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1.
Timothy M. Dellapenna Mead A. Allison Gary A. Gill Ronald D. Lehman Kent W. Warnken 《Estuarine, Coastal and Shelf Science》2006,69(3-4):519
To address the relative importance of shrimp trawling on seabed resuspension and bottom characteristics in shallow estuaries, a series of disturbance and monitoring experiments were conducted at a bay bottom mud site (2.5 m depth) in Galveston Bay, Texas in July 1998 and May 1999. Based on pre- and post-trawl sediment profiles of 7Be; pore water dissolved oxygen and sulfide concentration; and bulk sediment properties, it was estimated that the trawl rig, including the net, trawl doors, and “tickler chain,” excavate the seabed to a maximum depth of approximately 1.5 cm, with most areas displaying considerably less disturbance. Water column profile data in the turbid plume left by the trawl in these underconsolidated muds (85–90% porosity; <0.25 kPa undrained shear strength) demonstrate that suspended sediment inventories of up to 85–90 mg/cm2 are produced immediately behind the trawl net; an order of magnitude higher than pre-trawl inventories and comparable to those observed during a 9–10 m/s wind event at the study site. Plume settling and dispersion caused suspended sediment inventories to return to pre-trawl values about 14 min after trawl passage in two separate experiments, indicating particles re-settle primarily as flocs before they can be widely dispersed by local currents. As a result of the passage of the trawl rig across the seabed, shear strength of the sediment surface showed no significant increase, suggesting that bed armoring is not taking place and the trawled areas will not show an increase in critical shear stress. 相似文献
2.
Complexation of mercury by dissolved organic matter in surface waters of Galveston Bay, Texas 总被引：3，自引：0，他引：3
The chemical speciation of dissolved mercury in surface waters of Galveston Bay was determined using the concentrations of mercury-complexing ligands and conditional stability constants of mercury-ligand complexes. Two classes of natural ligands associated with dissolved organic matter were determined by a competitive ligand exchange-solvent solvent extraction (CLE-SSE) method: a strong class (Ls), ranging from 19 to 93 pM with an average conditional stability constant (KHgLs) of 1028, and a weak class (Lw) ranging from 1.4 to 9.8 nM with an average KHgLs of 1023. The range of conditional stability constants between mercury and natural ligands suggested that sulfides and thiolates are important binding sites for dissolved mercury in estuarine waters. A positive correlation between the estuarine distribution of dissolved glutathione and that of mercury-complexing ligands supported this suggestion. Thermodynamic equilibrium modeling using stability constants for HgL, HgClx, Hg(OH)x, and HgCl(OH) and concentrations of each ligand demonstrated that almost all of the dissolved mercury (> 99%) in Galveston Bay was complexed by natural ligands associated with dissolved organic matter. The importance of low concentrations of high-affinity ligands that may originate in the biological system (i.e., glutathione and phytochelatin) suggests that the greater portion of bulk dissolved organic matter may not be important for mercury complexation in estuarine surface waters. 相似文献
3.
P. W. Lehman 《Estuaries and Coasts》2004,27(2):311-324
Significant coherence among time series of environmental and biological production variables suggested mechanistic pathways
through which climate contributed to the downward shift in estuarine production (biomass) in northern San Francisco Bay estuary,
1975–1993. Climate directly and indirectly affected physical processes in the estuary through precipitation and its subsequent
impact on streamflow and physical variables affected by streamflow. Climate also directly influenced air temperature and wind
velocity. The influence if climate was evaluated through a climate index based on sea level pressure. A shift in this climate
index in the early 1980s coincided with changes in many environmental variables including water transparency, water temperature,
wind velocity, and rainfall. These physical changes were accompanied by a decrease in diatom, total zooplankton, andNeomysis mercedis carbon at the base of the food web throughout the estuary. Box-Jenkins time series coherence analysis was used to quantify
associations among these physical, chemical, and biological time series for nine regions of the estuary. These associations
were used to develop a conceptual model of mechanistic pathways that directly linked food web carbon production to climate.
Strong coherence among diatom, zooplankton, andN. mercedis carbon time series suggested climate also had an indirect impact on food web production through trophic cascade. Differing
mechanistic pathways among the nine regions of the estuary suggested climate was an important contributor to the spatial variability
in total food web production and trophic structure. 相似文献
4.
John T. Lehmann Robert Foy Donna A. Lehman 《Aquatic Sciences - Research Across Boundaries》2001,63(2):168-181
Physiological data indicate that a 3-compartment model appropriately represents ingestion, assimilation, and accumulation of somatic and reproductive tissue by an aquatic invertebrate predator. A secondary production model calibrated to growth and respiration data is used to constrain the model equations. Inverse solution methods are used to estimate rate constants for digestion and for net accrual of tissue by growing animals at different temperatures. Predicted time to 50% maximum digestion matches well with the empirical rate of digestion of prey enzymes in the predator gut. Model-derived digestion rates are consistent with the assimilation rate constant obtained by independent radiotracer methods, and with experimental detection time for prey enzymes. The model approach permits parameter estimation within a complex system of equations so that results can be compared with experimental data. 相似文献
5.
Peggy W. Lehman 《Estuaries and Coasts》1992,15(3):335-348
Long-term changes in chlorophyll concentration were predicted from environmental variables using Box-Jenkins transfer function models for the Sacramento and San Joaquin rivers and Suisun Bay. Data used for the analyses were collected continuously on a semimonthly or monthly basis over the 17-yr period between 1971 and 1987. Groups of highly correlated environmental variables were summarized along three environmental axes using principal component analysis. The first environmental axis summarized river flow and specific conductance. The second environmental axis summarized water transparency and the third environmental axis summarized air and water temperature. Chlorophyll concentration was significantly corss-correlated with environmental axes and individual environmental variables. Transfer function models developed to describe changes in chlorophyll concentration over time were characterized by lag responses and described between 41% and 51% of the data variation. Significant cross-correlations between environmental axes and the California climate index (CA SLP) were used to develop a conceptual model of the link between regional climate and estuarine production. 相似文献
6.
O.R. Lehman 《Advances in water resources》1980,3(2):87-89
Oxygen exchange between a sheltered ‘model’ farm pond (6 m diameter and 2.5 m deep) and the atmosphere averaged 0.33 m/day for measurements made from April to December. This value was similar to those reported for isolated surfaces in sheltered waters and in laboratory tanks but below those of large, inland waters. Low average wind velocity (0.7 m/sec) probably caused the low O2 exchange rate. Interactions between air and water temperatures and precipitation may be important factors governing short-term O2 exchange in highly sheltered waters. 相似文献
7.
Benot Noyelles 《中国天文和天体物理学报》2007,7(2):317-324
We describe and analyze observations of mutual events of Galilean satellites made at the Yunnan Observatory in February 2003 from CCD imaging for the first time in China. Astrometric positions were deduced from these photometric observations by modelling the relative motion and the photometry of the involved satellites during each event. 相似文献
8.
Evaluation of the response modification coefficient and collapse potential of special concentrically braced frames 总被引：1，自引：0，他引：1
Special concentrically braced frames (SCBFs) are commonly used for seismic design of buildings. Their large elastic stiffness and strength efficiently sustains the seismic demands during smaller, more frequent earthquakes. During large, infrequent earthquakes, SCBFs exhibit highly nonlinear behavior due to brace buckling and yielding and the inelastic behavior induced by secondary deformation of the framing system. These response modes reduce the system demands relative to an elastic system without supplemental damping using a response modification coefficient, commonly termed the R factor. More recently, procedures put forth in FEMAP695 have been made to quantify the R factor through a formalized procedure that accounts for collapse potential. The primary objective of the research in this paper was to evaluate the approach for SCBFs. An improved model for SCBFs that permits simulation of brace fracture was used to conduct response history analyses. A series of three‐story, nine‐story and 20‐story SCBFs were designed and evaluated. Initially, the FEMAP695 method was conducted to estimate collapse and the corresponding R factor. An alternate procedure for scaling the multiple acceleration records to the seismic design hazard was also evaluated. The results show significant variation between the two methods. Of the three variations of buildings studied, the largest vulnerability was identified for the three‐story building. To achieve a consistent margin of safety against collapse, a significantly lower R factor is required for the low‐rise SCBFs (three‐story), whereas the mid‐rise and high‐rise SCBFs (nine‐story and 20‐story) may continue to use the current value of 6, as provided in ASCE‐07. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
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10.
K. Hughen J. Southon S. Lehman C. Bertrand J. Turnbull 《Quaternary Science Reviews》2006,25(23-24):3216
An expanded Cariaco Basin 14C chronology is tied to 230Th-dated Hulu Cave speleothem records in order to provide detailed marine-based 14C calibration for the past 50,000 years. The revised, high-resolution Cariaco 14C calibration record agrees well with data from 230Th-dated fossil corals back to 33 ka, with continued agreement despite increased scatter back to 50 ka, suggesting that the record provides accurate calibration back to the limits of radiocarbon dating. The calibration data document highly elevated Δ14C during the Glacial period. Carbon cycle box model simulations show that the majority of observed Δ14C change can be explained by increased 14C production. However, from 45 to 15 ka, Δ14C remains anomalously high, indicating that the distribution of radiocarbon between surface and deep ocean reservoirs was different than it is today. Additional observations of the magnitude, spatial extent and timing of deep ocean Δ14C shifts are critical for a complete understanding of observed Glacial Δ14C variability. 相似文献