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51.
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Gary C. Barbee 《Ground Water Monitoring & Remediation》1994,14(1):129-140
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Nutrient enrichments and phytoplankton growth in the surface waters of the Louisiana Bight 总被引:1,自引:0,他引:1
Nitrate concentrations have increased twofold in the Mississippi River during the past three decades. The increased nitrogen loading to the Louisiana shelf has been postulated as a factor leading to eutrophication and the subsequent development of hypoxia west of the Mississippi River delta. While ratios of nitrogen:phosphorus and nitrogen:silica are relatively high in surface waters on the western Louisiana shelf, nitrogen has been posed as the ‘limiting’ nutrient in this region. Bioassays were performed with nutrient additions to surface waters collected from the Louisiana shelf to examine the potential for specific nutrient limitation. Experiments were conducted in March and September 1991, and May 1992. The growth responses of natural and cultured phytoplankton populations were determined by measuring the time course of in vivo and 3-(3,4 dichlorophenyl)-1, 1-dimethylurea (DCMU)-induced fluorescence, as well as initial and final chlorophylla concentrations. The results suggest that phosphate and silicate potentially limit phytoplankton growth during the winter-spring, particularly at low salinities. In late summer, in contrast, nitrogen limitation may be prominent at higher salinities. 相似文献
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Activities of 238U, 228Ra, 226Ra and 210Pb were determined in submarine hydrothermal massive sulfides by nondestructive, gamma-ray spectrometry. The samples were collected by the manned submersible DSRV “Alvin” from active hydrothermal fields on the Endeavour Segment and Axial Seamount of the Juan de Fuca Ridge. 210Pb activities are mostly below the equilibrium level with 226Ra, which is linearly correlated with the concentration of Ba, Sr and Ca in the deposits. The ratios of226Ra to the alkaline earth elements indicate that hydrothermal alteration of the underlying oceanic crust is the dominant source of Ra in the sulfide deposits.The 210Pb/Pb ratios measured in many of the sulfides are higher than the ratio obtained for a basalt source of 210Pb, probably because of 210Pb ingrowth from 226Ra within the sulfide deposits. An isochron approach employing ratios of 210Pb/Pb and 226Ra/Pb yielded an initial 210Pb/Pb ratio in the range of 0–0.57 dpm/μg Pb, implying that the crustal residence time of the hydrothermal fluid in the Endeavour Segment is very short ( < 10 years) and that basalt alteration is the only source of the 210Pb and Pb. For the hydrothermal fluid in Axial Seamount, a residence time of 10–20 years is estimated on the basis of the initial 210Pb/Pb and 228Ra/226Ra ratios of a sulfide chimney. Both residence time estimates are consistent with the results of previous studies. We have estimated the upper limits of sample ages ranging from tens to a hundred years for 12 samples, with the remaining seven samples indicating ages close to or older than 150 years, the practical limit of the 210Pb/Pb dating technique.A concentrically sectioned sulfide chimney from Axial Seamount yielded mean radial growth rates ranging from a few tenths to a few mm/year based on gradients of 210Pb/Pb and 228Ra/226Ra activity ratios, whereas a similarly sectioned chimney from the Endeavour Segment, although collected from the top of an active “black smoker” vent, displayed no 210Pb or 228Ra activity. Our results have implications for the duration and periodicity of hydrothermal circulation and associated mineral deposition at mid-ocean ridges. 相似文献
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Water samples were collected from 18 natural springs within the West Fork of the Obey River watershed. Overton County, Tennessee,
to determine if groundwater was adversely affected by runoff from abandoned surface coal mines Six springs were found to be
affected severely and deemed unfit as a source of potable water Water quality of the remaining springs was essentially unaffected
it appeared that proximity to surface mines, elevation at the outflow, and geology of the surrounding strata determined the
quality of the groundwater
The unit is jointly supported by Tennessee Technological University, the Tennessee Wildlife Resource Agency, and the US. Fish
and Wildlife Service 相似文献