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排序方式： 共有53条查询结果，搜索用时 15 毫秒
1.
Macrobenthic assemblages in Calcasieu Lake estuary (Louisiana) were sampled at 11 sites from October 1983 through November 1985. The sites were numerically dominated by subsurface-deposit feeders, consisting mostly of polychaetes. Greater densities of macrofauna were collected at the northern (upper) stations of the lake than were collected in West Cove or the southern stations. Abundances of polychaetes, oligochaetes, and amphipods at the upper lake stations accounted for most of the differences among stations. The numerical dominance by detritivores (97% of the fauna) and lack of strong sediment or salinity gradients across the estuary, resulted in an absence of temporal pattern in trophic structure of the macrofauna. 相似文献
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Phosphorus Burial in Sediments Along the Salinity Gradient of the Patuxent River,a Subestuary of the Chesapeake Bay (USA) 总被引：2，自引：0，他引：2
We used a sequential extraction technique and 210Pb dating to determine the chemical form and amount of particulate phosphorus (PP) that is retained during burial in 1-m-long
sediment cores collected along a salinity gradient from tidal freshwater to the mesohaline waters of the Patuxent River, a
subestuary of the Chesapeake Bay. PP buried in the study sites with salinity values ≤3 was similar in concentration and form
to PP entering the Patuxent from the watershed, suggesting efficient sequestration by the sediments at these low-salinity
sites. PP extracted with citrate–dithionite–bicarbonate was the dominant form of PP at all salinities and all depths, and
organic-P was the second most abundant fraction. We estimated that 81% of PP entering from the watershed is trapped in the
sediments of the upper Patuxent subestuary and that the subtidal sediments retain three times as much PP as the marshes adjacent
to the study sites. 相似文献
5.
Olivier Henry Michael Ablain Benoit Meyssignac Anny Cazenave Dallas Masters Steve Nerem Gilles Garric 《Journal of Geodesy》2014,88(4):351-361
Determining how the global mean sea level (GMSL) evolves with time is of primary importance to understand one of the main consequences of global warming and its potential impact on populations living near coasts or in low-lying islands. Five groups are routinely providing satellite altimetry-based estimates of the GMSL over the altimetry era (since late 1992). Because each group developed its own approach to compute the GMSL time series, this leads to some differences in the GMSL interannual variability and linear trend. While over the whole high-precision altimetry time span (1993–2012), good agreement is noticed for the computed GMSL linear trend (of $3.1\pm 0.4$ mm/year), on shorter time spans (e.g., ${<}10~\hbox {years}$ ), trend differences are significantly larger than the 0.4 mm/year uncertainty. Here we investigate the sources of the trend differences, focusing on the averaging methods used to generate the GMSL. For that purpose, we consider outputs from two different groups: the Colorado University (CU) and Archiving, Validation and Interpretation of Satellite Oceanographic Data (AVISO) because associated processing of each group is largely representative of all other groups. For this investigation, we use the high-resolution MERCATOR ocean circulation model with data assimilation (version Glorys2-v1) and compute synthetic sea surface height (SSH) data by interpolating the model grids at the time and location of “true” along-track satellite altimetry measurements, focusing on the Jason-1 operating period (i.e., 2002–2009). These synthetic SSH data are then treated as “real” altimetry measurements, allowing us to test the different averaging methods used by the two processing groups for computing the GMSL: (1) averaging along-track altimetry data (as done by CU) or (2) gridding the along-track data into $2^{\circ }\times 2^{\circ }$ meshes and then geographical averaging of the gridded data (as done by AVISO). We also investigate the effect of considering or not SSH data at shallow depths $({<}120~\hbox {m})$ as well as the editing procedure. We find that the main difference comes from the averaging method with significant differences depending on latitude. In the tropics, the $2^{\circ }\times 2^{\circ }$ gridding method used by AVISO overestimates by 11 % the GMSL trend. At high latitudes (above $60^{\circ }\hbox {N}/\hbox {S}$ ), both methods underestimate the GMSL trend. Our calculation shows that the CU method (along-track averaging) and AVISO gridding process underestimate the trend in high latitudes of the northern hemisphere by 0.9 and 1.2 mm/year, respectively. While we were able to attribute the AVISO trend overestimation in the tropics to grid cells with too few data, the cause of underestimation at high latitudes remains unclear and needs further investigation. 相似文献
6.
Measurements of phytoplankton distribution and production, and zooplankton abundance and biomass were made during the summer of 1979 along several shelf-slope transects in the Mid-Atlantic Bight. At the shelf-break, macrozooplankton (>200 μm) grazing was estimated to be sufficient to remove a substantial proportion of daily phytoplankton production. In contrast, on the shelf and in slope waters, where ciliates were abundant, estimates of macrozooplankton grazing indicated a consumption rate less than 15% of the daily primary production. Ciliate grazing, even at non-maximum rates, potentially could have consumed the entire daily primary production in all areas sampled. The findings indicate that the nature of the heterotrophic community is spatially variable in offshore waters even during summer conditions and could influence not only trophodynamic pathways but perhaps nutrient regeneration and recycling. This would be an important consideration in evaluating the fate of particle-bound chemcial species in the water column since fecal pellet producing zooplankton would affect rates of removal and sedimentation in a different manner than ciliates which produce non-compacted digestive debris. 相似文献
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Mountain Lake in Giles County, Virginia, USA, has a documented history of severe natural lake-level changes involving groundwater seepage that extends over the past 4,200?years. The natural lake was full during a large part of the twentieth century but dried up completely in September 2008 and levels have yet to recover. The objective of the study was to understand the hydrogeologic factors that influence lake-level changes using a daily water balance, electrical resistivity, water sampling and geochemical analysis, and well logging. Results from the water balance demonstrate the seasonal response to precipitation of a forested first-order drainage system in fractured rock. The resistivity surveys suggest discrete high-permeability areas may provide pathways for lake drainage. Imagery, well logs, and field observations appear to confirm the presence of a fault which crosses the Eastern Continental Divide to the east of the lake that had not previously been discussed in literature on the lake; the position of the lake within local and intermediate groundwater flow systems is considered. Historical data suggest that either significant precipitation or artificial intervention to mitigate seepage would be required for lake-level recovery in the near future. 相似文献
9.
A global eddy-admitting ocean/sea-ice simulation driven over 1958–2004 by daily atmospheric forcing is used to evaluate spatial
patterns of sea level change between 1993 and 2001. In the present study, no data assimilation is performed. The model is
based on the Nucleus for European Models of the Ocean code at the 1/4° resolution, and the simulation was performed without
data assimilation by the DRAKKAR project. We show that this simulation correctly reproduces the observed regional sea level
trend patterns computed using satellite altimetry data over 1993–2001. Generally, we find that regional sea level change is
best simulated in the tropical band and northern oceans, whereas the Southern Ocean is poorly simulated. We examine the respective
contributions of steric and bottom pressure changes to the total regional sea level changes. For the steric component, we
analyze separately the contributions of temperature and salinity changes as well as upper and lower ocean contributions. Generally,
the model results show that most regional sea level changes arise from temperature changes in the upper 750 m of the ocean.
However, contributions of salinity changes and deep steric changes can be locally important. We also propose a map of ocean
bottom pressure changes. Finally, we assess the robustness of such a model by comparing this simulation with a second simulation
performed by MERCATOR-Ocean based on the same core model, but differing by its short length of integration (1992–2001) and
its surface forcing data set. The long simulation presents better performance over 1993–2001 than the short simulation, especially
in the Southern Ocean where a long adjustment time seems to be needed.
In memory of my little brother Jean-Eudes, whose thirst for science filled out the rich discussions we had about my investigations
and his job as user-service provider for MERCATOR-Ocean. 相似文献
10.
An ichthyoplankton survey (18 stations in seven sampling sectors) was conducted in Narragansett Bay in 1990 to provide information on abundance, distribution, and seasonal occurrence of eggs and larvae of estuarine fishes, including seasonal migrants. An additional goal was to examine changes in species composition, abundance, and distribution occurring since the last baywide survey in 1972–73. The taxonomic composition of eggs and larvae in 1990 (41 species in 25 families from 684 plankton samples) and in 1972–73 (43 species in 28 families from 6900 samples) was similar. Maximum abundance of fish eggs occurred in June and larvae in July, minimum abundance in September to February. Species diversity was greatest in May–July and lowest during January in both surveys. However, egg and larval densities in 1990 were considerably lower than in 1972–73. Bay anchovy, tautog, and cunner accounted for 86% of the eggs and 87% of the larvae in the bay in 1990. These three species accounted for only 55% of the eggs and 51% of the larvae in 1972–73, with menhaden accounting for another 18% of the eggs and 34% of the larvae. Searobins, scup, and butterfish eggs were common in 1973 (19%) but rare in 1990 (2%). Ichthyoplankton abundance for several of the most abundant species was significantly lower (p<0.05) in the Providence River, upper bay, and Greenwich Bay in 1990 than in 1972–73. Density of fish eggs and larvae in the lower portions of the bay was lower in 1990 for some species but not others. Distribution data suggested a general down-bay shift in density in 1990. *** DIRECT SUPPORT *** A01BY085 00015 相似文献