articles+ search results

We assess the performance of three different algorithms for estimating surface ocean currents from two linear array HF radar systems. The delay-and-sum beamforming algorithm, commonly used with beamforming systems, is compared with two direction-finding algorithms: Multiple Signal Classification (MUSIC) and direction finding using beamforming (Beamscan). A 7-month dataset from two HF radar sites (CSW and GTN) on Long Bay, South Carolina (United States), is used to compare the different methods. The comparison is carried out on three locations (midpoint along the baseline and two locations with in situ Eulerian current data available) representing different steering angles. Beamforming produces surface current data that show high correlation near the radar boresight (R2 ≥ 0.79). At partially sheltered locations far from the radar boresight directions (59° and 48° for radar sites CSW and GTN, respectively) there is no correlation for CSW (R2 = 0) and the correlation is reduced significantly for GTN (R2 = 0.29). Beamscan performs similarly near the radar boresight (R2 = 0.8 and 0.85 for CSW and GTN, respectively) but better than beamforming far from the radar boresight (R2 = 0.52 and 0.32 for CSW and GTN, respectively). MUSIC's performance, after significant tuning, is similar near the boresight (R2 = 0.78 and 0.84 for CSW and GTN) while worse than Beamscan but better than beamforming far from the boresight (R2 = 0.42 and 0.27 for CSW and GTN, respectively). Comparisons at the midpoint (baseline comparison) show the largest performance difference between methods. Beamforming (R2 = 0.01) is the worst performer, followed by MUSIC (R2 = 0.37) while Beamscan (R2 = 0.76) performs best. [ABSTRACT FROM AUTHOR]

Marine biology, limnology, Biologie marine, limnologie, Ecology, Ecologie, Environment, Environnement, Sciences biologiques et medicales, Biological and medical sciences, Sciences biologiques fondamentales et appliquees. Psychologie, Fundamental and applied biological sciences. Psychology, Ecologie animale, vegetale et microbienne, Animal, plant and microbial ecology, Ecologie animale et végétale, Animal and plant ecology, Synécologie, Synecology, Ecosystèmes dulçaquicoles, Fresh water ecosystems, Ecosystèmes saumâtres, Brackish water ecosystems, Biogéochimie, Biogeochemistry, Biogeoquímica, Dénitrification, Denitrification, Desnitrificación, Marée, Tides, Marea, Milieu eau douce, Freshwater environment, Medio agua dulce, Milieu saumâtre, Brackish water environment, Medio salobre, Zone humide, Wetland, and Zona húmeda

Tidal freshwater wetlands (TFW) alter nitrogen concentrations in river water, but the role of these processes on a river's downstream nitrogen delivery is poorly understood. We examined spatial and temporal patterns in denitrification in TFW of four rivers in North Carolina, USA and evaluated the relative importance of denitrification rate and inundation on ecosystem-scale N2 efflux. An empirical model of TFW denitrification was developed to predict N2 efflux using a digital topographic model of the TFW, a time series of water level measurements, and a range of denitrification rates. Additionally, a magnitude-frequency analysis was performed to investigate the relative importance of storm events on decadal patterns in N2 efflux. Spatially, inundation patterns exerted more influence on N2 efflux than did the range of denitrification rate used. Temporal variability in N2 efflux was greatest in the lower half of the tidal rivers (near the saline estuary) where inundation dynamics exerted more influence on N2 efflux than denitrification rate. N2 efflux was highest in the upper half of the rivers following storm runoff, and under these conditions variation in denitrification rate had a larger effect on N2 efflux than variability in inundation. The frequency-magnitude analysis predicted that most N2 efflux occurred during low flow periods when tidal dynamics, not storm events, affected TFW inundation. Tidal hydrology and riparian topography are as important as denitrification rate in calculating nitrogen loss in TFW; we present a simple empirical model that links nitrogen transport in rivers with loss due to denitrification in TFW.

Geology, Géologie, Sciences exactes et technologie, Exact sciences and technology, Terre, ocean, espace, Earth, ocean, space, Sciences de la terre, Earth sciences, Hydrologie. Hydrogéologie, Hydrology. Hydrogeology, Hydrologie, Hydrology, Géologie de l'ingénieur et de l'environnement. Géothermie, Engineering and environment geology. Geothermics, Risques naturels: prévisions, dommages, etc, Natural hazards: prediction, damages, etc, Amérique du Nord, North America, America del norte, Bassin versant, drainage basins, Cuenca, Eau pluie, rain water, Agua llovediza, Eau surface, surface water, Agua superficie, Ecoulement, flow, Hydraulique, hydraulics, Hidraúlica, Hydrodynamique, hydrodynamics, Hidrodinámica, Ile, islands, Isla, Ligne partage eau, drainage divide, Ouragan, hurricanes, Huracán, Pluie, rainfall, Lluvia, Profondeur, depth, Profundidad, Vent, winds, Viento, Vitesse, velocity, Velocidad, Végétation, vegetation, Vegetación, Zone humide, wetlands, Terreno húmedo, Hurricane, Water velocity, Wetland, and Wind velocity

Between 2001 and 2005, seven category 3 or higher major hurricanes made landfall within the US. The hydrologic impacts of these distinct climatic phenomena frequently occurring in wetland watersheds, however, are not well understood. The focus of this study was to evaluate the impacts of hurricane wind and rainfall conditions on water velocity and water elevations within the study wetland, the Florida Everglades. Specifically water velocity data was measured near two tree islands (Gumbo Limbo (GL) and Satin Leaf (SL)) and wind speed, water elevation, and rainfall were obtained from nearby wind observation stations. During the direct impacts of the hurricanes (Hurricanes Katrina and Wilma), water speed, flow direction, and hydraulic gradients were altered, and the extent of variation was positively related to wind characteristics, with significant alterations in flow direction at depth during Hurricane Wilma due to higher wind speeds. After the direct impacts, the longer lasting effect of hurricanes (time scale of a few days) resulted in altered flow speeds that changed by 50% or less. These longer lasting changes in flow speeds may be due to the redistribution of emergent vegetation.

Marine biology, limnology, Biologie marine, limnologie, Ecology, Ecologie, Environment, Environnement, Sciences biologiques et medicales, Biological and medical sciences, Sciences biologiques fondamentales et appliquees. Psychologie, Fundamental and applied biological sciences. Psychology, Ecologie animale, vegetale et microbienne, Animal, plant and microbial ecology, Ecologie animale et végétale, Animal and plant ecology, Synécologie, Synecology, Généralités, General aspects, Eau surface, Surface water, Agua superficie, Ecoulement eau, Water flow, Flujo agua, Hydrobiologie, Hydrobiology, Hidrobiología, Hydrodynamique, Hydrodynamics, Hidrodinámica, Milieu aquatique, Aquatic environment, Medio acuático, Parc national, National park, Parque nacional, Paysage, Landscape, Paisaje, Transport, Transporte, Everglades, flow, hydrodynamics, sawgrass, slough, and spike rush

Over the last one hundred years, compartmentalization and water management activities have reduced water flow to the ridge and slough landscape of the Everglades. As a result, the once corrugated landscape has become topographically and vegetationally uniform. The focus of this study was to quantify variation in surface flow in the ridge and slough landscape and to relate flow conditions to particulate transport and deposition. Over the 2002-2003 and 2003-2004 wet seasons, surface velocities and particulate accumulation were measured in upper Shark River Slough in Everglades National Park. Landscape characteristics such as elevation, plant density and biomass also were examined to determine their impact on flow characteristics and material transport. The results of this study demonstrate that the release of water during the wet season not only increases water levels, but also increased flow speeds and particulate transport and availability. Further, flow speeds were positively and significantly correlated with water level thereby enhancing particulate transport in sloughs relative to ridges especially during peak flow periods. Our results also indicate that the distribution of biomass in the water column, including floating plants and periphyton, affects velocity magnitude and shape of vertical profiles, especially in the sloughs where Utricularia spp. and periphyton mats are more abundant. Plot clearing experiments suggest that the presence of surface periphyton and Utricularia exert greater control over flow characteristics than the identity (i.e., sawgrass or spike rush) or density of emergent macrophytes, two parameters frequently incorporated into models describing flow through vegetated canopies. Based on these results, we suggest that future modeling efforts must take the presence of floating biomass, such as Utricularia, and presence of periphyton into consideration when describing particulate transport.

Geology, Géologie, Sciences exactes et technologie, Exact sciences and technology, Terre, ocean, espace, Earth, ocean, space, Sciences de la terre, Earth sciences, Hydrologie. Hydrogéologie, Hydrology. Hydrogeology, Amérique du Nord, North America, America del norte, Accumulation, accumulation, Altération, alteration, Alteración, Arbre, trees, Concentration, concentration, Concentración, Creusement chenal, channelization, Croissance, growth, Densité, density, Densidad, Distribution dimension, size distribution, Ile, islands, Isla, Matière en suspension, suspended materials, Materia en suspensión, Parc national, national parks, Particule, particles, Paysage, landscapes, Paisaje, Préservation, preservation, Rivière, rivers, Río, Suspension, suspension, Suspensión, Transport sédiment, sediment transport, Turbidité, turbidity, Turbiedad, Vitesse, velocity, Velocidad, Végétation, vegetation, Vegetación, Zone humide, wetlands, Terreno húmedo, Everglades, Restoration, Suspended solids, Tree islands, and Water velocity

Tree islands in the Shark River Slough of the Everglades National Park (ENP), in the southern state of Florida in the United States, are part of a wetland system of densely vegetated ridges interspersed within relatively open sloughs. Human alteration of this system has had dramatic negative effects on the landscape of the region and restoration efforts will require adjusting the hydrology of the region to assure the preservation of these important ecologic features. The primary objectives of this study were to document the hydrology in the vicinity of tree islands in ENP by measuring velocities in time and space and by characterizing suspended sediments. The results of such measurements were interpreted with respect to factors that may limit tree island growth. The measurements were conducted in the vicinity of three tree islands known as Black Hammock (BH), Gumbo Limbo (GL), and an unnamed island that was named for this study as Satin Leaf (SL). Acoustical Doppler Velocity (ADV) meters were used for measuring the low velocities of the Everglades water flow. Properties of suspended sediments were characterized through measurements of particle size distribution, turbidity, concentration and particle density. Mean velocities observed at each of the tree islands varied from 0.9 to 1.4 cm/s. Slightly higher mean velocities were observed during the wet season (1.2-1.6 cm/s) versus the dry season (0.8-1.3cm/s). Maximum velocities of more than 4 cm/s were measured in areas of Cladium jamaicense die-off and at the hardwood hammock (head) of the islands. At the island's head, water is channelized around obstructions such as tree trunks in relatively rapid flow, which may limit the lateral extent of tree island growth. Channelization is facilitated by shade from the tree canopy, which limits the growth of underwater vegetation thereby minimizing the resistance to flow and limiting sediment deposition. Suspended sediment concentrations were low (0.5-1.5 mg/L) at all study sites and were primarily of organic origin. The mean particle size of the suspended sediments was 3 μm with a distribution that was exponential. Critical velocities needed to cause re-suspension of these particles were estimated to be above the actual velocities observed. Sediment transport within the water column appears to be at a near steady state during the conditions evaluated with low rates of sediment loss balanced by presumably the release of equivalent quantities of particles of organic origin. Existing hydrologic conditions do not appear to transport sufficient suspended sediments to result in the formation of tree islands. Of interest would be to collect hydrologic and sediment transport data during extreme hydrologic events to determine if enough sediment is transported under these conditions to promote sufficient sediment accumulations.

Marine biology, limnology, Biologie marine, limnologie, Ecology, Ecologie, Environment, Environnement, Sciences biologiques et medicales, Biological and medical sciences, Sciences biologiques fondamentales et appliquees. Psychologie, Fundamental and applied biological sciences. Psychology, Ecologie animale, vegetale et microbienne, Animal, plant and microbial ecology, Ecologie animale et végétale, Animal and plant ecology, Synécologie, Synecology, Ecosystèmes saumâtres, Brackish water ecosystems, Estuaire, Estuaries, Estuario, Marais salé, Salt marsh, Salina, Milieu saumâtre, Brackish water environment, Medio salobre, Sable, Sand, Arena, Nouvelle Zélande Ile Nord, North Island, and Nueva Zelanda Isla Norte

The objective of this study was to determine if the placement of dredged material on sediment-starved back barrier marshes in southeastern North Carolina could offset submergence without negatively affecting function. Clean sediment was placed in thickness from 0 to 10 cm on deteriorated and non-deteriorated marsh plots. Original stem densities were greater in non-deteriorated plots (256 stems m-2) compared to deteriorated sites (149 stems m-2). By the second growing season (after sediment additions), stem densities in the deteriorated plots (308 stems m-2) approached levels in the non-deteriorated plots (336 stems m-2). Sediment additions to both non-deteriorated and deteriorated plots resulted in a higher redox potential with plots receiving the most sediment exhibiting the highest Eh values. In deteriorated plots, placement of dredged material had the greatest effect on plant density, but also affected soil oxidation-reduction potential and sediment deposition (or mobility). Following sediment placement, substrate texture and composition incrementally returned to prefill conditions due to a combination of bioturbation and sedimentation. Where infaunal differences occurred, they were generally less abundant in deteriorated plots, but responses to sediment addition were variable. Sediment addition had little effect on the non-deteriorated plots, suggesting that the disposal of certain types of dredged material in marshes may be useful to mitigate the effects of marsh degradation without adversely affecting non-deteriorating marsh.

Marine biology, limnology, Biologie marine, limnologie, Ecology, Ecologie, Environment, Environnement, Oceanography, Océanographie, Sciences biologiques et medicales, Biological and medical sciences, Sciences biologiques fondamentales et appliquees. Psychologie, Fundamental and applied biological sciences. Psychology, Ecologie animale, vegetale et microbienne, Animal, plant and microbial ecology, Ecologie animale et végétale, Animal and plant ecology, Synécologie, Synecology, Ecosystèmes saumâtres, Brackish water ecosystems, Milieu saumâtre, Brackish water environment, Medio salobre, Modèle, Models, Modelo, Plateforme continentale, Continental shelf, Plataforma continental, Sédiment, Sediments, Sedimento, Transport, Transporte, Baie Carolina, Baie Onslow, Hurricane Isabel, bottom boundary layer model, cross-shelf transport, and sediment transport

Hurricane Isabel made landfall on the southeastern U.S. coast along the lower Outer Banks of North Carolina near Cape Lookout, NC on September 18, 2003. An instrumented quadrapod frame moored approximately 43.5 km off the coast of Wilmington, NC in Onslow Bay, at a depth of approximately 30 m, was present on the shelf when the hurricane passed approximately 160 km from Onslow Bay. The attached instrumentation includes a downward-looking Pulse-Coherent Acoustic Doppler Profiler (PC-ADP) and an upward-looking Acoustic Doppler Current Profiler (ADCP). Simultaneous measurements of flow velocities from the surface to the seabed along with acoustic backscatter measurements and seabed elevation were obtained during this storm event. Three days prior to the direct effects of the hurricane, long-period swells (14-17 s) began to impact the area causing sediment transport to occur on the mid-shelf. Shear velocities calculated from a bottom boundary layer model ranged from approximately 6 to 10 cm s-1 over the course of the sediment transport event. Based on shear velocity calculations, bedload and suspended sediment transport were occurring at the site 72 h prior to the passage of the hurricane as long-period swells reached heights of 4 m. The seabed altimeter recorded large fluctuations during the storm but was unable to trace elevation changes due to the presence of a highly concentrated layer of suspended sediments in the lower 10 cm of the bottom boundary layer. Large amounts of suspended sediment transport occurred in the along-shelf direction towards the southwest after tropical storm force winds of 30 m s-1 began to directly affect the area and wind-driven currents were generated. Wind-driven, subtidal currents responded rapidly to local wind forcing and shifted from 15 cm s-1 towards the southwest to 16 cm s-1 towards the northeast. Model calculated velocity profiles compared well with measured current velocity profiles. Boxcores collected pre- and post-storm show a storm layer of approximately 5 cm and a sediment trap that was attached to the frame 30 cm above the bed collected approximately 400 cm3 of fine to medium sands that were suspended during the storm.

Marine biology, limnology, Biologie marine, limnologie, Ecology, Ecologie, Environment, Environnement, Oceanography, Océanographie, Sciences biologiques et medicales, Biological and medical sciences, Sciences biologiques fondamentales et appliquees. Psychologie, Fundamental and applied biological sciences. Psychology, Ecologie animale, vegetale et microbienne, Animal, plant and microbial ecology, Ecologie animale et végétale, Animal and plant ecology, Synécologie, Synecology, Ecosystèmes marins, Sea water ecosystems, Bivalvia, Invertebrata, Mollusca, Crassostrea virginica, Milieu marin, Marine environment, Medio marino, Qualité eau, Water quality, Calidad agua, Bioremediation, North Carolina, Oysters, and Tidal creeks

The Eastern oyster, Crassostrea virginica, may improve water quality by filtering large quantities of particulate matter (both organic and inorganic) and nutrients from the overlying water column. Additionally, oyster reefs alter hydrodynamic conditions, further increasing the removal of particulate matter from the water column. This study examined the effects of small-scale oyster additions on sediment loading, chlorophyll a, nutrient concentrations, and flow in small tidal creeks. Two reefs were established in Hewletts Creek, New Hanover County, North Carolina. Total suspended solids (TSS), chlorophyll a, and ammonium were measured upstream and downstream of each created reef and in an adjacent control channel that lacked a reef. Data were collected monthly during ebb tides over a 10-month period between September 2000 and June 2001. In the first month after initial reef placement, mean TSS concentrations downstream of reef placement were slightly lower than those upstream of the reef. Although not statistically significant, TSS concentrations downstream of the reefs were less than upstream concentrations for five out of nine and five out of seven post-reef sampling months for the upland and the lower creek sites, respectively. Chlorophyll a concentrations were not significantly affected by initial reef placement (2 3 m), but were reduced substantially after reef enlargement (3 × 4 m) in one of the experimental creeks. Reef placement resulted in significant increases in ammonium concentrations downstream of the transplanted-reefs. In addition, deposition of feces and pseudofeces by the oysters resulted in accumulation of finer-grained materials in the treated channel relative to the control channels. Oyster filtration was most effective three hours following high tide, when the ratio of flow discharge to reef surface area was the highest. This work demonstrates that small oyster reefs established and maintained in some small tributary channels can reduce TSS and chlorophyll a concentrations and that the magnitude of the effect may vary over the course of the tidal cycle.