems.     

PHYCOBILIPROTEIN COMPONENTS AND CHARACTERISTICS OF THE PHYCOBILISOME FROM A THERMOPHILIC CYANOBACTERIUM MYXOSARCINA CONCINNA1

A phycocyanin (PC) and three allophycocyanin (AP) components (designated PC, AP1, AP2, and AP3) were prepared from Myxosarcina concinna Printz phycobilisomes by the native gradient PAGE performed in a neutral buffer system combined with the ion exchange column chromatography on DEAE‐DE52 cellulose. PC contained one β subunit () and two α ones ( and ), and it carried two rod linkers ( and ) and one rod‐core linker (). AP1 and AP3 were characterized as peripheral core APs, whereas AP2 was an inner‐core one. AP2 and AP3 were demonstrated to function as the terminal emitters. Each of the three APs contained two β subunits ( and ), two α subunits ( and ) and an inner‐core linker (). AP2 and AP3 had another subunit of the allophycocyanin B (AP‐B) type () belonging to the β subunit group, and AP1 and AP3 carried their individual specific core linkers ( and ), respectively. No AP component was shown to associate with the core‐membrane linker L_CM. The functions of the linker polypeptides in the phycobilisome (PBS) construction are discussed.     

Are geothermal streams important sites of nutrient uptake in an agricultural and urbanising landscape (Rotorua,New Zealand)?

1. Lakes in the Rotorua region of New Zealand are affected by eutrophication from urbanisation and agricultural land use. Some lake tributaries contain geothermally influenced waters, and it is currently unknown whether geothermal tributaries are active sites of nutrient cycling or represent point sources of nutrients to the lakes. 2. Using government data sets, we characterised the physicochemical conditions of geothermal and non‐geothermal streams. We then measured ecosystem metabolism and reach‐scale uptake of nitrate (), ammonium () and phosphate () in summer 2010 (n = 8 streams). Finally, we used government data to compare annual nutrient flux from geothermal and non‐geothermal surface water inputs to Lake Rotoiti. 3. As expected, geothermal streams had higher temperature, conductivity and nutrient concentrations and lower pH. However, primary production, community respiration and uptake rates in geothermal streams were not different from those in their non‐geothermal counterparts. Uptake rates of were higher in geothermal streams, and uptake was below detection in geothermal streams, probably due to the saturation by naturally high concentrations. 4. A comparison of Lake Rotoiti inputs suggested that geothermal streams are not significant sources of and , while geothermal inputs of represent an average of 46% of total flux from Lake Rotoiti tributaries. 5. Despite their high temperature and low pH, geothermal streams are active sites of photosynthesis, respiration and and cycling, indicating dynamic biofilm communities. 6. Management options for geothermal streams, if any, should focus on retention (e.g. uptake or coupled nitrification and denitrification) but could prove challenging given the persistent, naturally occurring high flux.     

Turgor and osmotic relations of the desert shrub Larrea tridentata

Abstract Leaf water relations characteristics of creosote bush, Larrea tridentata, were studied in view of previous reports that its leaves commonly experience zero or negative turgor under dry conditions. Leaf turgor loss point () was determined by a pressure-volume method for samples subjected to a hydration procedure and for untreated samples. Hydration caused to increase by as much as 3 M Pa. Hydration of samples also caused changes in other leaf water relations characteristics such as symplastic solute content, tissue elasticity and symplasmic water fraction, but total leaf solute content was unchanged. Comparison of our field plant water potential data with values of obtained by the two methods resulted in predictions of turgor loss during part or all of a diurnal cycle based on hydrated samples, and turgor maintenance (at least 0.3 MPa) based on untreated samples. Pooled data for obtained from both partially hydrated and untreated samples showed that L. tridentata maintains fairly constant levels of turgor over a wide range of leaf water potential. Dilution of cell contents by apoplastic water introduced significant errors in psychrometric determinations of osmotic potential in both frozen and thawed leaf tissue and expressed cell sap. Use of these values of osmotic potential resulted in predictions of zero turgor at all plant water potentials measured in the field.     

Biological traits and life table of the exotic Harmonia axyridis compared with Hippodamia variegata,and Adalia bipunctata (Col., Coccinellidae)

Abstract: As part of an environmental risk assessment study of exotic natural enemies used in inundative biological control, life‐history characteristics of Harmonia axyridis (Pallas), Hippodamia variegata (Goeze) and Adalia bipunctata (L.) (Col., Coccinellidae) were quantified under laboratory conditions at 25°C on Myzus persicae (Sulzer) as prey. Comparative studies showed significant differences among pre‐adult development times: H. axyridis developed slower ( = 19.8 days) than H. variegata ( = 18.1 days) and A. bipunctata ( = 18.4 days). Differences were also evident in the duration of egg, larval and pupal stages. No measurable differences among the three species were found for fecundity, oviposition rate and adult longevity. Harmonia axyridis exhibited the longest pre‐oviposition ( = 7.4 days) and interoviposition ( = 3.6 days) periods and the shortest oviposition period ( = 13.7 days). The Bieri model was used to describe age‐specific fecundity for the three species of coccinellids. The intrinsic rate of increase (r_m), net reproductive rate (R₀) and mean generation time (T) were higher for H. variegata (r_m = 0.114, R₀ = 52.75, T = 41.88 days) than for H. axyridis (r_m = 0.089, R₀ = 26.27, T = 38.81 days) or A. bipunctata (r_m = 0.081, R₀ = 18.49, T = 40.06 days). Our findings show that the biological traits of H. axyridis do not seem to be factors that may contribute to the invasiveness of this coccinellid.     

Effects of different Nitrogen forms and osmotic stress on water use efficiency of rice (Oryza sativa)

A hydroponic experiment with simulated water stress induced by polyethylene glycol (PEG) was conducted in greenhouse to study the effects of different nitrogen (N) forms (; and the mixture of and ) on water stress tolerance and water use efficiency (WUE and WUE_T) of different rice cultivars. Two rice cultivars (cv. ‘Shanyou 63’ hybrid indica and ‘Yangdao 6’ indica, China) were grown under non‐water‐ or water‐stressed condition [10% (w/v) PEG, molecular weight 6000] with different N forms for 3 weeks. Under non‐water stress, the biomass of Shanyou 63 was 50.0% and 64.3% and of Yangdao 6 was 6.9% and 87.8% higher under the supply of mixture of and than either under the sole supply of or , respectively; under water stress, the biomass of both rice cultivars decreased in all three nitrogen forms compared with non‐water stress; however, the inhibitory effect of water stress on biomass varied between and nutrition; the reduction of dry matter was significantly higher in than in nutrition. Compared with non‐water stress, under water stressed condition, WUE of both two rice cultivars significantly decreased in supply; WUE did not vary in and the mixture supply. It is concluded that (a) the resistance of water stress of rice seedlings is related to nitrogen form; (b) under water stress, could maintain a higher WUE compared with ; (c) hybrid indica rice seedlings have a higher water stress tolerance than indica rice seedlings.