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31.
The dissolved organic matter in aquatic systems consists of a heterogeneous mixture of compounds with wide ranging chemical properties and diverse origins. the presence of organic matter (OM) in surface and ground waters has important implications with regard to the mobility and therefore fate of many potential organic and inorganic pollutants. the majority of the analytical approaches in current use require exhaustive pre-treatment steps which may lead to the formation of artefacts within the sample. an analytical technique based upon gel permeation chromatography is proposed as a more reliable procedure for the characterisation of naturally occurring dissolved organic matter. 相似文献
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Summary Lake Pontchartrain is part of a brackish coastal estuarine system which serves as an important economic and recreational resource for the New Orleans region. Seafood extraction, shell dredging and leisure time activities are the major uses occurring on Lake Pontchartrain. In the past several decades, man has severely altered this system through urbanization, industrial activity, levée construction and subsequent destruction of wetlands surrounding the lake. There is a growing awareness of the environmental crisis facing Lake Pontchartrain, advanced by recent fish kills, detection of toxic chemicals, curtailment of recreational opportunities and the report of dead zones in the lake. This study summarizes a series of international environmental management techniques and examines the utilization of a regional structure for water resources management in the Lake Pontchartrain Basin.Dr. Fritz Wagner is Director and Professor of the School of Urban and Regional Studies at the University of New Orleans and David Hart was a Research Assistant in the same school, and is now employed in a local engineering and planning company. 相似文献
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Age-structured modeling reveals long-term declines in the natality of western Steller sea lions. 总被引:1,自引:0,他引:1
Since the mid-1970s, the western Steller sea lion (Eumetopias jubatus), inhabiting Alaskan waters from Prince William Sound west through the Aleutian Islands, has declined by over 80%. Changing oceanographic conditions, competition from fishing operations, direct human-related mortality, and predators have been suggested as factors driving the decline, but the indirect and interactive nature of their effects on sea lions have made it difficult to attribute changes in abundance to specific factors. In part, this is because only changes in abundance, not changes in vital rates, are known. To determine how vital rates of the western Steller sea lion have changed during its 28-year decline, we first estimated the changes in Steller sea lion age structure using measurements of animals in aerial photographs taken during population surveys since 1985 in the central Gulf of Alaska (CGOA). We then fit an age-structured model with temporally varying vital rates to the age-structure data and to total population and pup counts. The model fits indicate that birth rate in the CGOA steadily declined from 1976 to 2004. Over the same period, survivorship first dropped severely in the early 1980s, when the population collapsed, and then survivorship steadily recovered. The best-fitting model indicates that in 2004, the birth rate in the central Gulf of Alaska was 36% lower than in the 1970s, while adult and juvenile survivorship were close to or slightly above 1970s levels. These predictions and other model predictions concerning population structure match independent field data from mark-recapture studies and photometric analyses. The dominant eigenvalue for the estimated 2004 Leslie matrix is 1.0014, indicating a stable population. The stability, however, depends on very high adult survival, and the shift in vital rates results in a population that is more sensitive to changes in adult survivorship. Although our modeling analysis focused exclusively on the central Gulf of Alaska, the western Gulf of Alaska and eastern Aleutians show a similar pattern of declining pup fraction with no increase in the juvenile, or pre-breeding, fraction. This suggests that declining birth rate may be a problem for western Steller sea lions across the Gulf of Alaska and into the Aleutian Islands. 相似文献
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Setting of larvae of the oyster Crassostrea virginica was monitored in the James River, Virginia, USA from 1963 to 1980. Setting patterns were similar in two ways to those described prior to 1960 (before the onset of the oyster pathogen Haplosporidium nelsoni (MSX) in Chesapeake Bay): (1) setting intensity (average number of spat per shell) was greater at stations in the lower than upper estuary, and (2) on the average, 60 to 80% of the total annual set at each station occurred during a 6-week period from mid-August through September. However, annual setting intensity from 1963–1980 was lower than previously recorded, and annual sets occurred as a series of discrete pulses rather than continuously throughout the season. Pulses were each approximately 1 to 2 weeks in duration and separated by a period of diminished or no setting. Cross-correlation analysis of annual setting patterns among stations revealed three zones in the James River: the upper estuary and entire southwest side, the lower estuary, and a mid-estuary transition zone. Setting pulses tended to be synchronous at stations within each zone, but occurred 1 to 2 weeks later at stations in downriver than in upriver zones. The location of zones is related to known aspects of water circulation in the James River estuary. Moreover, pulse setting itself may be related to the absence of strong vertical salinity gradients accompanying the fortnightly stratification-destratification process.Contribution No. 1213 from the Virginia Institute of Marine Science, School of Marine Science, The College of William and Mary 相似文献