Interactive effects of nutrient additions and predation on infaunal communities

Nutrient additions represent an important anthropogenic stress on coastal ecosystems. At moderate levels, increased nutrients may lead to increased primary production and, possibly, to increased biomass of consumers although complex trophic interactions may modify or mask these effects. We examined the influence of nutrient additions and interactive effects of trophic interactions (predation) on benthic infaunal composition and abundances through small-scale field experiments in 2 estuaries that differed in ambient nutrient conditions. A blocked experimental design was used that allowed an assessment of direct nutrient effects in the presence and absence of predation by epibenthic predators as well as an assessment of the independent effects of predation. Benthic microalgal, production increased with experimental nutrient additions and was greater when infaunal abundances were lower, but there were no significant interactions between these factors. Increased abundances of one infaunal taxa,Laeonereis culveri, as well as the grazer feeding guild were observed with nutrient additions and a number of taxa exhibited higher abundances with predator exclusion. In contrast to results from freshwater systems there were no significant interactive effects between nutrient additions and predator exclusion as was predicted. The infaunal responses observed here emphasize the importance of both bottom-up (nutrient addition and primary producer driven) and top-down (predation) controls in structuring benthic communities. These processes may work at different spatial and temporal scales, and affect different taxa, making observation of potential interactive effects difficult.     

Age and Metallicity Gradients in the Galactic Bulge

We test for age and metallicity gradients in the Galactic Bulgebetween the two low extinction windows Baade's window (l = 1^° .1b=−4^° .8) and Sagittarius-I (l=1^° .3 b=−2^° .7). We derive ametallicity difference of ≤ 0.2 dex between BW andSGR-I window. This corresponds to a metallicity gradient of 1.3 dexkpc^-1. This steep gradient is reconcilable with the existence of ashort scale length inner component to the Bulge, most likely thatprominent in the NIR, which perhaps forms a separate entitysuperimposed on the larger, optical Bulge. Through number counts incolour-magnitude diagram around the turnoff for several fields andclusters, extending over a range in galactic longitude we find no evidencefor a significant young stellar population in the GalacticBulge. Previous suggestions of the existence of a significant youngstellar population have most likely incorrectly identified theforeground disk stars. This revised version was published online in July 2006 with corrections to the Cover Date.     

Partitioning of organic matter in continental margin sediments among density fractions

Hydrodynamic processes sort and redistribute organic matter (OM) and minerals on continental margins. Density fractionations were conducted on sediments from diverse margins (Mexico margin, Gulf of Mexico, Mississippi River delta, Eel River margin) to investigate the nature, provenance and age of OM among density fractions. Mass, elemental (C and N), lignin, and surface area distributions, as well as stable carbon and radiocarbon isotopic compositions were measured. The lowest density fractions (< 1.6 g cm^− 3) contained the highest organic carbon (OC) (up to 45%) and lignin concentrations (up to 8 mg g^− 1) due to abundant woody debris, whereas high density fractions (> 2.5 g cm^− 3) were OC-poor (%OC < 0.5) mineral material. Most sediment mass was found in the mesodensity fractions (1.6 to 2.5 g cm^− 3) that contained the highest proportion of OC (up to ~ 75%) for each sediment. Stable carbon isotope compositions (δ¹³C − 25.5‰ to − 22.9‰) show terrigenous OC as a significant component of density isolates from the river-dominated sediments (Gulf of Mexico, Mississippi River, and Eel margin), whereas the Mexico margin, least influenced by riverine input, was dominated by autochthonous marine OC (δ¹³C ~ − 21.5‰). Radiocarbon compositions of density fractions indicate significant pre-aged OC (Δ¹⁴C as low as − 900‰) in river-influenced sediments but not on the Mexico margin (Δ¹⁴C > − 200‰). Ratios of vanillic acid to vanillin (Ad/Al)v among lignin oxidation products increase with increasing particle density suggesting variable lignin sources or selective degradation of lignin among the different density fractions.     

Accurate Trace Element Reporting in Corundum: Development of Secondary Ion Mass Spectrometry Relative Sensitivity Factors

The attractive physical and chemical properties of corundum lend to this material’s importance in both its natural and synthetic forms. However, much of the quantitative work performed on this material is plagued by unknown inaccuracy as non‐matrix‐matched reference materials are used. To conduct accurate quantitative analysis using SIMS, matrix‐specific relative sensitivity factors (RSFs) were determined for eighteen trace elements in corundum using dose‐verified ion implants. The RSF values ranged from 2.56 × 10²² to 3.29 × 10²⁴ cm^‐1 with total combined uncertainty values ranging from 7 to 10%. The RSF values, which are related to ionisation potentials, showed trends consistent with expectations for an insulating oxide. The developed values were applied to calibrate reference materials for LA‐ICP‐MS and to study other natural and synthetic corundum samples. A measurement reference material calibrated for Mg, Si, Ti, V, Fe and Ga produced consistent results over ten sessions in 4 years with relative standard deviations per trace element of 5% or less, confirming the repeatability of our process. A key finding was that calibrating LA‐ICP‐MS with NIST SRM 610 and 612 glasses to analyse corundum resulted in under‐reporting trace elements Be, Ti, V, Fe, Co, Ni and Ga compared with using matrix‐matched reference materials.     

Scale-invariance of soil moisture variability and its implications for the frequency-size distribution of landslides

Power spectral analyses of soil moisture variability are carried out from scales of 100 m to 10 km on the microwave remotely-sensed data from the Washita experimental watershed during 1992. The power spectrum S(k) has an approximate power-law dependence on wave number k with the exponent −1.8. This behavior is consistent with the behavior of a stochastic differential equation for soil moisture at a point, and it has important consequences for the frequency-size distribution of landslides. We present the cumulative frequency-size distributions of landslides induced by precipitation in Japan and Bolivia as well as landslides triggered by the 1994 Northridge, California earthquake. Large landslides in these regions, despite being triggered by different mechanisms, have a cumulative frequency-size distribution with a power-law dependence on area with an exponent ranging from −1.5 to −2. We use a soil moisture field with the above statistics in conjunction with a slope stability analysis to model the frequency-size distribution of landslides. In our model, landslides occur when a threshold shear stress dependent on cohesion, pore pressure, internal friction and slope angle is exceeded. This implies a threshold dependence on soil moisture and slope angle since cohesion, pore pressure and internal friction are primarily dependent on soil moisture. The cumulative frequency-size distribution of domains of shear stress greater than a threshold value with soil moisture modeled as above and topography modeled as a Brownian walk is a power-law function of area with an exponent of −1.8 for large landslide areas. This distribution is similar to that observed for landslides. The effect of strong ground motion from earthquakes lowers the shear stress necessary for failure, but does not change the frequency-size distribution of failed areas. This is consistent with observations. This work suggests that remote sensing of soil moisture can be of great importance in monitoring landslide hazards and proposes a specific quantitative model for landslide hazard assessment.     

Recharge seasonality based on stable isotopes: Nongrowing season bias altered by irrigation in Nebraska

The sustainability of groundwater resources for agricultural and domestic use is dependent on both the groundwater recharge rate and the groundwater quality. The main purpose of this study was to improve the understanding of the timing, or seasonality, of groundwater recharge through the use of stable isotopes. Based on 768 groundwater samples collected from aquifers underlying natural resources districts in Nebraska, the isotopic composition of groundwater (δ²H and δ¹⁸O) was compared with that of precipitation by (a) mapping the isotopic composition of groundwater samples and (b) mapping a seasonality index for groundwater. Results suggest that for the majority of the state, groundwater recharge has a nongrowing season signature (October–April). However, the isotopic composition of groundwater suggests that in some intensively irrigated areas, human intervention in the water cycle has shifted the recharge signature towards the growing season. In other areas, a different human intervention (diversion of Platte River water for irrigation) has likely produced an apparent but possibly misleading nongrowing season recharge signal because the Platte River water differs isotopically from local precipitation. These results highlight the need for local information even when interpreting isotopic data over larger regions. Understanding the seasonality of recharge can provide insight into the optimal times to apply fertilizer, specifically in highly conductive soils with high leaching potential. In areas with high groundwater nitrate concentrations, this information is valuable for protecting the groundwater from further degradation. Although previous studies have framed nongrowing season recharge within the context of future climate change, this study also illustrates the importance of understanding how historical human intervention in the water cycle has affected groundwater recharge seasonality and subsequent implications for groundwater recharge and quality.     

IACT observations of gamma-ray bursts: prospects for the Cherenkov Telescope Array

Gamma rays at rest frame energies as high as 90 GeV have been reported from gamma-ray bursts (GRBs) by the Fermi Large Area Telescope (LAT). There is considerable hope that a confirmed GRB detection will be possible with the upcoming Cherenkov Telescope Array (CTA), which will have a larger effective area and better low-energy sensitivity than current-generation imaging atmospheric Cherenkov telescopes (IACTs). To estimate the likelihood of such a detection, we have developed a phenomenological model for GRB emission between 1 GeV and 1 TeV that is motivated by the high-energy GRB detections of Fermi-LAT, and allows us to extrapolate the statistics of GRBs seen by lower energy instruments such as the Swift-BAT and BATSE on the Compton Gamma-ray Observatory. We show a number of statistics for detected GRBs, and describe how the detectability of GRBs with CTA could vary based on a number of parameters, such as the typical observation delay between the burst onset and the start of ground observations. We also consider the possibility of using GBM on Fermi as a finder of GRBs for rapid ground follow-up. While the uncertainty of GBM localization is problematic, the small field-of-view for IACTs can potentially be overcome by scanning over the GBM error region. Overall, our results indicate that CTA should be able to detect one GRB every 20–30 months with our baseline instrument model, assuming consistently rapid pursuit of GRB alerts, and provided that spectral breaks below ~100 GeV are not a common feature of the bright GRB population. With a more optimistic instrument model, the detection rate can be as high as 1 to 2 GRBs per year.     

A three-step floating catchment area method for analyzing spatial access to health services

Gravity-based spatial access models have been widely used to estimate spatial access to healthcare services in an attempt to capture the interaction of various factors. However, these models are inadequate in informing health resource allocation work due to their inappropriate assumption of healthcare demand. For the purpose of effective healthcare resource planning, this article proposes a three-step floating catchment area (3SFCA) method to minimize the healthcare-demand overestimation problem. Specifically, a spatial impedance-based competition scheme is incorporated into the enhanced two-step floating catchment area (E2SFCA) method to account for a reasonable model of healthcare supply and demand. A case study of spatial access to primary care physicians along the Austin–San Antonio corridor area in central Texas showed that the proposed method effectively minimizes the overestimation of healthcare demand and reflects a more balanced geographic pattern of spatial access than E2SFCA. In addition, by using an adjusted spatial access index, the 3SFCA method indicates strong potential for identifying health professional shortage areas. The study concludes that 3SFCA is a promising method to provide health professionals and decision makers with useful healthcare accessibility information.     

The generation of a diverse suite of Late Pleistocene and Holocene basalt through dacite lavas from the northern Cascade arc at Mount Baker,Washington

Mt. Baker is a dominantly andesitic stratovolcano in the northern Cascade arc. In this study, we show that the andesites are not all derived from similar sources, and that open-system processes were dominant during their petrogenesis. To this end, we discuss petrographic observations, mineral chemistry, and whole rock major and trace element chemistry for three of Mt. Baker’s late Pleistocene to Holocene lava flow units. These include the basalt and basaltic andesite of Sulphur Creek (SC) (51.4–55.8 wt% SiO₂, Mg# 57–58), the Mg-rich andesite of Glacier Creek (GC) (58.3–58.7 wt% SiO₂, Mg# 63–64), and the andesite and dacite of Boulder Glacier (BG) (60.2–64.2 wt% SiO₂, Mg# 50–57). Phenocryst populations in all units display varying degrees of reaction and disequilibrium textures along with complicated zoning patterns indicative of open-system processes. All lavas are medium-K and calc-alkaline, but each unit displays distinctive trace element and REE characteristics that do not correlate with the average SiO₂ content of the unit. The mafic lavas of SC have relatively elevated REE abundances with the lowest (La/Yb)_N (~4.5). The intermediate GC andesites (Mg- and Ni-rich) have the lowest REE abundances and the highest (La/Yb)_N (~6.7) with strongly depleted HREE. The more felsic BG lavas have intermediate REE abundances and (La/Yb)_N (~6.4). The high-Mg character of the GC andesites can be explained by addition of 4% of a xenocrystic olivine component. However, their depleted REE patterns are similar to other high-Mg andesites reported from Mt. Baker and require a distinct mantle source. The two dominantly andesitic units (GC and BG) are different enough from each other that they could not have been derived from the same parent basalt. Nor could either of them have been derived from the SC basalt by crystal fractionation processes. Crystal fractionation also cannot explain the compositional diversity within each unit. Compositional diversity within the SC unit (basalt to basaltic andesite) can, however, be successfully modeled by mixing of basalt with compositions similar to the dacites in the BG unit. Given that the BG dacites erupted at ~80–90 ka, and a similar composition was mixed with the SC lavas at 9.8 ka, the process that produced this felsic end-member must have been repeatedly active for at least 70 ka.