Evaluating Ecological Equivalence of Created Marshes: Comparing Structural Indicators with Stable Isotope Indicators of Blue Crab Trophic Support     

Chris Llewellyn  Megan La Peyre 《Estuaries and Coasts》2011,34(1):172-184

This study sought to examine ecological equivalence of created marshes of different ages using traditional structural measures of equivalence, and tested a relatively novel approach using stable isotopes as a measure of functional equivalence. We compared soil properties, vegetation, nekton communities, and δ ¹³C and δ ¹⁵N isotope values of blue crab muscle and hepatopancreas tissue and primary producers at created (5–24 years old) and paired reference marshes in SW Louisiana. Paired contrasts indicated that created and reference marshes supported equivalent plant and nekton communities, but differed in soil characteristics. Stable isotope indicators examining blue crab food web support found that the older marshes (8 years+) were characterized by comparable trophic diversity and breadth compared to their reference marshes. Interpretation of results for the youngest site was confounded by the fact that the paired reference, which represented the desired end goal of restoration, contained a greater diversity of basal resources. Stable isotope techniques may give coastal managers an additional tool to assess functional equivalency of created marshes, as measured by trophic support, but may be limited to comparisons of marshes with similar vegetative communities and basal resources, or require the development of robust standardization techniques.  相似文献   

A hybrid scenario for gas giant planet formation in rings     

Richard H. Durisen  Kai Cai  Megan K. Pickett 《Icarus》2005,173(2):417-424

The core-accretion mechanism for gas giant formation may be too slow to create all observed gas giant planets during reasonable gas disk lifetimes, but it has yet to be firmly established that the disk instability model can produce permanent bound gaseous protoplanets under realistic conditions. Based on our recent simulations of gravitational instabilities in disks around young stars, we suggest that, even if instabilities due to disk self-gravity do not produce gaseous protoplanets directly, they may create persistent dense rings that are conducive to accelerated growth of gas giants through core accretion. The rings occur at and near the boundary between stable and unstable regions of the disk and appear to be produced by resonances with discrete spiral modes on the unstable side.  相似文献   

The nature of a gravitationally lensed submillimetre arc in MS0451.6−0305: two interacting galaxies at z∼ 2.9?     

Colin Borys  Scott Chapman  Megan Donahue  Greg Fahlman  Mark Halpern  Jean-Paul Kneib  Peter Newbury  Douglas Scott  Graham P. Smith 《Monthly notices of the Royal Astronomical Society》2004,352(3):759-767

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Classical Meteor Light Curve Morphology     

Martin Beech  Megan Hargrove 《Earth, Moon, and Planets》2004,95(1-4):389-394

We investigate the morphological variation of classical meteor light curves, under the constant velocity assumption, for a series of idealized atmospheric density profiles. We look specifically at the t _rise /t _fall ratio, which compares the rise time to maximum brightness against the time to fall from maximum brightness. We demonstrate that for a classical meteoroid undergoing rapid ablation in an isothermal atmosphere that t _rise/t _fall > 1, indicating that all such light curves are late peaked. For a classical meteoroid ablating in a region over which the density is constant, t _rise/t _fall≡ 0, and the light curve is necessarily downward concave in the height vs. intensity diagram. If ablation occurs over a region in which the density increases linearly with decreasing height, then t _rise/t _fall=1/(√5 – 1) ≈ 0.81, indicative of an early peaked, near symmetric light curve.  相似文献   

How societal expectations,new digital tools,and the COVID crisis are driving a step-change in human centric remediation     

John Horst  Carly Foster  Steven Perry  Ajani Stewart  Megan Klar  Beto Altamirano  Prasoon Sinha 《Ground Water Monitoring & Remediation》2021,41(3):13-21

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Repeated high flows drive morphological change in rivers in recently deglaciated catchments     

Lawrence J. B. Eagle  Jonathan L. Carrivick  Alexander M. Milner  Lee E. Brown  Megan J. Klaar 《地球表面变化过程与地形》2021,46(7):1294-1310

Climate change is decreasing glacier cover and increasing the frequency and magnitude of precipitation-driven high flows and floods in many regions of the world. Precipitation may become the dominant water source for river systems in recently deglaciated catchments, with major rainfall events driving significant changes in river channel morphology. Few studies, however, have examined river channel response to repeated precipitation-driven high flows. In this study, we measured the geomorphological condition of four low-order rivers in recently deglaciated catchments (70–210 years ice free) before and after a series of repeated precipitation-driven high flows during summer 2014. High flows drove substantial initial morphological change, with up to 75% change in baseflow channel planform position and active channel form change from pre- to post-high flow. Post-high flow years were associated with increased instream wood and geomorphological complexity at all but the youngest river. Channel changes were part of an active relaxation stage at all rivers, where channels continued to migrate, and complexity varied through time. Overall, these measurements permit us to propose a conceptual model of the role of geomorphologically effective high flows in the context of paraglacial adjustment theory. Specifically, we suggest that older rivers in recently deglaciated catchments can undergo a short-term (<10 years) increase in the rate of geomorphological development as a result of the recruitment of instream wood and channel migration during and following repeated precipitation-driven high flows. Enhancing our knowledge of these geomorphological and paraglacial processes in response to high flows is important for the effective management of riverine water and ecosystem resources in rapidly changing environments.  相似文献   

Responses of soil and water chemistry to mountain pine beetle induced tree mortality in Grand County,Colorado, USA     

David W. Clow  Charles Rhoades  Jennifer Briggs  Megan Caldwell  William M. Lewis 《Applied Geochemistry》2011

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A Modeling and Field Approach to Identify Essential Fish Habitat for Juvenile Bay Whiff (Citharichthys spilopterus) and Southern Flounder (Paralichthys lethostigma) Within the Aransas Bay Complex, TX     

Bridgette F. Froeschke  Gregory W. Stunz  Megan M. Reese Robillard  Jason Williams  John T. Froeschke 《Estuaries and Coasts》2013,36(5):881-892

The goal of this study was to use an ecosystem-based approach to consider the effect of environmental conditions on the distribution and abundance of juvenile bay whiff and southern flounder within the Aransas Bay Complex, TX, USA. Species habitat models for both species were developed using boosted regression trees. Juvenile bay whiff were associated with low temperatures (<15 °C, 20–23 °C), moderate percent dry weight of sediments (25–60 %), salinity >10, and moderate to high dissolved oxygen (6–9 mg O₂/l, 10–14 mg/l). Juvenile southern flounder were associated with low temperatures (<15 °C), low percent dry weight of sediment (<25 %), seagrass habitat, shallow depths (<1.2 m), and high dissolved oxygen (>8 mg O₂/l). Our results indicate that conservation measures should focus along the eastern side of Aransas Bay and the north corner of Copano Bay to protect essential fish habitat. These findings provide a valuable new tool for fisheries managers to aid in the sustainable management of bay whiff and southern flounder and provide crucial information needed to prioritize areas for habitat conservation.  相似文献   

Synthesized Model of Geospatial Thinking     

Amy Lobben  Megan Lawrence 《The Professional geographer》2013,65(3):307-318

Since the National Research Council (2006) report Learning to Think Spatially formalized geospatial thinking, researchers and educators have recognized the importance of investigating and understanding geospatial thinking. Conceptual frameworks have been developed and applied to individual research projects. Although useful in these contexts and potentially extendable to other related inquiries, they also overlap and conflict with one another. Moreover, the separate frameworks are built on different constructs, resulting in a disparate rather than a cohesive theoretical foundation for geospatial thinking. This article synthesizes existing frameworks and generates a model that represents conceptual advances and provides a foundation for research question generation.  相似文献   

Potential effects of obliquity change on gas hydrate stability zones on Mars     

Margaret J. Root  Megan E. Elwood Madden 《Icarus》2012,218(1):534-544

Methane hydrate dissociation due to obliquity-driven temperature change has been suggested as a potential source of atmospheric methane plumes recently observed on Mars. This work uses both equilibrium and time-dependent models to determine how geothermal gradients change on Mars as a result of obliquity and predict how these changes affect gas hydrate stability zones (HSZs). The models predict that the depth to the HSZ decreases with increasing latitude for both CO₂ and CH₄ hydrate, with CO₂ hydrate occurring at shallower depths than CH₄ hydrate over all latitudes. The depth of the HSZ increases as surface temperatures warm and decreases as surface temperatures cool with changing obliquity, with the largest change in HSZ volume predicted near the equator and the poles. Therefore, changes in the depth to the HSZ may cause hydrate dissociation near the equator and poles as the geothermal gradient moves in and out of the hydrate stability field over hundreds of thousands of years. Sublimation of overlying ice containing diffused methane could account for recent observations of seasonal methane plumes on Mars. In addition, near-surface gas hydrate reservoirs may be preserved at mid-latitudes due to minimal changes in surface temperature with obliquity over geologic time scales. Comparisons of the predicted changes in the HSZ with hydrate dissociation and diffusion rates reveal that metastable hydrate may also remain in the near subsurface, especially at high latitudes, for millions to billions of years. The presence of methane hydrate in the near subsurface at midlatitudes could be an important analytical target for future Mars missions, as well as serving as a source of fuel for future spacecraft.  相似文献