National-scale estimation of potentially harmful element ambient background concentrations in topsoil using parent material classified soil:stream–sediment relationships

Regulatory authorities require estimates of ambient background concentrations (ABCs) of potentially harmful elements (PHEs) in topsoil; such data are currently not available in many countries. High resolution soil geochemical data exist for only part of England and Wales, whilst stream sediment data cover the entire landscape. A novel methodology is presented for estimating soil equivalent ABCs for PHEs from high-resolution (HR) stream sediment geochemical data grouped by common parent materials (PM), using arsenic (As) as an example. Geometric mean (GM) values for local PM groups are used to investigate different approaches for transforming sediment to soil equivalent concentrations. Holdout validation is used to assess: (i) the optimum number of samples for calculating local GM values, and (ii) the optimum scale at which to group data when using linear regression analysis to estimate GM soil ABCs from local sediment geochemical values. Holdout validation showed that the smallest differences were generally observed when five observations were used to calculate the GM and that these should be grouped over the smallest possible area in order to encompass soils over PMs with elevated GM As concentrations. Geometric mean ABCs are estimated and mapped for As in mineral soil across all of England and Wales within delineations of PM polygons. Errors for the estimation of soil equivalent GM As ABCs based on sediment data for an independent validation set were of a similar magnitude to those from holdout validation applied to the original data suggesting the approach is robust. The estimates of soil equivalent ABCs suggest that As exceeds the regulatory threshold used in risk assessments for residential land use (20 mg kg⁻¹) across 16% of the landscape of England and Wales. The applicability of the method for cognate landscapes, and potential refinements is discussed.     

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.     

Determination of part-per-trillion levels of atmospheric dimethyl sulfide by isotope dilution gas chromatography/mass spectrometry

Stable isotopic dilution was applied to the determination of dimethyl sulfide (DMS) in ambient air at the low part-per-trillion by volume (pptrv) levels. Perdeuterated DMS was used as an internal standard in the gas chromatography/mass spectrometry determination. The isotopically labelled internal standard provided insensitivity to possible losses of DMS in sampling and analysis. The lower limit of detection (LLD) was 1 pptrv with a sample acquisition time of 2 min.     

Vicariance and dispersal: Confrontation or compatibility?

Current methods in biogeography are briefly reviewed and definitions of dispersal discussed. The need for countenancing the possibilities of operation of both the vicariance and dispersal modes is stressed, and for taking into account geological, climatological and floristic information as well as various aspects of the biology and information contained in the distribution patterns and phylogenies of the group under study. Methods for assessing the likelihood of vicariance and dispersal as causal processes, and possible indications of which of these has predominated, are also considered.     

Colonisation of the Krakatau islands by invertebrates

Arrival of invertebrates on the Krakatau islands, and the development of invertebrate assemblages there since 1883, are reviewed. Many data are imprecise, and difficulties of interpretation are outlined. A few well-known groups, such as butterflies, are used to suggest that much colonisation so far has been by relatively generalist species,and that the rate of addition of new species may become slower as the less vagile specialists characteristic of later successional vegetation may find natural colonisation harder. Studies of the assemblages developing in association with particular plant species are likely to be helpful in assessing future trends of invertebrate colonisation.     

Evolution of the Nearshore Bed Envelope

The temporal growth of the envelope of bed motion owing to the migration of bedforms, which can be considered a proxy for maximum object burial depth, is examined using five different data sets. These data sets support the hypothesis that the envelope of bed motion will grow as an exponential taper, quickly at first, tapering off and approaching an asymptotic value. This growth is largest and fastest in the surf zone where wave and current flows are strong. Within the surf zone, envelopes owing solely to the migration of megaripples (bedforms with heights from 20 to 40 cm and lengths from 1 to 5 m) grow for about 8 d and reach an asymptote of about 40 cm. When wave energy becomes larger ( 1 m), bed envelopes are dominated by migrating sand bars and approach an asymptote of 3-4 m, but only after 2-12 years (depending on the beach). In addition, the frequency of object burial (the percentage of time that an object would be buried by the crests of migrating bedforms) is highest in the surf zone and grows rapidly with time.     

Utilisation of different carbon sources in a shallow estuary identified through stable isotope techniques

Organic carbon in estuarine sediments can have many different sources. Terrestrial, riverine, estuarine and marine C pools may all contribute to and influence the organic C (C_org) inventory of the estuarine sediments and the differing stable isotope signatures of the sources are reflected in the sediment's overall ¹³C content. Ecological interpretations of sedimentary isotope data may, however, be limited by the fact the total C_org inventory of a sediment may not be an accurate representation of the fraction that is labile and being actively turned over by the sedimentary community. To gain a better understanding of sedimentary C_org dynamics in estuaries and the relationship between the sedimentary C pool and the C_org undergoing mineralisation, we studied three components of an estuarine system: (1) the sedimentary C_org inventory on a transect from the mouth to the upper end of the estuary, (2) temporal changes of sedimentary C_org at one station throughout a year, and (3) the δ¹³C of respired CO₂ compared to the δ¹³C of available source material and sedimentary C_org in a novel application of methods developed for soil science. Our experiments demonstrated that material of marine origin dominated the studied estuary. At the time-series station, material of marine origin dominated the sedimentary C_org throughout the 1-yr study period. δ¹³C values of CO₂ released from the sediment differed significantly from the sedimentary C_org inventory at all study sites, but also clearly reflected differences between the main sections of the estuary. These results suggest that δ¹³C measurements of respired CO₂ are promising as a tool to advance our understanding of C cycling in estuaries, and highlight that the sedimentary C_org pool alone may not be a satisfactory indicator of OM utilisation in estuarine sediments.