Hypoxia in Chesapeake Bay, 1950–2001: Long-term change in relation to nutrient loading and river flow     

James?D.?HagyEmail author  Walter?R.?Boynton  Carolyn?W.?Keefe  Kathryn?V.?Wood 《Estuaries and Coasts》2004,27(4):634-658

A 52-yr record of dissolved oxygen in Chesapeake Bay (1950–2001) and a record of nitrate (NO₃ ⁻) loading by the Susquehanna River spanning a longer period (1903, 1945–2001) were assembled to describe the long-term pattern of hypoxia and anoxia in Chesapeake Bay and its relationship to NO₃ ⁻ loading. The effect of freshwater inflow on NO₃ ⁻ loading and hypoxia was also examined to characterize its effect at internannual and longer time scales. Year to year variability in river flow accounted for some of the observed changes in hypoxic volume, but the long-term increase was not due to increased river flow. From 1950–2001, the volume of hypoxic water in mid summer increased substantially and at an accelerating rate. Predicted anoxic volume (DO<0.2 mg I⁻¹) at average river flow increased from zero in 1950 to 3.6×10⁹ m³ in 2001. Severe hypoxia (DO<1.0 mg I⁻¹) increased from 1.6×10⁹ to 6.5×10⁹ m³ over the same period, while mild hypoxia (DO<2.0 mg I⁻¹) increased from 3.4×10⁹ to 9.2×10⁹ m³. NO₃ ⁻ concentrations in the Susquehanna River at Harrisburg, Pennsylvania, increased up to 3-fold from 1945 to a 1989 maximum and declined through 2001. On a decadal average basis, the superposition of changes in river flow on the long-term increase in NO₃ ⁻ resulted in a 2-fold increase in NO₃ ⁻ loading from the Susquehanna River during the 1960s to 1970s. Decadal average loads were subsequently stable through the 1990s. Hypoxia was positively correlated with NO₃ ⁻ loading, but more extensive hypoxia was observed in recent years than would be expected from the observed relationship. The results suggested that the Bay may have become more susceptible to NO₃ ⁻ loading. To eliminate or greatly reduce anoxia will require reducing average annual total nitrogen loading to the Maryland mainstem Bay to 50×10⁶ kg yr⁻¹, a reduction of 40% from recent levels.  相似文献   

A watershed perspective on nutrient enrichment,science, and policy in the Patuxent River,Maryland: 1960–2000     

Christopher?F.?D'EliaEmail author  Walter?R.?Boynton  James?G.?Sanders 《Estuaries and Coasts》2003,26(2):171-185

The Patuxent River, Maryland, is a nutrient-overenriched tributary of the Chesapeake Bay. Nutrient inputs from sewage outfalls and nonpoint sources (NPS) have grown substantially during the last four decades, and chlorophylla levels have increased markedly with concomitant reductions in water quality and dissolved oxygen concentrations. The Patuxent has gained national attention because it was one of the first river basins in the U.S. for which basin-wide nutrient control standards were developed. These included a reduction in NPS inputs and a limit on both nitrogen (N) and phosphorus (P) loadings in sewage discharges intended to return the river to 1950s conditions. Full implementation of point source controls occurred by 1994, but population growth and land-use changes continue to increase total nutrient loadings to the river. The present paper provides the perspectives of scientists who participated in studies of the Patuxent River and its estuary over the last three decades, and who interacted with policy makers as decisions were made to develop a dual nutrient control strategy. Although nutrient control measures have not yet resulted in dramatic increases in water quality, we believe that without them, more extensive declines in water quality would have occurred. Future reductions will have to come from more effective NPS controls since future point source loading will be difficult to further reduce with present technology. Changing land use will present a challenge to policy makers faced with sprawling population growth and accelerated deforestation.  相似文献   

Hard X-ray and gamma-ray flares on the Sun: Stereoscopic effects near the limb from observations on the 2001 <Emphasis Type="Italic">Mars Odyssey</Emphasis> spacecraft and near-Earth spacecraft     

M. A. Livshits  V. A. Chernetskii  I. G. Mitrofanov  A. S. Kozyrev  M. L. Litvak  A. B. Sanin  V. I. Tret’yakov  W. Boynton  K. Shinohara  D. Hamara 《Astronomy Reports》2005,49(11):916-931

We present the first results of data on solar flares detected by the HEND instrument onboard the 2001 Mars Odyssey interplanetary spacecraft during its flight to Mars and in orbit around Mars. The instruments carried by the spacecraft, which was developed at the Space Research Institute of the Russian Academy of Sciences, included a scintillation detector with two crystals, enabling the detection of photons with energies from tens of keV to 2.5 MeV with high time resolution. Several dozen flares were detected on both the sides visible from the Earth and back side of the Sun, supplementing other available data in a number of cases. A joint analysis of the HEND data together with data obtained in near-Earth orbits enabled the detection of variations in the integrated fluxes of photons with energies exceeding 80 keV during observations of flares near the limb from various directions. Two events were analyzed in great detail: the setting of a region displaying frequent very short flares on May 20, 2001, and the rising of the group 10486, which displayed numerous flare phenomena on the limb followed by extremely high activity in October–November 2003. These variations appear in simultaneous observations of limb M flares made at angles differing by only 8°–10°. Analyses of observations of rising sources obtained on two spacecraft lead to similar results. This indicates that the vast majority of emission at energies exceeding 80 keV arises at altitudes of no higher than seven to ten thousand kilometers. We briefly consider the powerful solar-disk gamma-ray flare of August 25, 2001. In this case, there are some differences in the behavior of the hard radiation in the decay phase for observations made at angles differing by 25°, which is most likely due to differences in the instrumental responses to radiation with this spectrum. The absence of hard radiation at great heights in the region of the “cusp” places some constraints on our picture of the physical processes occurring in powerful solar flares.  相似文献   

Compositional evidence regarding the influx of interplanetary materials onto the lunar surface     

John T. Wasson  William V. Boynton  Chen-Lin Chou  Philip A. Baedecker 《Earth, Moon, and Planets》1975,13(1-3):121-141

Siderophilic element/Ir ratios are higher in mature lunar soils from highlands sites than in those from mare sites. We infer that the population of materials responsible for the early intense bombardment of the Moon had high ratios, and that the population responsible for the essentially constant flux has low ratios. No group of chondrites has siderophile/Ir ratios identical to those in the mare or highlands soils; CM chondrites are the most similar, and CM-like materials may account for a major fraction of Earth-crossing materials during the past 3.7 b.y.Siderophile/Ir ratios may be used to determine the amount of highlands regolith in soils or breccias from the mare-highlands interface areas (Apollo 15 and 17), and to infer the time of formation of highlands breccias whose sideropbiles originated in mature soils. Arguments are summarized against the viewpoint that the siderophiles in most highlands breccias originated in basin-forming projectiles. Differences in mature soil siderophile concentrations at Apollo 14 and 16 indicate a substantially greater concentration at the latter site immediately following the Imbrium event.Siderophile concentrations are used to estimate mean regolith depths at the landing sites; as relative values these are more precise than estimates based on seismic or crater observations. The longlived flux is calculated to be 2.9 g cm^–2 b.y.^–1 averaged over the past 3.7 b.y. A consideration of the relationship between mass fluence and time indicates that the mass flux decreased with a half-life of about 40 m.y. immediately following the Imbrium event.  相似文献   

Element distribution in size fractions of Apollo-16 soils: Evidence for element mobility during regolith processes     

W.V. Boynton  C.-L. Chou  R.W. Bild  P.A. Baedecker  J.T. Wasson 《Earth and Planetary Science Letters》1976,29(1):21-33

Three Apollo-16 soils, 61220, 63500 and 65500, having diverse properties were separated into six size fractions and analyzed for 8 volatiles and siderophiles. Relative concentrations of an additional 20 elements were determined in 61220 and 63500. The volatile elements Cd, Zn, In and Ga increase in concentration with decreasing grain size; in the finest fractions the increase is roughly parallel to the increase in specific surface area, and a surface correlation is inferred. The total increase from coarsest (177–500 μm) to finest (<5 μm) fraction is by factors of 10–20 for Zn and Cd in soils 61220 and 63500. Concentrations of elements that are good indicators of KREEP (Sm), mare basalts (Fe) and anorthosites (Ca) show nearly no dependence on size. The preservation of the observed surface correlation throughout regolith evolution suggests that the volatile elements are labile on the lunar surface.Concentration-size distributions of siderophiles show peaks in the 80–300 μm range for each soil, independent of whether they are dominantly extralunar (Ni, Ge, Au, Ir) or lunar (Co) in origin. If this peak results from agglutinate formation, a viable mechanism must allow for incorporation of the extralunar siderophiles. Alternatively, the peak may result from a continuous growth of metal grain size during the evolution of the regolith.  相似文献   

Distribution of 28 elements in size fractions of lunar mare and highlands soils     

William V. Boynton  John T. Wasson 《Geochimica et cosmochimica acta》1977,41(8):1073-1082

Four volatile, six siderophile and 18 generally lithophile elements were determined in six sieve fractions of mare soil 15100 and seven sieve fractions of highlands soil 66080; 15100 is a moderately and 66080 a highly mature soil.Two size fractions of 66080 were subjected to leaching with HCI and etching with HF. Leaching removed ca. 25% of the rare earths in both the 500-177 μm and 62-20 μm fractions; the soluble phase, probably a phosphate, is enriched in light rare earths relative to the bulk soil. The leach and etch removed a larger portion of Zn and Cd than expected on the basis of surface concentrations inferred from size distribution data apparently because of selective dissolution of minor volatile-rich phases.Lithophile concentrations in 66080 are nearly independent of grain size. In 15100 decreasing grain sizes show moderately increasing amounts of KREEP and anorthosite related elements, and decreasing amounts of basalt related elements. In 66080 a maximum in siderophile concentration occurs at ca 150 μm, as previously observed in our studies of 61220, 63500 and 65500. This peaking appears to result from a gradual increase with time in the size of metal grains as a result of welding during micrometeorite impacts. The coarse fraction maximum is not observed in the siderophile data for 15100, probably because of the much smaller fluence of extralunar projectiles at the Apollo 15 site. A modest rise in siderophile concentrations in the smallest size fractions of all soils probably results from recondensation of impact-vaporized materials.The concentrations of highly volatile Zn, Cd and In in 15100 and 66080 show a marked increase with decreasing size, but the fine/coarse ratios are about a factor of two lower than those in soils 61220 and 63500. The lower ratio in 66080 results entirely from higher concentrations in the coarser fractions. It appears that this is a reflection of the higher maturity of 66080, and that the volume-correlated component in lunar soils increases with increasing near-surface residency. The high amount of volume-correlated component in 15100 may be related to the more efficient formation of agglutinates in basalt-rich soils. The observed increase in rare gas and volatile metal concentration with decreasing grain size results from an increasing bias in surface exposure of fine grain sizes, probably as a result of the adhesion of smaller to larger grains.  相似文献   

A review of recent developments in estuarine scalar flux estimation   总被引：1，自引：0，他引：1  

D. A. Jay  R. J. Uncles  J. Largeir  W. R. Geyer  J. Vallino  W. R. Boynton 《Estuaries and Coasts》1997,20(2):262-280

The purpose of this contribution is to review recent developments in calculation of estuarine scalar fluxes, to suggest avenues for future improvement, and to place the idea of flux calculation in a broader physical and biogeochemical context. A scalar flux through an estuarine cross section is the product of normal velocity and scalar concentration, sectionally integrated and tidally averaged. These may vary on interannual, reasonal, tidal monthly, and event time scales. Formulation of scalar fluxes in terms of an integral scalar conservation expression shows that they may be determined either through “direct” means (measurement of velocity and concentration) or by “indirect” inference (from changes in scalar, inventory and source/sink terms). Direct determination of net flux at a cross section has a long and generally discouraging history in estuarine oceanography. It has proven difficult to extract statistically significant net (tidally averaged) fluxes from much larger flood and ebb transports, and the best mathematical representation of flux mechanisms is unclear. Observations further suggest that both lateral and vertical variations in scalar transport through estuarine cross sections are large, while estuarine circulation theory has focused on two-dimensional analyses that treatment either vertical or lateral variations but not both. Indirect estimates of net fluxes by determination of the other relevant terms in an integral scalar conservation balance may be the best means of determining scalar import-export in systems with residence times long relative to periods of tidal monthly fluctuations. But this method offers, little insight into the interaction of circulation modes and scalar fluxes, little help in verifying predictive models, and may also be difficult to apply in some circumstances. Thus, the need to understand, measure, and predict anthropogenic influences on transport or carbon, nutrient, suspended matter, trace metals, and other substances across the land-margin brings a renewed urgency to the issue of how to best carry out estuarine scalar flux determination. An interdisciplinary experiment is suggested to test present understanding, available instrument, and numerical models.  相似文献   

GPS and GLONASS Product News     

Frank Boynton  Keith McDonald 《GPS Solutions》1998,2(1):62-65

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GRS evidence and the possibility of paleooceans on Mars     

James M. Dohm  Victor R. Baker  William V. Boynton  Alberto G. Fairén  Justin C. Ferris  Michael Finch  Roberto Furfaro  Trent M. Hare  Daniel M. Janes  Jeffrey S. Kargel  Suniti Karunatillake  John Keller  Kris Kerry  Kyeong J. Kim  Goro Komatsu  William C. Mahaney  Dirk Schulze-Makuch  Lucia Marinangeli  Gian G. Ori  Javier Ruiz  Shawn J. Wheelock 《Planetary and Space Science》2009,57(5-6):664-684

The Gamma Ray Spectrometer (Mars Odyssey spacecraft) has revealed elemental distributions of potassium (K), thorium (Th), and iron (Fe) on Mars that require fractionation of K (and possibly Th and Fe) consistent with aqueous activity. This includes weathering, evolution of soils, and transport, sorting, and deposition, as well as with the location of first-order geomorphological demarcations identified as possible paleoocean boundaries. The element abundances occur in patterns consistent with weathering in situ and possible presence of relict or exhumed paleosols, deposition of weathered materials (salts and clastic minerals), and weathering/transport under neutral to acidic brines. The abundances are explained by hydrogeology consistent with the possibly overlapping alternatives of paleooceans and/or heterogeneous rock compositions from diverse provenances (e.g., differing igneous compositions).  相似文献   

Sediment-water oxygen and nutrient exchanges along the longitudinal axis of Chesapeake Bay: Seasonal patterns,controlling factors and ecological significance   总被引：2，自引：0，他引：2  

Jean L. W. Cowan  Walter R. Boynton 《Estuaries and Coasts》1996,19(3):562-580

Sediment-water oxygen and nutrient (NH₄ ⁺, NO₃ ^?+NO₂ ^?, DON, PO₄ ^3?, and DSi) fluxes were measured in three distinct regions of Chesapeake Bay at monthly intervals during 1 yr and for portions of several additional years. Examination of these data revealed strong spatial and temporal patterns. Most fluxes were greatest in the central bay (station MB), moderate in the high salinity lower bay (station SB) and reduced in the oligohaline upper bay (station NB). Sediment oxygen consumption (SOC) rates generally increased with increasing temperature until bottom water concentrations of dissolved oxygen (DO) fell below 2.5 mg l^?1, apparently limiting SOC rates. Fluxes of NH₄ ⁺ were elevated at temperatures >15°C and, when coupled with low bottom water DO concentrations (<5 mg l^?1), very large releases (>500 μmol N m^?2 h^?1) were observed. Nitrate + nitrite (NO₃ ^?+NO₂ ^?) exchanges were directed into sediments in areas where bottom water NO₃ ^?+NO₂ ^? concentrations were high (>18 μM N); sediment efflux of NO₃ ^?+NO₂ ^? occurred only in areas where bottom water NO₃ ^?+NO₂ ^? concentrations were relatively low (<11 μM N) and bottom waters well oxygenated. Phosphate fluxes were small except in areas of hypoxic and anoxic bottom waters; in those cases releases were high (50–150 μmol P m^?2 h^?1) but of short duration (2 mo). Dissolved silicate (DSi) fluxes were directed out of the sediments at all stations and appeared to be proportional to primary production in overlying waters. Dissolved organic nitrogen (DON) was released from the sediments at stations NB and SB and taken up by the sediments at station MB in summer months; DON fluxes were either small or noninterpretable during cooler months of the year. It appears that the amount and quality of organic matter reaching the sediments is of primary importance in determining the spatial variability and interannual differences in sediment nutrient fluxes along the axis of the bay. Surficial sediment chlorophyll-a, used as an indicator of labile sediment organic matter, was highly correlated with NH₄ ^?, PO₄ ^3?, and DSi fluxes but only after a temporal lag of about 1 mo was added between deposition events and sediment nutrient releases. Sediment O:N flux ratios indicated that substantial sediment nitrification-denitrification probably occurred at all sites during winter-spring but not summer-fall; N:P flux ratios were high in spring but much less than expected during summer, particularly at hypoxic and anoxic sites. Finally, a comparison of seasonal N and P demand by phytoplankton with sediment nutrient releases indicated that the sediments provide a substantial fraction of nutrients required by phytoplankton in summer, but not winter, especially in the mid bay region.  相似文献