We conducted a quantitative assessment of estuarine ecosystem responses to reduced phosphorus and nitrogen loading from sewage
treatment facilities and to variability in freshwater flow and nonpoint nutrient inputs to the Patuxent River estuary. We
analyzed a 19-year dataset of water quality conditions, nutrient loading, and climatic forcing for three estuarine regions
and also computed monthly rates of net production of dissolved O2 and physical transport of dissolved inorganic nitrogen (DIN) and phosphorus (DIP) using a salt- and water-balance model.
Point-source loading of DIN and DIP to the estuary declined by 40–60% following upgrades to sewage treatment plants and correlated
with parallel decreases in DIN and DIP concentrations throughout the Patuxent. Reduced point-source nutrient loading and concentration
resulted in declines in phytoplankton chlorophyll-a (chl-a) and light-saturated carbon fixation, as well as in bottom-layer O2 consumption for upper regions of the estuary. Despite significant reductions in seaward N transport from the middle to lower
estuary, chl-a, turbidity, and surface-layer net O2 production increased in the lower estuary, especially during summer. This degradation of water quality in the lower estuary
appears to be linked to a trend of increasing net inputs of DIN into the estuary from Chesapeake Bay and to above-average
river flow during the mid-1990s. In addition, increased abundance of Mnemiopsis leidyi significantly reduced copepod abundance during summer from 1990 to 2002, which favored increases in chl-a and allowed a shift in total N partitioning from DIN to particulate organic nitrogen. These analyses illustrate (1) the value
of long-term monitoring data, (2) the need for regional scale nutrient management that includes integrated estuarine systems,
and (3) the potential water quality impacts of altered coastal food webs. 相似文献
The occasional occurrence of brief but intense bursts of cosmic gamma-rays was disclosed through a systematic search of data acquired from the Vela satellites. Confirmation of the nature of the events and additional detail of their characteristics has subsequently been provided by 15 other groups of experimenters with instruments on 13 spacecraft. Thirty-nine such events have been identified from data spanning a period of four and a half years. The record of intensity as a function of time varies considerably for different events, with total durations ranging from 0.1 to 60s. Time-integrated flux density ranges from 10–6 to 10–3 erg cm–2. Spectral measurements have been accomplished by several groups of experimenters, showing a broad maximum in the energy distribution at about 150 keV. The distribution of source directions implies either near galactic or extragalactic locations. The existing data are not sufficient to distinguish between the various models proposed to explain the phenomenon; no model is completely consistent with all observed characteristics.Work performed under the auspices of the United States Energy Research and Development Administration.Paper presented at the COSPAR Symposium on Fast Transients in X-and Gamma-Rays, held at Varna, Bulgaria, 29–31 May, 1975. 相似文献
Little is known about centennial- to millennial-scale climate variability during interglacial times, other than the Holocene. We here present high-resolution evidence from anoxic (unbioturbated) sediments in the eastern Mediterranean Sea that demonstrates a sustained ∼800-yr climate disturbance in the monsoonal latitudes during the Eemian interglacial maximum (∼125 ka BP). Results imply that before and after this event, the Intertropical Convergence Zone (ITCZ) penetrated sufficiently beyond the central Saharan watershed (∼21°N) during the summer monsoon to fuel flooding into the Mediterranean along the wider North African margin, through fossil river/wadi systems that to date have been considered only within a Holocene context. Relaxation in the ITCZ penetration during the intra-Eemian event curtailed this flux, but flow from the Nile - with its vast catchment area - was not affected. Previous work suggests a concomitant Eurasian cooling event, with intensified impact of the higher-latitude climate on the Mediterranean basin. The combined signals are very similar to those described for the Holocene cooling event around 8 ka BP. The apparent type of concurrent changes in the monsoon and higher-latitude climate may reflect a fundamental mechanism for variability in the transfer of energy (latent heat) between the tropics and higher latitudes. 相似文献
The Remote Sensing Core Curriculum (RSCC) was initiated in 1993 to meet the demands for a college-level set of resources to enhance the quality of education across national and international campuses. The American Society of Photogrammetry and Remote Sensing adopted the RSCC in 1996 to sustain support of this educational initiative for its membership and collegiate community. A series of volumes, containing lectures, exercises, and data, is being created by expert contributors to address the different technical fields of remote sensing. The RSCC program is designed to operate on the Internet taking full advantage of the World Wide Web (WWW) technology for distance learning. The issues of curriculum development related to the educational setting, with demands on faculty, students, and facilities, is considered to understand the new paradigms for WW-influenced computer-aided learning. The WWW is shown to be especially appropriate for facilitating remote sensing education with requirements for addressing image data sets and multimedia learning tools. The RSCC is located at http://www.umbc.edu/rscc 相似文献
Vein structures are typically the earliest expression of brittle deformation within sediments. These mud-filled veins, which characteristically occur regularly spaced within bed-parallel arrays, form in sediments that possess a strong interlocking particle framework. Downslope creep has been proposed to explain the origins of vein structures, however, a recent suggestion that they are generated by the passage of earthquake shear waves through sediments explains aspects of their morphology, and their dominant occurrence at active convergent margins. Their coexistence with less disruptive “ghost veins” in Peru margin sediments, and their almost normal attitude to bedding, however, suggests that vein structures were formed by processes more complex than downslope creep, or seismically induced shearing alone.
Experimental earthquake simulation was undertaken by laterally shaking a box containing crushed diatomite. Fractures were induced almost normal to the horizontal shaking direction, and to a lesser extent as antithetic Riedel shears, both of which closely resembled vein structures. The fracturing process during shaking may be viewed as a progressive fragmentation of the diatomite, in which new fractures form half-way between pre-existing ones. Thus fracture spacings are progressively halved. Shear zones oriented at a low angle to the shaking direction were also generated, combining with the high-angle fractures to form structures very similar to those observed in Peru margin sediments. When shaken, fines added to the diatomite segregated into planar zones that resembled ghost veins, half-way between fractures. The alternating pattern of fractures and fines indicated that a standing pressure wave had been created within the box during shaking. The fractures were created by alternating compression and extension at the antinodes, while the fines concentrated in zones of minimum grain movement around the nodal planes. This suggests that vein structures are initiated by the combined action of shear and pressure waves within a sediment. The strain waves may be seismic in origin, or may also form in downslope movement system. 相似文献
Rhabdosargus holubi is a small (maximum weight=2.4?kg) yet important fishery species in the estuaries of the south-east coast of South Africa. Little is known of its biology and specifically its growth rate, which is essential for sustainable management of the fishery. We examined and counted the opaque zones in the sectioned otoliths of 134 R. holubi to determine its age and growth parameters. The otoliths from two recaptured fish marked with oxytetracycline confirmed that one opaque zone was deposited annually. The species reached a maximum age of 18 years and growth was adequately described by a von Bertalanffy growth function of the form: Lt = 358.1 (1 – e?0.24(t+0.77)) mm fork length. There were no significant differences between any of the male and female growth parameters (likelihood ratio test: p = 0.3). The growth was slow (omega index: ω = 86.56); however, despite this, the unique life history of R. holubi may provide a degree of resilience to heavy fishing pressure in estuaries. 相似文献
The West Coast dusky kob Argyrosomus coronus is a commercially exploited fish with a distribution confined to the Angola–Benguela Frontal Zone (ABFZ) of the southeastern Atlantic Ocean. A previous study revealed that during a recent period of local warming the species extended its distribution into Namibian waters, where it hybridised with the resident and congeneric Argyrosomus inodorus. Environmental changes are a major threat to marine biodiversity and when combined with overfishing have the potential to accelerate the decline of species. However, little is known regarding the evolutionary history and population structure of A. coronus across the ABFZ. We investigated genetic diversity, population structure and historical demographic changes using mtDNA control region sequences and genotypes at six nuclear microsatellite loci, from 180 individuals. A single, genetically homogeneous population was indicated across the distributional range of A. coronus (φST = 0.041, FST = 0.000, D = 0.000; p > 0.05). These findings imply that the oceanographic features within the ABFZ do not appear to significantly influence population connectivity in A. coronus, which simplifies management of the species. However, reconstruction of the demographic history points to a close link between the evolutionary history of A. coronus and the environmental characteristics of the ABFZ. This outcome suggests the species’ vulnerability to the rapid environmental changes being observed across this region, and highlights a pressing need for transboundary management to mitigate the impacts of climate change in this global hotspot of seawater temperature changes. 相似文献
Currently available systems for monitoring both negative and positive pore pressures have certain restrictions as to their use. A system design is described that seeks to overcome some of the potentially more restrictive elements insofar as site investigations in tropical slopes are concerned. In particular, the system uses an integrated approach to both pore pressure and ancillary (e.g. raingauge) monitoring. Initial trials of the instrumentation suggest its suitability for pore pressure recording where high temporal resolution is needed in both negative and positive pressures. 相似文献