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1.
In many areas of the North American mid-Atlantic coast, seagrass beds are either in decline or have disappeared due, in part, to high turbidity that reduces the light reaching the plant surface. Because of this reduction in the areal extent of seagrass beds there has been a concomitant diminishment in dampening of water movement (waves and currents) and sediment stabilization. Due to ongoing declines in stocks of suspension-feeding eastern oysters (Crassostrea virginica) in the same region, their feeding activity, which normally serves to improve water clarity, has been sharply reduced. We developed and parameterized a simple model to calculate how changes in the balance between sediment sources (wave-induced resuspension) and sinks (bivalve filtration, sedimentation within seagrass beds) regulate turbidity. Changes in turbidity were used to predict the light available for seagrass photosynthesis and the amount of carbon available for shoot growth. We parameterized this model using published observations and data collected specifically for this purpose. The model predicted that when sediments were resuspended, the presence of even quite modest levels of eastern oysters (25 g dry tissue weight m?2) distributed uniformly throughout the modeled domain, reduced suspended sediment concentrations by nearly an order of magnitude. This increased water clarity, the depth to which seagrasses were predicted to grow. Because hard clams (Mercenaria mercenaria) had a much lower weight-specific filtration rate than eastern oysters; their influence on reducing turbidity was much less than oysters. Seagrasses, once established with sufficiently high densities (>1,000 shoots m?2), damped waves, thereby reducing sediment resuspension and improving light conditions. This stabilizing effect was minor compared to the influence of uniformly distributed eastern oysters on water clarity. Our model predicted that restoration of eastern oysters has the potential to reduce turbidity in shallow estuaries, such as Chesapeake Bay, and facilitate ongoing efforts to restore seagrasses. This model included several simplifiying assumptions, including that oysters were uniformly distributed rather than aggregated into offshore reefs and that oyster feces were not resuspended. 相似文献
2.
Three quarters of the global human population will live in coastal areas in the coming decades and will continue to develop these areas as population density increases. Anthropogenic stressors from this coastal development may lead to fragmented habitats, altered food webs, changes in sediment characteristics, and loss of near-shore vegetated habitats. Seagrass systems are important vegetated estuarine habitats that are vulnerable to anthropogenic stressors, but provide valuable ecosystem functions. Key to maintaining these habitats that filter water, stabilize sediments, and provide refuge to juvenile animals is an understanding of the impacts of local coastal development. To assess development impacts in seagrass communities, we surveyed 20 seagrass beds in lower Chesapeake Bay, VA. We sampled primary producers, consumers, water quality, and sediment characteristics in seagrass beds, and characterized development along the adjacent shoreline using land cover data. Overall, we could not detect effects of local coastal development on these seagrass communities. Seagrass biomass varied only between sites, and was positively correlated with sediment organic matter. Epiphytic algal biomass and epibiont (epifauna and epiphyte) community composition varied between western and eastern regions of the bay. But, neither eelgrass (Zostera marina) leaf nitrogen (a proxy for integrated nitrogen loading), crustacean grazer biomass, epifaunal predator abundance, nor fish and crab abundance differed significantly among sites or regions. Overall, factors operating on different scales appear to drive primary producers, seagrass-associated faunal communities, and sediment properties in these important submerged vegetated habitats in lower Chesapeake Bay. 相似文献
3.
Lawrence P. Sanford 《Estuaries and Coasts》1994,17(1):148-165
Moored instruments were used to make observations of near bottom currents, waves, temperature, salinity, and turbidity at shallow (3.5 m and 5.5 m depth) dredged sediment disposal sites in upper Chesapeake Bay during the winters of 1990 and 1991 to investigate time-varying characteristics of resuspension processes over extended periods. Resulting time series data show the variability of two components of the suspended sediment concentration field. Background suspended sediment concentrations varied inversely with salinity and in direct relation to Susquehanna River flow. Muddy bottom sediments were also resuspended locally by both tidal currents and wind-wave forcing, resulting in short-term increases and decreases in suspended concentration, with higher peak concentrations near the bottom. In both years, episodes of wave-forced resuspension dominated tidal resuspension on an individual event basis, exceeding most tidal resuspension peaks by a factor of 3 to 5. The winds that generated the waves responsible for the observed resuspension events were not optimal for wave generation, however. Application of a simple wind-wave model showed that much greater wave-forced resuspension than that observed might be generated under the proper conditions. The consolidated sediments investigated in 1990 were less susceptible to both tidal and wave-forced resuspension than the recently deposited sediments investigated in 1991. There was also some indication that wave-forced resuspension increased erodibility of the bottom sediments on a short-term basis. Wave-forced resuspension is implicated as an important part of sediment transport processes in much of Chesapeake Bay. Its role in deeper, narrower, and more tidally energetic estuaries is not as clear, and should be investigated on a case-by-case basis. 相似文献
4.
We investigated spatial correlations between wave forcing, sea level fluctuations, and shoreline erosion in the Maryland Chesapeake Bay (CB), in an attempt to identify the most important relationships and their spatial patterns. We implemented the Simulating WAves Nearshore (SWAN) model and a parametric wave model from the USEPA Chesapeake Bay Program (CBP) to simulate wave climate in CB from 1985 to 2005. Calibrated sea level simulations from the CBP hydrodynamic model over the same time period were also acquired. The separate and joint statistics of waves and sea level were investigated for the entire CB. Spatial patterns of sea level during the high wave events most important for erosion were dominated by local north-south winds in the upper Bay and by remote coastal forcing in the lower Bay. We combined wave and sea level data sets with estimates of historical shoreline erosion rates and shoreline characteristics compiled by the State of Maryland at two different spatial resolutions to explore the factors affecting erosion. The results show that wave power is the most significant influence on erosion in the Maryland CB, but that many other local factors are also implicated. Marshy shorelines show a more homogeneous, approximately linear relationship between wave power and erosion rates, whereas bank shorelines are more complex. Marshy shorelines appear to erode faster than bank shorelines, for the same wave power and bank height. A new expression for the rate of shoreline erosion is proposed, building on previous work. The proposed new relationship expresses the mass rate of shoreline erosion as a locally linear function of the difference between applied wave power and a threshold wave power, multiplied by a structure function that depends on the ratio of water depth to bank height. 相似文献
5.
Tidally driven flows, waves, and suspended sediment concentrations were monitored seasonally within a Zostera marina seagrass (eelgrass) meadow located in a shallow (1–2 m depth) coastal bay. Eelgrass meadows were found to reduce velocities approximately 60 % in the summer and 40 % in the winter compared to an adjacent unvegetated site. Additionally, the seagrass meadow served to dampen wave heights for all seasons except during winter when seagrass meadow development was at a minimum. Although wave heights were attenuated across the meadow, orbital motions caused by waves were able to effectively penetrate through the canopy, inducing wave-enhanced bottom shear stress (τ b ). Within the seagrass meadow, τ b was greater than the critical stress threshold (=0.04 Pa) necessary to induce sediment suspension 80–85 % of the sampling period in the winter and spring, but only 55 % of the time in the summer. At the unvegetated site, τ b was above the critical threshold greater than 90 % of the time across all seasons. During low seagrass coverage in the winter, near-bed turbulence levels were enhanced, likely caused by stem–wake interaction with the sparse canopy. Reduction in τ b within the seagrass meadow during the summer correlated to a 60 % reduction in suspended sediment concentrations but in winter, suspended sediment was enhanced compared to the unvegetated site. With minimal seagrass coverage, τ b and wave statistics were similar to unvegetated regions; however, during high seagrass coverage, sediment stabilization increased light availability for photosynthesis and created a positive feedback for seagrass growth. 相似文献
6.
With increased shoreline hardening and development, it is important to understand the ecological processes occurring in these and adjacent coastal habitats. A common species found associated with these hard-substrate habitats in Chesapeake Bay is the grass shrimp, Palaemonetes pugio. Caging experiments were conducted from June to August 2010 to examine the effects of shrimp on the recruitment and development of hard-substrate communities. Experiments were conducted at two low-salinity sites within Chesapeake Bay and one high-salinity site in an adjacent coastal bay in Virginia. The addition of grass shrimp reduced recruitment of polychaetes and scyphistomae of the sea nettle, Chrysaora quinquecirrha, and increased recruitment of encrusting bryozoans and the oyster, Crassostrea virginica. After 12?weeks, sea nettles at one low-salinity site, dominated predator-exclusion treatments. At the high-salinity site, oysters dominated when shrimp were present. Although it is unclear whether the results of short-term caging studies can be applied across larger temporal and spatial scales, the significant effects of grass shrimp on two important Chesapeake Bay species suggests that increases in hard-substrate habitat could have broader impacts within this and other systems. 相似文献
7.
The natural aging process of Chesapeake Bay and its tributary estuaries has been accelerated by human activities around the shoreline and within the watershed, increasing sediment and nutrient loads delivered to the bay. Riverine nutrients cause algal growth in the bay leading to reductions in light penetration with consequent declines in sea grass growth, smothering of bottom-dwelling organisms, and decreases in bottom-water dissolved oxygen as algal blooms decay. Historically, bay waters were filtered by oysters, but declines in oyster populations from overfishing and disease have led to higher concentrations of fine-sediment particles and phytoplankton in the water column. Assessments of water and biological resource quality in Chesapeake Bay and tributaries, such as the Potomac River, show a continual degraded state. In this paper, we pay tribute to Owen Bricker’s comprehensive, holistic scientific perspective using an approach that examines the connection between watershed and estuary. We evaluated nitrogen inputs from Potomac River headwaters, nutrient-related conditions within the estuary, and considered the use of shellfish aquaculture as an in-the-water nutrient management measure. Data from headwaters, nontidal, and estuarine portions of the Potomac River watershed and estuary were analyzed to examine the contribution from different parts of the watershed to total nitrogen loads to the estuary. An eutrophication model was applied to these data to evaluate eutrophication status and changes since the early 1990s and for comparison to regional and national conditions. A farm-scale aquaculture model was applied and results scaled to the estuary to determine the potential for shellfish (oyster) aquaculture to mediate eutrophication impacts. Results showed that (1) the contribution to nitrogen loads from headwater streams is small (about 2 %) of total inputs to the Potomac River Estuary; (2) eutrophic conditions in the Potomac River Estuary have improved in the upper estuary since the early 1990s, but have worsened in the lower estuary. The overall system-wide eutrophication impact is high, despite a decrease in nitrogen loads from the upper basin and declining surface water nitrate nitrogen concentrations over that period; (3) eutrophic conditions in the Potomac River Estuary are representative of Chesapeake Bay region and other US estuaries; moderate to high levels of nutrient-related degradation occur in about 65 % of US estuaries, particularly river-dominated low-flow systems such as the Potomac River Estuary; and (4) shellfish (oyster) aquaculture could remove eutrophication impacts directly from the estuary through harvest but should be considered a complement—not a substitute—for land-based measures. The total nitrogen load could be removed if 40 % of the Potomac River Estuary bottom was in shellfish cultivation; a combination of aquaculture and restoration of oyster reefs may provide larger benefits. 相似文献
8.
Ana L. Hernández Cordero Rochelle D. Seitz Romuald N. Lipcius Caitlin M. Bovery David M. Schulte 《Estuaries and Coasts》2012,35(5):1340-1345
Bay scallop (Argopecten irradians) populations existed in Chesapeake Bay until 1933, when they declined dramatically due to a loss of seagrass habitat. Since then, there have been no documented populations within the Bay. However, some anecdotal observations of live bay scallops within the lower Bay suggest that restoration of the bay scallop is feasible. We therefore tested whether translocated adults of the southern bay scallop, Argopecten irradians concentricus, could survive during the reproductive season in vegetated and unvegetated habitats of the Lynnhaven River sub-estuary of lower Chesapeake Bay in the absence of predation. Manipulative field experiments evaluated survival of translocated, caged adult scallops in eelgrass Zostera marina, macroalgae Gracilaria spp., oyster shell, and rubble plots at three locations. After a 3-week experimental period, scallop survival was high in vegetated habitats, ranging from 98% in their preferred habitat, Z. marina, to 90% in Gracilaria spp. Survival in Z. marina was significantly higher than that in rubble (76%) and oyster shell (78%). These findings indicate that reproductive individuals can survive in vegetated habitats of lower Chesapeake Bay when protected from predators and that establishment of bay scallop populations within Chesapeake Bay may be viable. 相似文献
9.
Research on the effects of declining abundances of the Eastern oyster (Crassostrea virginica) in Chesapeake Bay and other estuaries has primarily focused on the role of oysters in filtration and nutrient dynamics,
and as habitat for fish or fish prey. Oysters also play a key role in providing substrate for the overwintering polyp stage
of the scyphomedusa sea nettle,Chrysaora quinquecirrha, which is an important consumer of zooplankton, ctenophores, and icthyoplankton. Temporal trends in sea nettle abundances
in visual counts from the dock at Chesapeake Biological Laboratory, trawls conducted in the mesohaline portion of the Patuxent
River, and published data from the mainstem Chesapeake Bay indicate that sea nettles declined in the mid 1980s when overfishing
and increased disease mortality led to sharp decreases in oyster landings and abundance. Climate trends, previously associated
with interannual variation in sea nettle abundances, do not explain the sharp decline. A potentially important consequence
of declining sea nettle abundances may be an increase in their ctenophore prey (Mnemiopsis leidyi), and a resultant increase in predation on icthyoplankton and oyster larvae. Increased predation on oyster larvae by ctenophores
may inhibit recovery of oyster populations and reinforce the current low abundance of oysters in Chesapeake Bay. 相似文献
10.
Christopher P. Buzzelli 《Estuaries and Coasts》1998,21(4):659-672
The fringing environments of lower Chesapeake Bay include sandy shoals, seagrass meadows, intertidal mud flats, and marshes. A characterization of a fringing ecosystem was conducted to provide initialization and calibration data for the development of a simulation model. The model simulates primary production and material exchange in the littoral zone of lower Chesapeake Bay. Carbon (C) and nitrogen (N) properties of water and sediments from sand, seagrass, intertidal silt-mud, and intertidal marsh habitats of the Goodwin Islands (located within the Chesapeake Bay National Estuarine Research Reserve in Virginia, CBNERR-VA) were determined seasonally. Spatial and temporal differences in sediment microalgal biomass among the habitats were assessed along with annual variations in the distribution and abundance ofZostera marina L. andSpartina alterniflora Loisel. Phytoplankton biomass displayed some seasonality related to riverine discharge, but sediment microalgal biomass did not vary spatially or seasonally. Macrophytes in both subtidal and intertidal habitats exhibited seasonal biomass patterns that were consistent with other Atlantic estuarine ecosystems. Marsh sediment organic carbon and inorganic nitrogen differed significantly from that of the sand, seagrass, and silt habitats. The only biogeochemical variable that exhibited seasonality was low marsh NH4 +. The subtidal sediments were consistent temporally in their carbon and nitrogen content despite seasonal changes in seagrass abundance. Eelgrass has a comparatively low C:N ratio and is a potential N sink for the ecosystem. Changes in the composition or size of the vegetated habitats could have a dramatic influence over resource partitioning within the ecosystem. A spatial database (or geographic information system, GIS) of the Goodwin Islands site has been initiated to track long-term spatial habitat features and integrate model output and field data. This ecosystem characterization was conducted as part of efforts to link field data, geographic information, and the dynamic simulation of multiple habitats. The goal of these efforts is to examine ecological structure, function, and change in fringing environments of lower Chesapeake Bay. 相似文献
11.
A series of cruises was carried out in the estuarine turbidity maximum (ETM) region of Chesapeake Bay in 1996 to examine physical and biological variability and dynamics. A large flood event in late January shifted the salinity structure of the upper Bay towards that of a salt wedge, but most of the massive sediment load delivered by the Susquehanna River appeared to bypass the ETM zone. In contrast, suspended sediments delivered during a flood event in late October were trapped very efficiently in the ETM. The difference in sediment trapping appeared to be due to increases in particle settling speed from January to October, suggesting that the fate of sediments delivered during large events may depend on the season in which they occur. The ETM roughly tracked the limit of salt (defined as the intersection of the 1 psu isohaline with the bottom) throughout the year, but it was often separated significantly from the limit of salt with the direction of separation unrelated to the phase of the tide. This was due to a lag of ETM sediment resuspension and transport behind rapid meteorologically induced or river flow induced motion of the salt limit. Examination of detailed time series of salt, suspended sediment, and velocity collected near the limit of salt, combined with other indications, led to the conclusion that the convergence of the estuarine circulation at the limit of salt is not the primary mechanism of particle trapping in the Chesapeake Bay ETM. This convergence and its associated salinity structure contribute to strong tidal asymmetries in sediment resuspension and transport that collect and maintain a resuspendable pool of rapidly settling particles near the salt limit. Without tidal resuspension and transport, the ETM would either not exist or be greatly weakened. In spite of this repeated resuspension, sedimentation is the ultimate fate of most terrigenous material delivered to the Chesapeake Bay ETM. Sedimentation rates in the ETM channel are at least an order of magnitude greater than on the adjacent shoals, probably due to focusing mechanisms that are poorly understood. 相似文献
12.
Seagrass both disappeared and recovered within 4 yr in one region of northern Indian River Lagoon (IRL). For the specific
area referred to as Turnbull Bay, a relatively pristine area of the IRL, over 100 ha of seagrass completely disappeared from
1996 to 1997 and then recovered by 2000. Based on lagoon-wide mapping from aerial photographs taken every 2–3 years since
1986, coverage of seagrass in Turnbull Bay declined from 124 ha in 1989 to 34 ha by 1999 and increased to 58 ha in 2003. Bi-annual
monitoring of fixed seagrass transects tells a more detailed story. Species composition along the Turnbull transect shifted
fromHalodule wrightii toRuppia maritima beginning in 1995, and macroalgal abundance increased. By the summer of 1997, seagrass completely disappeared along the transect,
as well as in most of the surrounding areas in Turnbull Bay; macroalgae covered much of the sediment surface. No significant
water quality changes were detected. Light attenuation and suspended solid values did increase after the seagrass disappeared.
Porewater sulfide concentrations, taken after all the grass was gone in 1997, were high (2,000 μM), but did improve by 1998
(1,200 μM). Seagrass recovery was rapid and occurred in the reverse sequence of species loss. Seedlings ofR. maritima were the first colonizers, then patches ofH. wrightii appeared. In 2000,Halophila engelmannii returned in the deeper water (>0.6m). By the summer of 2000, the beds had completely recovered. We conclude that this demise
was a natural event caused by a long-term buildup of seagrass biomass and a thick (10–15 cm) layer of organic detritus and
ooze. We surmise that such a crash and subsequent recovery may be a natural cycle of decline and recovery within this semirestricted,
poorly-flushed area. The frequency of this cycle remains uncertain. 相似文献
13.
Neelima Thankappan Noujas Varangalil Thomas Kachapally Varghese Kurian Njaliplackil Philipose 《Natural Hazards》2018,90(3):1177-1199
Shoreline changes are largely dependent on coastal morphology. South-west coast of India is a high energy coast characterised by monsoon high waves, steep beach face and medium-sized beach sand. Waves are generally from west and west south-west during rough monsoon season and from south-west during fair weather season. Shoreline change along this coast is studied with reference to coastal morphological features. Various morphological features, modifications and chronological positions of shoreline are analysed with the information derived from multidated satellite imageries, toposheets and GPS shoreline mapping along with extended field survey. Image processing and GIS techniques have been used for the analysis of data and presentation of results. Sediment accumulation on the leeward side of artificial structures such as harbour breakwaters and groynes is used as a sediment transport indicator. Artificial structures such as seawalls, groynes and harbour breakwaters modify morphology. Shoreline south of headlands/promontories and breakwaters are stable or accreting due to net northerly longshore sediment transport while erosion tendency is observed on the north side. Lateritic cliffs fronting the sea or with seasonal beach undergo slumping and cliff edge retreat as episodic events. Spits adjoining tidal inlets are prone to shoreline variations due to oscillations of inlet mouth. Interventions in the form of inlet stabilization and construction of coastal protection structures trigger erosion along adjoining coasts. Seawalls constructed along highly eroding coasts get damaged, whereas those constructed along monsoon berm crest with frontal beaches for protection against monsoon wave attack are retained. Fishing gaps within seawalls are areas of severe temporary erosion during rough monsoon season. Accretion or erosion accompanies construction of harbour breakwaters in a stable coastal plain. Close dependence of shoreline changes on morphology necessitates detailed understanding of impacts on morphology prior to introducing any intervention in the coastal zone. 相似文献
14.
Lyle M. Varnell 《Estuaries and Coasts》2014,37(2):508-523
A long-term (1948–2010) shoreward energy history of upper tidal shorelines in lower Chesapeake Bay was developed using a simple calculation of kinetic energy from corresponding wind and tide data. These data were primarily used to determine the likelihood of shoreline energy increases coincident with local sea level rise. Total annual shoreward energy ranged from 620 kJ/m of shoreline in 1950 to 17,785 kJ/m of shoreline in 2009. No clear linear trends are apparent, but mean annual energy shows an increase from 2,732 kJ/m before 1982 to 6,414 kJ/m since then. This increase in mean energy was accompanied by more numerous spikes of comparatively higher annual energy. Shoreward energy delivered to lower Chesapeake Bay’s upper tidal shorelines was enabled by an increasing amount of time per year that tidal height exceeds mean high water, accompanied by increasing heights of tidal anomalies. An index termed the Hydrologic Burden was developed that incorporates the combination of time and tidal height that demonstrates this increasing trend. Although opportunities for greater shoreward energy increased as the Hydrologic Burden increased, and even though there is evidence that greater energy was delivered to the shorelines during the latter time series, energy per hour delivery was shown not to have increased, and may have decreased, due to a steady reduction in average wind speed in lower Chesapeake Bay since the mid-1980s. Energy delivery in lower Chesapeake Bay was primarily from the northeast, and energy delivery over the time series is shown to organize symmetrically around a point between the northeast and north–northeast directions. This is evidence of a self-organizational phenomenon that transcends changes in local wind and tide dynamics. 相似文献
15.
Macrobenthic community indices were examined for their ability to characterize the influence of shoreline alteration and watershed
land use in nearshore estuarine environments of the Chesapeake Bay, U.S.A. Twenty-three watersheds were surveyed in 2002 and
2003 for nearshore macrobenthic assemblages, environmental parameters (i.e., dissolved oxygen, pH, total suspended solids,
salinity, and sediment composition), shoreline condition, and land use. Two indices of macrobenthic biological integrity,
benthic index of biological integrity in the nearshore (B-IBIN) and abundance biomass comparison (W-value), were evaluated for associations with environmental and shoreline condition,
and riparian and watershed land use. Comparisons between nearshore measures of the B-IBI with offshore values (>2 m; Chesapeake
Bay benthic index of biological integrity [B-IBICB]) were conducted to assess the ability of the index to reflect land use patterns at near and far proximities to shore. Nearshore
macrobenthic communities were represented by a total of 94 species (mean number of species =9.2 ± 0.4 sample−1), and were dominated by the phyla Arthropoda, Annelida, and Mollusca. Temporal variability in environmental conditions and
macrobenthic abundance and biomass may be attributable to the notable increase in precipitation in 2003 that led to nutrient
influxes and algal blooms. For the biotic indices applied in the nearshore, the highest scores were associated with forested
watersheds (W-value, B-IBIN). Ecological thresholds were identified with nonparametric change-point analysis, which indicated a significant reduction
in B-IBIN and W-value scores when the amount of developed shoreline exceeded 10% and developed watershed exceeded 12%, respectively. 相似文献
16.
Jimena Samper-Villarreal Peter J. Mumby Megan I. Saunders Chris Roelfsema Catherine E. Lovelock 《Estuaries and Coasts》2018,41(6):1732-1743
Blue carbon initiatives require accurate monitoring of carbon stocks. We examined sources of variability in seagrass organic carbon (Corg) stocks, contrasting spatial with short temporal scales. Seagrass morphology and sediment Corg stocks were measured from biomass and shallow sediment cores collected in Moreton Bay, Australia. Samples were collected between 2012 and 2013, from a total of 77 sites that spanned a gradient of water turbidity. Environmental measures of water quality between 2000 and 2013 revealed strong seasonal fluctuations from summer to winter, yet seagrass biomass exhibited no temporal variation. There was no temporal variability in Corg stocks, other than below ground biomass stocks were slightly higher in June 2013. Seagrass locations were grouped into riverine, coastal, and seagrass loss locations and short temporal variability of Corg stocks was analysed within these categories to provide clearer insights into temporal patterns. Above ground Corg stocks were similar between coastal and riverine meadows. Below ground Corg stocks were highest in coastal meadows, followed by riverine meadows. Sediment Corg stocks within riverine meadows were much higher than at coastal meadows and areas of seagrass loss, with no difference in sediment Corg stocks between these last two categories. Riverine seagrass meadows, of higher turbidity, had greater total Corg stocks than meadows in offshore areas irrespective of time. We suggest that Corg stock assessment should prioritise sampling over spatial gradients, but repeated monitoring over short time scales is less likely to be warranted if environmental conditions remain stable. 相似文献
17.
Field measurements of the vertical structure of near-bed suspended sediment concentrations were obtained from arrays of fast response optical backscatter suspended solids sensors to examine the time-dependent response of sediment resuspension to waves and currents and the constraints imposed by bedforms. Data were recorded from both a nonbarred, marine shoreface and a barred lacustrine shoreface, under both shoaling and breaking waves (significant heights of 0·25–1·50m; peak periods of 3 and 8 s) and in water depths of 0·5–5·0 m. Sediment concentrations are positively correlated with increasing elevation above the bed, but lagged in time. The time lag varies directly with separation distance between measurement locations and inversely with the horizontal component of the near-bed oscillatory velocity. Both the presence of wave groups and the settling velocities of the sediment particules in suspension influence the temporal changes in concentration at a given elevation. Sediment concentrations appear to respond more slowly to the incident wind-wave forcing with distance away from the bed as a result of two factors: (1) the sequential increase in concentration induced by a succession of large waves in a group; and (ii) the relative increase in finer sediments with smaller settling velocities. Bedforms interact with the near-bed horizontal currents to impose a distinct constraint upon the timing of suspension events relative to the phase of the fluid motion, and, therefore, the vertical structure of the suspended sediment concentration at a range of time scales. The near-bed concentrations appear to be strongly dependent upon the vertical convection of sediment associated with the ejection from the wave boundary layer of separation vortices generated in the lee of ripple crests. Concentration gradients in the presence of vortex ripples are large, as are the correlation between concentrations measured at different elevations within the fluid. 相似文献
18.
A 6.500-meter reach of western Chesapeake Bay shoreline (lower Mayo Peninsula) lost about 1.1×106 cubic meters of sediment (equivalent to 170 cubic meters lost per meter of shoreline) between 1846 and 1932, when the first
aerial photographs show the shoreline already substantially protected by a system of groins and intermittent bulkheading.
These structures have eliminated the fastland as a source of erodable material, and have starved the supply of sand for littoral
drift, thus limiting the extent of the beaches to the remaining groin fields.
Volumes of sediment involved in these impacts are small in the overall sediment budget. Bulkheads produce no deficit in the
budget since scouring of the beaches on their seaward sides makes up for the decreased erosion of protected fastland. Groins
trap little of the potential littoral drift (computed to be about 104 cubic meters per meter of shoreline per year). The sand supply in the remaining beaches is nearly equivalent to the annual
loss of sediment from the entire shoreline system due to the long-term rate of erosion of the shoreline and nearshore between
1846 and 1932. 相似文献
19.
John E. Olney 《Estuaries and Coasts》1983,6(1):20-35
The seasonal abundance and spatial distribution of eggs and early larvae of the bay anchovy,Anchoa mitchilli, and the weakfish,Cynoscion regalis, were determined from plankton collections taken during 1971–1976 in the lower Chesapeake Bay. Eggs and larvae of the bay anchovy,Anchoa mitchilli, dominated the ichthyoplankton, making up 96% of the total eggs and 88% of all larvae taken. A comparison of egg and larval densities from the lower Chesapeake Bay to existing data from other East Coast estuaries suggested that Chesapeake Bay is a major center of spawning activity for this species.Anchoa mitchilli spawning commenced in May when mean water column temperatures approached 17°C and abruptly ceased after August. Eggs and early larvae presented a continuous distribution throughout the study area during these months. Eggs and larvae of several sciaenid species, especiallyC. regalis, ranked second in numerical abundance. Larval weakfish were consistently taken in late summer of each sampling year but peak abundance and distribution was observed in August 1971. Sciaenid eggs exhibited a distinct polyhaline distribution with greatest concentrations observed at the Chesapeake Bay entrance or along the Bay eastern margin. Analysis of sciaenid egg morphometry and larval occurrence suggested spawning activity of at least four species. Additional important species represented by eggs and/or larvae in the lower Chesapeake Bay wereHypsoblennius hentzi, Gobiosoma ginsburgi, Trinectes maculatus, Symphurus plagiusa andParalichthys dentatus with the remaining species occurring infrequently. 相似文献
20.
The delivery, flux and fate of terrigenous sediment entering the Great Barrier Reef lagoon has been a focus of recent studies and represents an ongoing environmental concern. Wave‐induced bed stress is the most significant mechanism of sediment resuspension in the Great Barrier Reef, and field data and mathematical modelling indicates that the combined effects of short‐period wind waves, longer period swell waves, and tidal and wind‐driven currents can often exceed the critical bed stress for resuspension. Suspended‐sediment concentrations at 20 m water depth indicate resuspension seldom occurs on the middle shelf under normal wave conditions. Non‐cyclonic turbidity events are generally confined to the inner shelf. The wave climate in the southern sector of the central Great Barrier Reef lagoon is the most erosive, and resuspension of outer shelf sediments was hindcast for recorded cyclones. Wind‐driven, longshore currents are fundamental to the northward movement of sediment, and the annual northward mass flux from embayments undergoing resuspension in the Burdekin region is estimated to be one order of magnitude larger than the mass of sediment introduced by a moderate flood plume. Strong onshore winds are estimated to generate significant three‐dimensional bottom return currents on approximately 30–70 days per year, forming a potentially significant offshore‐directed sediment flux during high suspended‐sediment concentration events on the inner shelf. 相似文献