The formation of the Oort cloud in open cluster environments

We study the influence of an open cluster environment on the formation and current structure of the Oort cloud. To do this, we have run 19 different simulations of the formation of the Oort cloud for 4.5 Gyrs. In each simulation, the Solar System spends its first 100 Myrs in a different open cluster environment before transitioning to its current field environment. We find that, compared to forming in the field environment, the inner Oort cloud is preferentially loaded with comets while the Sun resides in the open cluster and that most of this material remains locked in the interior of the cloud for the next 4.4 Gyrs. In addition, the outer Oort cloud trapping efficiencies we observe in our simulations are lower than previous formation models by about a factor of 2, possibly implying an even more massive early planetesimal disk. Furthermore, some of our simulations reproduce the orbits of observed extended scattered disk objects, which may serve as an observational constraint on the Sun's early environment. Depending on the particular open cluster environment, the properties of the inner Oort cloud and extended scattered disk can vary widely. On the other hand, the outer portions of the Oort cloud in each of our simulations are all similar.     

The integrated optical spectrum of the Crab nebula and evidence for its fading synchrotron continuum

A flux-calibrated optical spectrum integrated over the entire Crab nebula was obtained by making drift scans with a long-slit spectrograph. Compared to observations obtained over the past 40 years, these new data confirm an earlier controversial result that the [O iii ]  λλ4959, 5007  equivalent width is increasing with time, although the rate of ∼0.9 per cent yr⁻¹ is somewhat slower than that measured previously. Additionally, the Hβ equivalent width is increasing at a comparable rate, but the measured fluxes of both Hβ and [O  iii ] have changed less than their respective equivalent widths. The different rates of change in the measured fluxes and equivalent widths of these lines suggest that the optical synchrotron continuum from the Crab nebula is indeed fading rapidly. The apparent decline is consistent with a rate around  −0.5 (±0.2)  per cent yr⁻¹ at wavelengths near 5000  Å inferred independently from measurements of the optical continuum flux during the same time period.     

Spontaneous Formation of Magnetic Flux Concentrations in Stratified Turbulence

The negative effective magnetic pressure instability discovered recently in direct numerical simulations (DNSs) may play a crucial role in the formation of sunspots and active regions in the Sun and stars. This instability is caused by a negative contribution of turbulence to the effective mean Lorentz force (the sum of turbulent and non-turbulent contributions) and results in the formation of large-scale inhomogeneous magnetic structures from an initially uniform magnetic field. Earlier investigations of this instability in DNSs of stably stratified, externally forced, isothermal hydromagnetic turbulence in the regime of large plasma ?? are now extended into the regime of larger scale separation ratios where the number of turbulent eddies in the computational domain is about 30. Strong spontaneous formation of large-scale magnetic structures is seen even without performing any spatial averaging. These structures encompass many turbulent eddies. The characteristic time of the instability is comparable to the turbulent diffusion time, L ²/?? _t, where ?? _t is the turbulent diffusivity and L is the scale of the domain. DNSs are used to confirm that the effective magnetic pressure does indeed become negative for magnetic field strengths below the equipartition field. The dependence of the effective magnetic pressure on the field strength is characterized by fit parameters that seem to show convergence for larger values of the magnetic Reynolds number.     

Groundwater-level response to land-use change and the implications for salinity management in the West Moorabool River catchment, Victoria, Australia

The connection between the removal of native vegetation, rising water tables and increasing stream salinity has been established for many catchments across Australia. However, the West Moorabool River in south west Victoria is an example of a catchment where there has been little discernable effect on groundwater levels following land clearing. Over the past 150 years, a significant portion of the catchment has been cleared of dense forest for agricultural development. Historic standing water-level records from 1870–1871 and 1881 are compared with contemporary measurements (1970s to 2007) recorded in the government bore databases. The data show that the earliest recorded groundwater levels are well within the seasonal range of values observed today. By integrating geology and hydrogeology with historical observations of groundwater levels, climate data and land use, the contemporary field observations of stream salinity are linked to the changed water use and shift in rainfall. In contrast to the normally accepted axiom, reafforestation as a management strategy to mitigate the rising salinity in the West Moorabool River catchment would seem inappropriate.     

Evaluating local population dynamics of the American lobster,Homarus americanus,with trap‐based mark‐recapture methods and seabed mapping

Seabed mapping, spatially referenced trapping, and mark‐recapture methods have all been useful tools in ecological studies of lobsters and other benthic animals. Here we integrate the three methods to evaluate local population dynamics and movements of American lobsters, Homarus americanus, in coastal fishing grounds in Maine, United States. The study was conducted on five study areas of different size, and used two different sampling protocols. At one site (1 km² in area) we used a monthly mark‐recapture sampling interval over a 6‐month period, only tagging a subsample of the catch. At four smaller sites (0.3 km²) we used a shorter‐term approach, sampling at 3–4‐day intervals for a 2‐week duration, tagging the entire catch. Tagging data were analysed with a modified Jolly‐Seber model adapted for continuous sampling to estimate population abundance, gains (immigration), and losses (emigration and mortality). Side‐scan sonar surveys of the seabed combined with diver‐based population surveys, stratified by substrate type, provided an independent comparison to mark‐recapture‐based estimates of abundance over the same areas. Spatial referencing of trap catch also allowed us to relate catch rates and lobster movements directly to seabed features. The longer‐term tagging data on the larger study area provided abundance estimates that were more consistent with the diver observations, and estimates of gains and losses statistically more robust, than those derived from the shorter‐term effort on the smaller sites. The flux of lobsters followed the well known seasonal movements on these fishing grounds, with gains and losses from the larger study area ranging over 1000 individuals per day, and an estimated mid‐summer peak density of >65 000 lobsters per km² (individuals >50 mm carapace length). This approach may lend itself to broader application with the American lobster.     

Method‐dependent influence of environmental variables on reef fish assemblages when comparing trap and video surveys

Despite substantial survey effort and a large body of literature on abiotic and biotic factors in temperate reef ecosystems, knowledge of the complex and interactive effects of environmental variables on those communities is limited. Various survey methods have been developed to study environmental predictors of biodiversity, but there remains a gap in our understanding of how survey results are influenced by environmental factors. Here, we surveyed the fish assemblage associated with southeastern U.S. temperate marine reefs with simultaneous, paired trap, and camera gears throughout a ~50,000 km² area during 2011–2013 and assessed the influence of environmental variables on the trap‐ and video‐surveyed assemblages. Predictor variables in the multivariate general linear models included depth, temperature, month, year, location, substrate relief, percent sessile biota, biota type, and turbidity. Depth and latitude had the greatest influence on the fish assemblage for both gears. The influence of habitat variables differed between methods and percent biota explained more variation in the fish assemblage when assessed by traps, while substrate relief and biota type explained more variation in the fish assemblage when assessed by video. In general, habitat complexity was positively related to the abundance of fishes in the video survey, but there was a negative relationship in the trap survey. Differences between gears were species‐specific and the influences of environmental variables were similar for some species such as Haemulon plumierii and Hyporthodus niveatus. The methods presented here can be used to assess method‐dependent differences in fish assemblages, which is a necessary precursor to assess the effect of environmental variables on the accuracy of surveys.     

Fine‐scale diet of the Australian sea lion (Neophoca cinerea) using DNA‐based analysis of faeces

We applied DNA‐based faecal analysis to determine the diet of female Australian sea lions (n = 12) from two breeding colonies in South Australia. DNA dietary components of fish and cephalopods were amplified using the polymerase chain reaction and mitochondrial DNA primers targeting the short (~100 base pair) section of the 16S gene region. Prey diversity was determined by sequencing ~50 amplicons generated from clone libraries developed for each individual. Faecal DNA was also combined and cloned from multiple individuals at each colony and fish diversity determined. Diets varied between individuals and sites. Overall, DNA analysis identified a broad diversity of prey comprising 23 fish and five cephalopod taxa, including many species not previously described as prey of the Australian sea lion. Labridae (wrasse), Monacanthidae (leatherjackets) and Mullidae (goat fish) were important fish prey taxa. Commonly identified cephalopods were Octopodidae (octopus), Loliginidae (calamary squid) and Sepiidae (cuttlefish). Comparisons of fish prey diversity determined by pooling faecal DNA from several samples provided a reasonable but incomplete resemblance (55–71%) to the total fish diversity identified across individual diets at each site. Interpretation of diet based on the recovery of prey hard‐parts identified one cephalopod beak (Octopus sp.) and one fish otolith (Parapriacanthus elongatus). The present study highlights the value of DNA‐based analyses and their capabilities to enhance information of trophic interactions.     

Global extent and distribution of artificial, residential waterways in estuaries

Artificial residential waterways are now widespread in the world’s estuaries. We used the global mapping tool, Google Earth, to determine that there are nearly 4000 linear km of artificial waterways globally, covering an area of 270 km². Residential waterways constructed as open, flow-through canal estates are at their greatest extent in North America (77% of global linear extent), where systems are typically longer and narrower, with more openings and dead-ends than systems elsewhere. The remaining canal estates are spread across all other continents except Antarctica: Asia (7%), Europe (7%), Oceania (7%), South America (0.9%), and Africa (0.6%). A relatively recent design change from open canals to artificial estuarine lakes with tidal barriers has occurred on all continents except Africa, most extensively in Australia (14 km² area, 57 independent systems). The extremely large expansion in artificial residential waterways aimed at increasing opportunities for waterfront living by humans has also modified and expanded estuarine habitat available to aquatic biota. Research can best underpin planning and management of these types of waterways by focussing on their value as habitat and their provision of other goods and services.     

The Southern Antarctic Circumpolar Current Front: physical and biological coupling at South Georgia

The coupling of physics and biology was examined along a 160 km long transect running out from the north coast of South Georgia Island and crossing the Southern Antarctic Circumpolar Current Front (SACCF) during late December 2000. Surface and near surface potential TS properties indicated the presence of three water types: a near-shore group of stations characterised by water which became progressively warmer and fresher closer to South Georgia, an offshore grouping in which sea surface temperatures and those at the winter water level were relatively warm (1.8°C and 0.5°C, respectively), and a third in which surface and winter water temperatures were cooler and reflected the presence of the SACCF. The transect bisected the SACCF twice, revealing that it was flowing in opposite directions, north-westward closest to South Georgia and south-eastwards at its furthest point from the island. The innermost limb was a narrow intense feature located just off the shelf break in 2000–3500 m of water and in which rapid surface baroclinic velocities (up to 35 cm s⁻¹) were encountered. Offshore in the outermost limb, shown subsequently to be a mesoscale eddy that had meandered south from the retroflected limb of the SACCF, flow was broader and slower with peak velocities around 20 cm s⁻¹. Chlorophyll a biomass was generally low (<1 mg m⁻³) over much of the transect but increased dramatically in the region of the innermost limb of the SACCF, where a deepening of the surface mixed layer was coincident with a subsurface chlorophyll maximum (7.4 mg m⁻³) and elevated concentrations down to 100 m. The bloom was coincident with depleted nutrient concentrations, particularly silicate, nitrate and phosphate, and although ammonium concentrations were locally depleted the bloom lay within an elevated band (up to 1.5 mmol m⁻³) associated with the frontal jet. Increased zooplankton abundance, higher copepod body carbon mass and egg production rates all showed a strong spatial integrity with the front. The population structure of the copepods Calanoides acutus and Rhincalanus gigas at stations within the front suggested that rather than simply resulting from entrainment and concentration within the jet, increased copepod abundance was the result of development in situ. Estimates of bloom duration, based on silicate and carbon budget calculations, set the likely duration between 82 and 122 d, a figure supported by the development schedule of the two copepod species. Given this timescale, model outputs from FRAM and OCCAM indicated that particles that occurred on the north side of South Georgia in December would have been in the central-southern Scotia Sea 2–3 months earlier, probably in sea ice affected regions.     

Measuring Concentrated Poverty in a Global Metropolis: Lessons from Los Angeles∗

The traditional definition of concentrated poverty—40 percent of a census tract population living below the federal poverty threshold—is problematic in light of burgeoning working poor populations, increasing inner-suburban poverty, and problems with the federal poverty threshold itself. This article questions the meaning of concentrated poverty as traditionally defined and measured through an analysis of social and built environments in selected Los Angeles area communities that meet the 40 percent threshold. Such neighborhoods face significant challenges, yet do not conform to stereotypes of concentrated poverty areas. Findings indicate the need for measures that recognize the increasingly heterogeneous forms and landscapes of poverty in globalizing urban regions.