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311.
Michael A. Mallin Matthew R. McIver Heather A. Wells Douglas C. Parsons Virginia L. Johnson 《Estuaries and Coasts》2005,28(5):750-760
The New River Estuary consists of a series of broad shallow lagoons draining a catchment area of 1,436 km2, located in Onslow County, North Carolina. During the 1980s and 1990s it was considered one of the most eutrophic estuaries
in the southeastern United States and sustained dense phytoplankton blooms, bottom water anoxia and hypoxia, toxic outbreaks
of the dinoflagellatePfiesteria, and fish kills. High nutrient loading, especially of phosphorus (P), from municipal and military sewage treatment plants
was the principal cause leading to the eutrophic conditions. Nutrient addition bioassay experiments showed that additions
of nitrogen (N) but not P consistently yielded significant increases in phytoplankton production relative to controls. During
1998 the City of Jacksonville and the U.S. Marine Corps Base at Camp Lejeune completely upgraded their sewage treatment systems
and achieved large improvements in nutrient removal, reducing point source inputs of N and P to the estuary by approximately
57% and 71%, respectively. The sewage treatment plant upgrades led to significant estuarine decreases in ammonium, orthophosphate,
chlorophylla, and turbidity concentrations, and subsequent increases in bottom water dissolved oxygen (DO) and light penetration. The
large reduction in phytoplankton biomass led to a large reduction in labile phytoplankton carbon, likely an important source
of biochemical oxygen demand in this estuary. The upper estuary stations experienced increases in average bottom water DO
of 0.9 to 1.4 mg l−1, representing an improvement in benthic habitat for shellfish and other organisms. The reductions in light attenuation and
turbidity should also improve the habitat conditions for growth of submersed aquatic vegetation, an important habitat for
fish and shellfish. 相似文献
312.
Heather Xiaoquan Zhang P. Mick Kelly Catherine Locke Alexandra Winkels W. Neil Adger 《Geoforum》2006,37(6):1066-1081
Migration is frequently portrayed as a negative force in its relationship with economic and social development. This negative perception is exhibited through describing population movements as either ‘forced’ (e.g. political and environmental refugees) or ‘voluntary’ movements (e.g. economic and uninhibited relocation). This paper examines the limitations of this conceptual dualism. It points out that the dualistic approach, widely used in the context of developing countries, simplifies a highly complex phenomenon by ignoring its essential heterogeneity and spatial and temporal dynamics. As such, it is limited in explaining and understanding the globally diversified, historically and politically contextualised situations. Focusing on the migratory experiences of contemporary Vietnam, the study identifies major patterns and trends of population mobility in the country in the past fifty years. It shows that despite the state’s continued attempts to reshape the spatial distribution of population over recent historical periods, the policy outcomes with respect to population mobility have been swayed as much by individuals and their families in pursuit of their own aspirations and livelihoods as by state plans. The Vietnam case has provided evidence of a much more complicated relationship between migration and livelihoods than the conceptual dichotomy assumes, and the opportunity for a richer set of policy options. We argue that the evidence from Vietnam, and elsewhere, warrants an integrated approach to studying migration, combining analysis at the macro- and micro-levels with the nexus lying at the critical decision-making point of the individual or household. 相似文献
313.
Christopher L. Kirkland R. Hugh Smithies Ailsa J. Woodhouse Heather M. Howard Michael T.D. Wingate Elena A. Belousova John B. Cliff Rosanna C. Murphy Catherine V. Spaggiari 《Gondwana Research》2013,23(2):759-781
The Hf and Nd isotopic evolution of the Musgrave Province, central Australia, is used to constrain the timing of crust formation and lithospheric organisation of Proterozoic Australia. The dataset from this region challenges two widely held tenets of Hf and Nd isotope systematics, namely; that crust formation events can only be identified as periods when crystallisation ages correspond to model ages, and that linear Hf evolution arrays away from depleted mantle (along crustal Lu/Hf or Sm/Nd slopes) reflect reworking of the same source.Hf isotopes in Musgrave Province zircon crystals indicate two major crust formation events at c. 1900 Ma and at 1600–1550 Ma. Although no juvenile rocks or crystals are known from c. 1900 Ma, radiogenic addition into the crust at this time is required to account for consistent Nd and Hf evolution patterns, which show no indication of an initially heterogeneous source. Oxygen isotopes in zircon grains confirm that much of the c. 1900 Ma Hf isotopic signal is not compromised by mixtures. Furthermore, the correspondence between mantle extraction and the commencement of reworking of Archean material supports new crust generation at c. 1900 Ma and a coupling between lower and upper crustal processes. The c. 1900 Ma timing of juvenile addition is dissimilar to that in the Albany–Fraser and Arunta Orogens and may reflect continental arc development on the margin of a southern continent.The general Hf isotopic evolution trend of the Musgrave Province apparently reflects reworking from a dominant c. 1900 Ma source with some additional unradiogenic and radiogenic input through time. However, in the 1220–1050 Ma interval this apparent isotopic evolution contrasts with geological observations that indicate input of voluminous mantle-derived material. Intracontinental rifts and other regions with sustained very-high temperature crustal recycling processes generate magmatic provinces with extreme HFSE-enrichment. This can have a profound influence on isotopic evolution trends, suppressing typical juvenile addition patterns. Isotopic mixture modelling indicates that a significant volume of mantle derived material can be accommodated within HFSE enriched magmas without diverging isotopic signatures from apparent reworking trends. In the Musgrave Province, the crust had become so HFSE enriched during the prolonged Musgrave Orogeny (1220–1150 Ma) that it was insensitive to mantle input, which is estimated to have been as much as 85% during this event. 相似文献
314.
David M. Bell Matthew J. Gregory Van Kane Jonathan Kane Robert E. Kennedy Heather M. Roberts Zhiqiang Yang 《Carbon balance and management》2018,13(1):15
Background
Satellite-based aboveground forest biomass maps commonly form the basis of forest biomass and carbon stock mapping and monitoring, but biomass maps likely vary in performance by region and as a function of spatial scale of aggregation. Assessing such variability is not possible with spatially-sparse vegetation plot networks. In the current study, our objective was to determine whether high-resolution lidar-based and moderate-resolution Landsat-base aboveground live forest biomass maps converged on similar predictions at stand- to landscape-levels (10 s to 100 s ha) and whether such differences depended on biophysical setting. Specifically, we examined deviations between lidar- and Landsat-based biomass mapping methods across scales and ecoregions using a measure of error (normalized root mean square deviation), a measure of the unsystematic deviations, or noise (Pearson correlation coefficient), and two measures related to systematic deviations, or biases (intercept and slope of a regression between the two sets of predictions).Results
Compared to forest inventory data (0.81-ha aggregate-level), lidar and Landsat-based mean biomass predictions exhibited similar performance, though lidar predictions exhibited less normalized root mean square deviation than Landsat when compared with the reference plot data. Across aggregate-levels, the intercepts and slopes of regression equations describing the relationships between lidar- and Landsat-based biomass predictions stabilized (i.e., little additional change with increasing area of aggregates) at aggregate-levels between 10 and 100 ha, suggesting a consistent relationship between the two maps at landscape-scales. Differences between lidar- and Landsat-based biomass maps varied as a function of forest canopy heterogeneity and composition, with systematic deviations (regression intercepts) increasing with mean canopy cover and hardwood proportion within forests and correlations decreasing with hardwood proportion.Conclusions
Deviations between lidar- and Landsat-based maps indicated that satellite-based approaches may represent general gradients in forest biomass. Ecoregion impacted deviations between lidar and Landsat biomass maps, highlighting the importance of biophysical setting in determining biomass map performance across aggregate scales. Therefore, regardless of the source of remote sensing (e.g., Landsat vs. lidar), factors affecting the measurement and prediction of forest biomass, such as species composition, need to be taken into account whether one is estimating biomass at the plot, stand, or landscape scale.315.
Rapid weathering and erosion rates in mountainous tropical watersheds lead to highly variable soil and saprolite thicknesses which in turn impact nutrient fluxes and biological populations. In the Luquillo Mountains of Puerto Rico, a 5-m thick saprolite contains high microorganism densities at the surface and at depth overlying bedrock. We test the hypotheses that the organisms at depth are limited by the availability of two nutrients, P and Fe. Many tropical soils are P-limited, rather than N-limited, and dissolution of apatite is the dominant source of P. We document patterns of apatite weathering and of bioavailable Fe derived from the weathering of primary minerals hornblende and biotite in cores augered to 7.5 m on a ridgetop as compared to spheroidally weathering bedrock sampled in a nearby roadcut.Iron isotopic compositions of 0.5 N HCl extracts of soil and saprolite range from about δ56Fe = 0 to ? 0.1‰ throughout the saprolite except at the surface and at 5 m depth where δ56Fe = ? 0.26 to ? 0.64‰. The enrichment of light isotopes in HCl-extractable Fe in the soil and at the saprolite–bedrock interface is consistent with active Fe cycling and consistent with the locations of high cell densities and Fe(II)-oxidizing bacteria, identified previously. To evaluate the potential P-limitation of Fe-cycling bacteria in the profile, solid-state concentrations of P were measured as a function of depth in the soil, saprolite, and weathering bedrock. Weathering apatite crystals were examined in thin sections and an apatite dissolution rate of 6.8 × 10? 14 mol m? 2 s? 1 was calculated. While surface communities depend on recycled nutrients and atmospheric inputs, deep communities survive primarily on nutrients released by the weathering bedrock and thus are tightly coupled to processes related to saprolite formation including mineral weathering. While low available P may limit microbial activity within the middle saprolite, fluxes of P from apatite weathering should be sufficient to support robust growth of microorganisms in the deep saprolite. 相似文献
316.