bluuebunny/crossref_metadata_2025_split huggingface dataset:
Land‐use influences macroinvertebrate community response following a pulse disturbance
Authors: Collier Kevin J. , Quinn John M. 
Published: July 2003

Summary1. We tested the hypothesis that interactions between disturbance types can influence invertebrate community response and recovery in two streams draining pasture (press‐pulse disturbance) and native forest (pulse disturbance) catchments before and after a one‐in‐28‐year flood. We also sampled drift and adult insects to gain insights into the relative importance of these two postdisturbance recolonisation pathways.2. Taxa numbers and total density declined markedly at the forested site after the flood, but there was a delayed response at the pasture site, reflecting greater initial resistance to this pulse disturbance among taxa adapted to the underlying press disturbance.3. Community composition was less stable at the pasture site where per cent abundance of taxa was highly variable prior to the flood and over the 2‐year postflood sampling period. After the flood, the pasture stream fauna was more heavily dominated by vagile taxa, including several chironomid species and hydroptilid caddisflies.4. Taxa numbers and densities recovered to preflood levels within 5–7 months at both sites, but a range of taxa‐specific responses was observed that took up to 18 months to recover to preflood densities. Community stability at the pasture site had not returned to preflood composition by 2 years postflood.5. Changes in drift densities of several common stream invertebrates at the pasture site reflected postflood changes in benthic densities and seasonally low drift in winter. Terrestrial invertebrates dominated drift at the pasture site for 3 months postflood whereas Ephemeroptera were most common at the native forest site.6. Flight patterns of selected adult aquatic insects showed a strongly seasonal pattern. Abundance of adults at the pasture site in the second year following the flood increased in line with the recovery of the non‐Dipteran benthic fauna. Significant upstream flight occurred for several caddisfly species at the native forest site, and weakly directional or downstream flight was evident for most common Plecoptera and Ephemeroptera.7. This study indicates that the magnitude and duration of responses to major pulse disturbances can depend on the presence or absence of an underlying press disturbance. This finding has implications for monitoring, and suggests that a knowledge of disturbance history beyond 2 years may be required to interpret mechanisms contributing to observed land‐use impacts.
Factors affecting ammonium uptake in streams – an inter‐biome perspective
Authors: Webster Jackson R. , Mulholland Patrick J. , Tank Jennifer L. , Valett H. Maurice , Dodds Walter K. , Peterson Bruce J. , Bowden William B. , Dahm Clifford N. , Findlay Stuart , Gregory Stanley V. , Grimm Nancy B. , Hamilton Stephen K. , Johnson Sherri L. , Martí Eugènia , Mcdowell William H. , Meyer Judy L. , Morrall Donna D. , Thomas Steven A. , Wollheim Wilfred M. 
Published: July 2003

Summary1. The Lotic Intersite Nitrogen eXperiment (LINX) was a coordinated study of the relationships between North American biomes and factors governing ammonium uptake in streams. Our objective was to relate inter‐biome variability of ammonium uptake to physical, chemical and biological processes.2. Data were collected from 11 streams ranging from arctic to tropical and from desert to rainforest. Measurements at each site included physical, hydraulic and chemical characteristics, biological parameters, whole‐stream metabolism and ammonium uptake. Ammonium uptake was measured by injection of 15N‐ammonium and downstream measurements of 15N‐ammonium concentration.3. We found no general, statistically significant relationships that explained the variability in ammonium uptake among sites. However, this approach does not account for the multiple mechanisms of ammonium uptake in streams. When we estimated biological demand for inorganic nitrogen based on our measurements of in‐stream metabolism, we found good correspondence between calculated nitrogen demand and measured assimilative nitrogen uptake.4. Nitrogen uptake varied little among sites, reflecting metabolic compensation in streams in a variety of distinctly different biomes (autotrophic production is high where allochthonous inputs are relatively low and vice versa).5. Both autotrophic and heterotrophic metabolism require nitrogen and these biotic processes dominate inorganic nitrogen retention in streams. Factors that affect the relative balance of autotrophic and heterotrophic metabolism indirectly control inorganic nitrogen uptake.
Combined stable isotope and gut contents analysis of food webs in plant‐dominated, shallow lakes
Authors: Jones J. Iwan , Waldron Susan 
Published: July 2003

Summary 1. To determine feeding links between primary producers, invertebrates and fish, stable isotope analyses and gut content analyses of fish were conducted on the components of four shallow, eutrophic to hypertrophic, plant‐dominated lakes.2. Although separation of basal resources was possible, the diets of both fish and invertebrates were broad, comprising food from different compartments (planktonic, epiphytic/benthic), as well as from different trophic levels.3. Mixing models were used to determine the extent to which periphyton production supported higher trophic levels. Only one species of invertebrate relied upon periphyton production exclusively.4. Fish density affected the diets of invertebrates. The response was different for planktonic and epiphytic/benthic invertebrates. The proportion of periphyton production in the diets of zooplankton appeared to increase with fish density, whilst it decreased for other invertebrates.5. As all zooplankton samples were collected in the open water at dusk, these results are further evidence for the diurnal horizontal migration of zooplankton. Although not conclusive, they are consistent with a behavioural response by invertebrates and zooplankton in the presence of fish.
An experimental study of the effects of nutrient supply and Chaoborus predation on zooplankton communities of a shallow tropical reservoir (Lake Brobo, Côte d'Ivoire)
Authors: Pagano Marc , Koffi Maryse Aka , Cecchi Philippe , Corbin Daniel , Champalbert Gisèle , Saint‐Jean Lucien 
Published: July 2003

Summary1. Based on two mesocosm experiments and 10 in vitro predation experiments, this work aimed to evaluate the impact of nutrient supply and Chaoborus predation on the structure of the zooplankton community in a small reservoir in Côte d'Ivoire.2. During the first mesocosm experiment (M1), P enrichment had no effect on phytoplankton biomass (chlorophyll a) but significantly increased the biomass of some herbivorous zooplankton species (Filinia sp, Ceriodaphnia affinis). During the second experiment (M2), N and P enrichment greatly increased phytoplankton biomass, rotifers and cladocerans (C. affinis, C. cornuta, Moina micrura and Diaphanosoma excisum). In both experiments, nutrient addition had a negative impact on cyclopoid copepods.3. Larger zooplankton, such as cladocerans or copepodites and adults of Thermocyclops sp., were significantly reduced in enclosures with Chaoborus in both mesocosm experiments, whereas there was no significant reduction of rotifers and copepod nauplii. This selective predation by Chaoborus shaped the zooplankton community and modified its size structure. In addition, a significant Chaoborus effect on chlorophyll a was shown in both experiments.4. The preference of Chaoborus for larger prey was confirmed in the predation experiments. Cladocerans D. excisum and M. micrura were the most selected prey. Rotifer abundance was not significantly reduced in any of the 10 experiments performed.5. In conclusion, both bottom‐up and top‐down factors may exert a structuring control on the zooplankton community. Nutrients favoured more strictly herbivorous taxa and disadvantaged the cyclopoid copepods. Chaoborus predation had a strong direct negative impact on larger crustaceans, favoured small herbivores (rotifer, nauplii) and seemed to cascade down to phytoplankton.
Hyporheic community composition in a gravel‐bed stream: influence of vertical hydrological exchange, sediment structure and physicochemistry
Authors: Olsen Dean A. , Townsend Colin R. 
Published: July 2003

Summary 1. We studied the relative contributions of the magnitude and direction of vertical hydrological exchange, subsurface sediment composition and interstitial physicochemistry in determining the distribution of hyporheic invertebrates in the Kye Burn, a fourth order gravel‐bed stream in New Zealand.2. In winter 2000 and summer 2001, we measured vertical hydrological gradient (VHG), dissolved oxygen, water temperature and water chemistry using mini‐piezometers, each installed in a different upwelling or downwelling zone. Next to every piezometer, a freeze core sample was taken to quantify the sediment, particulate organic matter and invertebrates.3. Dissolved oxygen concentration at 25 cm was high on both occasions (>9 mg L−1) but was higher in winter than summer. Interstitial water temperature was higher in down than upwellings and was substantially higher in summer than winter. Other features of the subsurface sediments and interstitial nitrate–nitrite concentrations were similar on both occasions and in up and downwellings. Interstitial ammonium and soluble reactive phosphorous concentrations were higher in winter than summer and ammonium was higher in up than downwelling areas.4. The proportion of fine sediment (63 μm–1 mm), sediment heterogeneity and VHG accounted for the greatest proportion of variance in invertebrate distributions in both summer and winter.5. The hyporheos was numerically dominated by early instar leptophlebiid mayfly nymphs and asellotan isopods. Water mites were a taxonomically diverse group with 13 genera. Taxonomic diversity (Shannon–Weaver), but not taxon richness, was higher in upwelling areas, reflecting lower numerical dominance by a few taxa in these locations.6. Sediment composition (particularly the amount of fine sediments) and vertical hydrological exchange determined the composition and distribution of the hyporheos. Patchiness in these factors is important in planning sampling regimes or field manipulations in the hyporheic zone.
Macrophytic, epipelic and epilithic primary production in a semiarid Mediterranean stream
Authors: Velasco J. , Millan A. , Vidal‐Abarca M. R. , Suarez M. L. , Guerrero C. , Ortega M. 
Published: July 2003

Summary 1. Primary production by Chara vulgaris and by epipelic and epilithic algal assemblages was measured in a semiarid, Mediterranean stream (Chicamo stream, Murcia, Spain) during one annual cycle.2. The rates of gross primary production (GPP) and community respiration (CR) were determined for each algal assemblage using oxygen change in chambers. The net daily metabolism (NDM) and the GPPd−1 : CR24 ratio were estimated by patch‐weighting the assemblage‐level metabolism values.3. Gross primary production and CR showed significant differences between assemblages and dates. The highest rates were measured in summer and spring, while December was the only month when there were no significant differences in either parameters between assemblages. GPP was strongly correlated with respiration, but not with algal biomass.4. Chara vulgaris showed the highest mean annual metabolic rates (GPP = 2.80 ± 0.83 gC m−2 h−1, CR = 0.76 ± 0.29 gC m−2 h−1), followed by the epilithic assemblage (GPP = 1.97 ± 0.73 gC m−2 h−1, CR = 0.41 ± 0.12 gC m−2 h−1) and epipelic algae (GPP = 1.36 ± 0.22 gC m−2 h−1, CR = 0.39 ± 0.06 gC m−2 h−1).5. The epipelic assemblage dominated in terms of biomass (82%) and areal cover (88%), compared with the other primary producers. Epipelic algae contributed 84% of gross primary production and 86% of community respiration in the stream.6. Mean monthly air temperature was the best single predictor of macrophyte respiration and of epipelic GPP and CR. However, ammonium concentration was the best single predictor of C. vulgaris GPP, and suspended solid concentration of epilithon GPP and CR.7. Around 70% of the variation in both mean GPP and mean CR was explained by the mean monthly air temperature alone. A multiple regression model that included conductivity, PAR and nitrates in addition to mean monthly air temperature, explained 99.99% of the variation in mean CR.8. Throughout the year, NDM was positive (mean value 7.03 gC m−2 day−1), while the GPP : CR24 ratio was higher than 1, confirming the net autotrophy of the system.
Predation and refugia: implications for Chaoborus abundance and species composition
Authors: Wissel B. , Yan N. D. , Ramcharan C. W. 
Published: July 2003

Summary1. Previous studies have suggested that the occurrence of larval Chaoborus in lakes may be affected by fish predation, pH, elevation, temperature, nutrient level, water transparency and interspecific competition, but so far, a detailed statistical evaluation of these findings has not been performed.2. The aim of this study was to apply regression and ordination techniques to a large data set of 56 lakes in order to test which variables related to lake morphology, water chemistry, and fish predation determine (1) the abundance of individual Chaoborus species and (2) their species composition.3. Individual Chaoborus species were influenced by very different sets of environmental factors. Nutrient levels positively affected the largest species, Chaoborus americanus, which was restricted to fishless lakes. Abundance of the smallest and most transparent species, C. punctipennis, seemed to be controlled more by the larger Chaoborus species than by fish. Larger chaoborids required low water clarity in order to co‐exist with fish, probably to increase refuge availability. Generally, small lakes (for C. flavicans/C. trivittatus) and shallow lakes (for C. punctipennis) supported higher abundances of Chaoborus.
Nutritional quality of leaf detritus altered by elevated atmospheric CO2: effects on development of mosquito larvae
Authors: Tuchman Nancy C. , Wahtera Kirk A. , Wetzel Robert G. , Russo Nicole M. , Kilbane Grace M. , Sasso Lisa M. , Teeri James A. 
Published: July 2003

Summary1. Populus tremuloides leaf litter was produced under elevated (ELEV = 720 ppm) and ambient (AMB = 360 ppm) atmospheric CO2 conditions. Leaf chemical quality was significantly altered by CO2 enrichment. ELEV leaves had significantly higher concentrations of phenolic compounds and lignins, and higher C : N ratios than AMB.2. Leaf litter was incubated in a headwater stream for 14 days to become colonised by microorganisms; aquatic bacterial productivity was significantly lower on ELEV than on AMB leaf litter. Colonised leaves were fed to four species of detritivorous mosquito larvae to assess their survivorship and development rates.3. Larval mortality was 2.2 times higher for Aedes albopictus fed ELEV litter when compared with AMB. Although mortality of A. triseriatus, A. aegypti and Armigeres subalbatus was not affected by treatment, larval development rate was delayed by 78, 25 and 27%, respectively, when fed ELEV litter.4. Increased mosquito mortality and/or delayed larval development rates are more likely to have negative implications for food web structure and productivity in ecosystems where immature stages of mosquitoes are an important food source of predators.
Comment on: ‘The 3D shear experiment over the Natih field in Oman: the effects of fracture‐filling fluids on shear propagation’ by C.M. van der Kolk, W.S. Guest and J.H.H.M. Potters
Authors: Hudson J.A. , Crampin Stuart 
Published: July 2003

ABSTRACTWe find errors in theory and application in the paper by van der Kolket al. (Geophysical Prospecting49, 179–197 (2001)) that invalidate their arguments for the cause of the reduction in the velocity of the vertically propagating slower split shear‐wave in the gas cap over the Natih field in Oman. We suggest that existing theories are adequate and can explain the anomaly.
Towards non‐linear inversion for characterization of time‐lapse phenomena through numerical modelling
Authors: Abubakar A. , Van Den Berg P.M. , Fokkema J.T. 
Published: July 2003

ABSTRACTWe compare two geophysical survey measurements of the same type made at different times in order to characterize the change in the geological medium during the elapsed time. The aim of this study is to develop a strategy using a full non‐linear inversion algorithm as the interpretation tool. In this way, not only the location and the form of the changes are recovered, but also the changes in the material parameters of the geological medium can be estimated. In order to solve this fully non‐linear problem, the so‐called ‘multiplicative regularized contrast source inversion’ (MR‐CSI) method is employed. The unique property of this iterative method is that it does not solve the forward problem at each iterative step. This makes it possible to use the non‐linear inversion algorithm for large‐scale computation problems. The numerical results show that by taking into account the non‐linear nature of the problem, interpretation of the time‐lapse data can be significantly improved, compared with that obtained using linear inversion.
Seismic velocity analysis in the scattering‐angle/azimuth domain
Authors: Brandsberg‐Dahl S. , Ursin B. , De Hoop M.V. 
Published: July 2003

ABSTRACTMigration velocity analysis is carried out by analysing the residual moveout and amplitude variations in common image point gathers (CIGs) parametrized by scattering angle and azimuth. The misfit criterion in the analysis is of the differential‐semblance type. By using angles to parametrize the imaging we are able to handle and exploit data with multiple arrivals, although artefacts may occur in the CIGs and need to be suppressed.The CIGs are generated by angle migration, an approach based on the generalized Radon transform (GRT) inversion, and they provide multiple images of reflectors in the subsurface for a range of scattering angles and azimuths. Within the differential semblance applied to these CIGs, we compensate for amplitude versus angle (AVA) effects. Thus, using a correct background velocity model, the CIGs should have no residual moveout nor amplitude variation with angles, and the differential semblance should vanish. If the velocity model is incorrect, however, the events in the CIGs will appear at different depths for different angles and the amplitude along the events will be non‐uniform. A standard, gradient‐based optimization scheme is employed to develop a velocity updating procedure. The model update is formed by backprojecting the differential semblance misfits through ray perturbation kernels, within a GRT inverse. The GRT inverse acts on the data, subject to a shift in accordance with ray perturbation theory.The performance of our algorithm is demonstrated with two synthetic data examples using isotropic elastic models. The first one allows velocity variation with depth only. In the second one, we reconstruct a low‐velocity lens in the model that gives rise to multipathing. The velocity model parametrization is based upon the eigentensor decomposition of the stiffness tensor and makes use of B‐splines.
CDP mapping in tilted transversely isotropic (TTI) media. Part I: Method and effectiveness
Authors: Qin Yilong , Zhang Zhongjie , Li Shoulin 
Published: July 2003

ABSTRACTTransverse isotropy with a tilted axis of symmetry (TTI) causes image distortion if isotropic models are assumed during data processing. A simple anisotropic migration approach needs long computational times and is sensitive to the signal‐to‐noise ratio. This paper presents an efficient, general approach to common‐depth‐point (CDP) mapping to image the subsurface in TTI media from qP‐wave seismic data by adding anisotropic and dip parameters to the velocity model. The method consists of three steps: (i) calculating traveltimes and positions of the CDP points; (ii) determining CDP trajectories; (iii) CDP imaging. A crucial step is the rapid computation of traveltimes and raypaths in the TTI media, which is achieved by the Fermat method, specially adapted for anisotropic layered media. The algorithm can image the subsurface of a given model quickly and accurately, and is suitable for application to a bending reflector. The effectiveness of the method is demonstrated by comparing the raypaths, the traveltimes and the results of CDP mapping, when assuming isotropic media, transversely isotropic media with a vertical axis of symmetry (TIV), and TTI media.
CDP mapping in tilted transversely isotropic (TTI) media. Part II: Velocity analysis by combining CDP mapping with a genetic algorithm
Authors: Qin Yilong , Zhang Zhongjie , Xu Shenhui 
Published: July 2003

ABSTRACTThis paper presents a method for velocity analysis in tilted transversely isotropic (TTI) media by combining CDP mapping with a genetic algorithm. CDP mapping is a velocity analysis method for determining anisotropic velocity but has difficulties due to the following factors: (i) it involves a non‐linear and multimodal objective function; (ii) it is prohibitively expensive in the evaluation of candidate solutions, which often involves the calculation of images in the depth domain; (iii) there is often a very large parameter space. Recognizing the global and multimodal nature of the problem, a genetic algorithm is employed to search for the optimal velocity model. The efficiency of the method contributes to two critical processes: rapid model evaluation, achieved by generating CDP mapping only in the neighbourhood of specific reflectors, and fast computation, based on Fermat's principle, of the CDP points and traveltimes in TTI media. The method produces subsurface structure images in the depth domain, and can also solve for Thomsen's anisotropic parameters (ɛ and δ), the vertical velocity and the dip of the symmetry axis in the model space, simultaneously.
Decorrugation and removal of directional trends of magnetic fields by the wavelet transform: application to archaeological areas
Authors: Fedi Maurizio , Florio Giovanni 
Published: July 2003

ABSTRACTThe problem of removing directional trends frequently occurs in the processing of magnetic data and also in the subsequent steps of data interpretation. The so‐called corrugations are typical directional trends occurring in levelled data, which may be removed in several ways. Classical techniques are based on high‐pass filtering of the data and successively filtering these transformed data with directional cosine filters. Other linear features are due to real sources, such as pipelines in shallow surveys or dike swarms in regional surveys. They should, nevertheless, be considered as noise, due to the fact that their effect is strong and tends to hide the field features related to structures of more interest. We deal with both kinds of problem, presenting the results of a study in an archaeological area of southern Italy. Decorrugation of magnetic field anomalies is performed using a method based on the excellent space–frequency localization properties of wavelet bases, allowing a very sharp filtering of the field along a selected direction. We compare this technique with the classical one in a synthetic case and find that the wavelet decorrugation is simpler and produces low distortion maps. Besides the field decorrugation, the wavelet approach was also shown to be useful in the subsequent enhancement of the measured field. In fact, we show that the wavelet analysis offers a unique framework where various filtering problems (directional, isotropic, global or local as well) may be easily solved. As regards the archaeological case, strong noisy effects from elongated sources (pipelines) were successfully removed in a sharp and local way.