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31.
32.
The prairie pothole region (PPR) in the north-central United States and south-central Canada constitutes the most important waterfowl breeding area in North America. Projected long-term changes in precipitation and temperature may alter the drivers of waterfowl abundance: wetland availability and emergent vegetation cover. Previous studies have focused on isolated wetland dynamics, but the implications of changing precipitation on managed, river-fed wetlands have not been addressed. Using a structured decision making (SDM) approach, we derived optimal water management actions for 20 years at four river-fed National Wildlife Refuges (NWRs) in North and South Dakota under contrasting increasing/decreasing (+/?0.4 %/year) inflow scenarios derived from empirical trends. Refuge pool depth is manipulated by control structures. Optimal management involves setting control structure heights that have the highest probability of providing a desired mix of waterfowl habitat, given refuge capacities and inflows. We found optimal seasonal control structure heights for each refuge were essentially the same under increasing and decreasing inflow trends of 0.4 %/year over the next 20 years. Results suggest managed pools in the NWRs receive large inflows relative to their capacities. Hence, water availability does not constrain management; pool bathymetry and management tactics can be greater constraints on attaining management objectives than climate-mediated inflow. We present time-dependent optimal seasonal control structure heights for each refuge, which are resilient to the non-stationary precipitation scenarios we examined. Managers can use this information to provide a desired mixture of wildlife habitats, and to re-assess management objectives in reserves where pool bathymetry prevents attaining the currently stated objectives. 相似文献
33.
34.
A colorimetric method for the determination of phosphorus in batches of geochemical survey samples is described. The powdered material is digested with a nitric-perchloric acid mixture, excess of acid is evaporated off and the residue is leached with water. Aliquots of the leachate are treated with vanadomolybdate reagent and the yellow phosphorus complex is extracted into isobutyl methyl ketone. The colour in the organic phase, which is stable for many days, is matched against a series of similarly prepared standards. Iron, other coloured ions, and titanium do not interfere. Standard deviations of ±0.02% about a mean value of 0.20% P2 O5 and ±0.4% about a mean of 11.6% P2 O5 were obtained. At least 40 powdered samples may be analysed per working day per person. 相似文献
35.
36.
S. Jane Arthur 《Astrophysics and Space Science》2002,281(1-2):267-270
The radiation produced by the gas cooling behind a fast supernova remnant shock in the interstellar medium is capable of ionizing
the undisturbed medium ahead of the shock wave. In this work I investigate the nonequilibrium evolution of these photoionized
precursor regions by means of radiation-hydrodynamic simulations of supernova remnant evolution.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
37.
Wyatt Mark C. Holland Wayne S. Greaves Jane S. Dent William R. F. 《Earth, Moon, and Planets》2003,92(1-4):423-434
Debris disks are found around some 15% of main sequencestars and their dust is thought to be continuallyreplenished in collisions between planetesimals inextrasolar Kuiper belts.While they were discovered in 1984 by IRAS, it is onlywith more recent imaging that their true nature has beenrevealed. This paper discusses recent debris disk images andtheir impact on our understanding of extrasolar systems.Importantly these images confirm the extrasolar Kuiper belthypothesis for most (but not all) debris diskcandidates and show that the planetesimals within thesedisks must have grown to at least a few km.Asymmetries in imaged disk structures also provide informationabout the planetary systems orbiting inside these planetesimalbelts. The impact of debris disk studies on our understandingof the evolution of our own Kuiper belt, as well as theirpotential to solve puzzles such as the origin of the missingmass and the outer edge of the Kuiper belt, is alsodiscussed. 相似文献
38.
Water quality in the Great Barrier Reef region: responses of mangrove, seagrass and macroalgal communities 总被引:1,自引:0,他引:1
Marine plants colonise several interconnected ecosystems in the Great Barrier Reef region including tidal wetlands, seagrass meadows and coral reefs. Water quality in some coastal areas is declining from human activities. Losses of mangrove and other tidal wetland communities are mostly the result of reclamation for coastal development of estuaries, e.g. for residential use, port infrastructure or marina development, and result in river bank destabilisation, deterioration of water clarity and loss of key coastal marine habitat. Coastal seagrass meadows are characterized by small ephemeral species. They are disturbed by increased turbidity after extreme flood events, but generally recover. There is no evidence of an overall seagrass decline or expansion. High nutrient and substrate availability and low grazing pressure on nearshore reefs have lead to changed benthic communities with high macroalgal abundance. Conservation and management of GBR macrophytes and their ecosystems is hampered by scarce ecological knowledge across macrophyte community types. 相似文献
39.
This survey provides baseline information on sediment characteristics, porewater, adsorbed and plant tissue nutrients from intertidal coastal seagrass meadows in the central region of the Great Barrier Reef World Heritage Area. Data collected from 11 locations, representative of intertidal coastal seagrass beds across the region, indicated that the chemical environment was typical of other tropical intertidal areas. Results using two different extraction methods highlight the need for caution when choosing an adsorbed phosphate extraction technique, as sediment type affects the analytical outcome. Comparison with published values indicates that the range of nutrient parameters measured is equivalent to those measured across tropical systems globally. However, the nutrient values in seagrass leaves and their molar ratios for Halophila ovalis and Halodule uninervis were much higher than the values from the literature from this and other regions, obtained using the same techniques, suggesting that these species act as nutrient sponges, in contrast with Zostera capricorni. The limited historical data from this region suggest that the nitrogen and phosphorus content of seagrass leaves has increased since the 1970s concomitant with changing land use practice. 相似文献
40.
Jane M. Reed Francesc Mesquita-Joanes Huw I. Griffiths 《Journal of Paleolimnology》2012,47(2):251-275
Diatoms (Bacillariophyceae; single-celled algae) and ostracods (Ostracoda; shelled microcrustacea) are known for their sensitivity
to salinity. In palaeolimnology, the potential has yet to be tested for quantifying past salinity, lake level, and by inference,
climate change, by application of multiple-indicator transfer functions. We used weighted averaging techniques to derive diatom
(n = 91; r
2
= 0.92) and ostracod (n = 53; r
2
= 0.83) conductivity transfer functions from modern diatom, ostracod and water chemistry data collected in lakes of central,
western and northern Turkey. Diatoms were better represented across the full gradient than ostracods, at intermediate levels
of conductivity in particular, but both transfer functions were statistically robust. Because transfer functions are not infallible,
we further tested the strength and simplicity of salinity response and the potential for identifying characteristic associations
of diatom and ostracod taxa in different parts of the salinity gradient, to improve palaeoclimate reconstruction. We identified
a subset of 51 samples that contained both diatoms and ostracods, collected at the same time from the same habitat. We used
Two-Way Indicator Species Analysis of a combined diatom-ostracod data set, transformed to achieve numerical equivalence, to
explore distributions in more detail. A clear ecological threshold was apparent at ~3 g l−1 salinity, rather than at 5 g l−1, the boundary used by some workers, equating to the oligosaline-mesosaline boundary. Other salinity boundaries were poorly
defined, indicating lack of a simple, well-defined salinity response. We did, however, define characteristic associations
of taxa, to facilitate the distinction to be drawn between a hydrologically open, fresh lake and an oligosaline lake, in palaeoenvironmental
reconstruction. Over the rest of the salinity gradient, we highlighted the potential for the multi-proxy approach to strengthen
ostracod-based reconstruction in particular, to overcome the problem of broad apparent tolerance ranges in common halophilic
taxa such as Limnocythere inopinata, which often dominate in low-diversity fossil assemblages. The combination of multi-proxy quantitative reconstruction, complemented
by qualitative understanding of ecological responses generated by the analysis, remains a powerful tool in Quaternary palaeoclimate
research. 相似文献