Sediment Accumulation Rates and Submersed Aquatic Vegetation (SAV) Distributions in the Mesohaline Chesapeake Bay, USA

This study assesses spatial and temporal sedimentological trends in four mesohaline Chesapeake Bay submersed aquatic vegetation (SAV) habitats, two with persistent SAV beds and two with ephemeral SAV beds, to determine their relationship to current and historical sediment characteristics??grain size, organic content, and accumulation rates. In general, grain size is similar among all sites, and subsurface sediment differs from surficial sediment only at one site where a thin surficial sand layer (??2?C3?cm) is present. This thin sand layer is not completely preserved in the longer-term sedimentary record even though it is critical to determining whether the sediment is suitable for SAV. Evidence for nearshore fining, similar to that observed in the deeper waters of the Bay, is present at the site where the shoreline has been hardened suggesting that locations with hardened shorelines limit exchange of coarser (sandy) material between the shore and nearshore environments. Whether the fining trend will continue to a point at which the sediment will become unsuitable for SAV in the future or whether some new type of equilibrium will be reached cannot be addressed with our data. Instead, our data suggest that SAV presence/absence is related to changes in sedimentary characteristics??persistent beds have relatively steady sediment composition, while ephemeral beds have finer sediments due to reduced sand input. Additionally, sediment accumulation rates in the persistent beds are ??9?mm/year, whereas rates in the ephemeral beds are ??3?mm/year. Thus, the ephemeral sites highlight two potential sedimentary controls on SAV distribution: the presence of a sufficiently thick surficial sand layer as previously postulated by Wicks (2005) and accumulation rates high enough to bury seeds prior to germination and/or keep up with sea-level rise.     

In Situ Effects of Shoreline Type and Watershed Land Use on Submerged Aquatic Vegetation Habitat Quality in the Chesapeake and Mid-Atlantic Coastal Bays

Submerged aquatic vegetation (SAV) is an ecologically and economically valuable component of coastal estuaries that acts as an early indicator of both degrading and improving water quality. This study aimed to determine if shoreline hardening, which is associated with increased population pressure and climate change, acts to degrade SAV habitat quality at the local scale. In situ comparisons of SAV beds adjacent to both natural and hardened shorelines in 24 subestuaries throughout the Chesapeake and Mid-Atlantic Coastal Bays indicated that shoreline hardening does impact adjacent SAV beds. Species diversity, evenness, and percent cover were significantly reduced in the presence of riprap revetment. A post hoc analysis also confirmed that SAV is locally affected by watershed land use associated with increased population pressure, though to a lesser degree than impacts observed from shoreline armoring. When observed over time, SAV recovery at the local level took approximately 3 to 4 years following storm impacts, and SAV adjacent to natural shorelines showed more resilience to storms than SAV adjacent to armored shorelines. The negative impacts of shoreline hardening and watershed development on SAV shown here will inform coastal zone management decisions as increasing coastal populations and sea level rise drive these practices.     

Effectiveness of Living Shorelines as an Erosion Control Method in North Carolina

Living shorelines are a shoreline stabilization strategy encompassing a range of vegetative to structural materials and serve as an alternative approach to the use of structures like bulkheads, which are known to aggravate erosion. Living shorelines are often installed with little to no long-term monitoring for effectiveness; specifically, there is a lack of quantitative data regarding their performance as a shoreline stabilization strategy. This study sought to assess the performance of living shorelines with sills, with respect to shoreline protection, by determining shoreline change rates (SCR) using geospatial analysis. Shoreline surveys were conducted using a real-time kinematic (RTK)-GPS unit at a total of 17 living shoreline projects and nine control segments at 12 sites along the coast of North Carolina. Current shoreline position was compared to historic (pre-installation) shoreline positions obtained from aerial imagery, dating to 1993. The average SCR among northern sites before installation was ??0.45?±?0.49 m year^?1, and in southern sites, it was ??0.21?±?0.52 m year^?1. After installation, average SCR was significantly less erosive at northern and southern sites with living shorelines, 0.17?±?0.47 and ??0.01?±?0.51 m year^?1, respectively. Of the 17 living shoreline project segments, 12 exhibited a reduction in the rate of erosion; of those 12, six were observed to be accreting. This study supports the convention that living shorelines can reduce the rate of erosion and potentially restore lost shore zone habitat.     

Temporal and Spatial Dynamics of Estuarine Shoreline Change in the Albemarle-Pamlico Estuarine System,North Carolina,USA

Many shoreline studies rely on historical change rates determined from aerial imagery decades to over 50 years apart to predict shoreline position and determine setback distances for coastal structures. These studies may not illustrate the coastal impacts of short-duration but potentially high-impact storm events. In this study, shoreline change rates (SCRs) are quantified at five different sites ranging from marsh to sediment bank shorelines around the Albemarle-Pamlico estuarine system (APES) for a series of historical (decadal to 50-year) and short-term (bimonthly) time periods as well as for individual storm events. Long-term (historical) SCRs of approximately ?0.5 ± 0.07 m year^?1 are observed, consistent with previous work along estuarine shorelines in North Carolina. Short-term SCRs are highly variable, both spatially and temporally, and ranged from 15.8 ± 7.5 to ?19.3 ± 11.5 m year^?1 at one of the study sites. The influence of wave climate on the spatial and temporal variability of short-term erosion rates is investigated using meteorological observations and coupled hydrodynamic (Delft3D) and wave (SWAN) models. The models are applied to simulate hourly variability in the surface waves and water levels. The results indicate that in the fetch-limited APES, wind direction strongly influences the wave climate at the study sites. The wave height also has an influence on short-term SCRs as determined from the wave simulations for individual meteorological events, but no statistical correlation is found for wave height and SCRs over the long term. Despite the significantly higher rates of shoreline erosion over short time periods and from individual events like hurricanes, the cumulative impact over long time periods is low. Therefore, while the short-term response of these shorelines to episodic forcing should be taken into account in management plans, the long-term trends commonly used in ocean shoreline management can also be used to determine erosion setbacks on estuarine shorelines.     

Threatened archaeological,historic, and cultural resources of the Georgia Coast: Identification,prioritization and management using GIS technology

Archaeological sites in beach and estuarine environments are continually threatened by diverse natural marine processes. Shoreline erosion, bluff retreat, and sea level rise all present potential for site destruction. Using historic maps, aerial imagery, and field survey methods in a GIS, 21 potentially significant archaeological sites on Georgia barrier islands were selected for determination of site‐specific rates of shoreline change using a powerful, new, moving‐boundary GIS analysis tool. A prioritized list of sites, based on the order of site loss from erosion, was generated to assist coastal managers in identifying and documenting sites most at risk. From the original selection of 21 sites, 11 sites were eroding, 8 shorelines were stable, and 2 shorelines were accreting. The methodology outlined here produces critical information on archaeological site loss rates and provides a straightforward means of prioritizing sites for detailed documentation. © 2010 Wiley Periodicals, Inc.     

Water quality associated with survival of submersed aquatic vegetation along an estuarine gradient

The decline of submersed aquatic vegetation (SAV) in tributaries of the Chesapeake Bay has been associated with increasing anthropogenic inputs, and restoration of the bay remains a major goal of the present multi-state “Bay Cleanup” effort. In order to determine SAV response to water quality, we quantified the water column parameters associated with success of transplants and natural regrowth over a three-year period along an estuarine gradient in the Choptank River, a major tributary on the eastern shore of Chesapeake Bay. The improvement in water quality due to low precipitation and low nonpoint source loadings during 1985–1988 provided a natural experiment in which SAV was able to persist upstream where it had not been for almost a decade. Mean water quality parameters were examined during the growing season (May–October) at 14 sites spanning the estuarine gradient and arrayed to show correspondence with the occurrence of SAV. Regrowth of SAV in the Choptank is associated with mean dissolved inorganic nitrogen <10 μM; mean dissolved phosphate <0.35 μM; mean suspended sediment <20 mg l^?1; mean chlorophylla in the water column <15 μg l^?1; and mean light attenuation coefficient (Kd) <2 m^?1. These values correspond well with those derived in other parts of the Chesapeake, particularly in the lower bay, and may provide managers with values that can be used as target concentrations for nutrient reduction strategies where SAV is an issue.     

Seasonal-Scale and Decadal-Scale Sediment-Vegetation Interactions on the Subaqueous Susquehanna River Delta,Upper Chesapeake Bay

Submersed aquatic vegetation (SAV) have been a prominent feature on the Susquehanna Flats, the shallow, subaqueous delta of the Susquehanna River, Maryland. SAV were absent from the Flats between 1972 and 2000, but have since recovered. While it is well established that SAV can improve water quality by promoting sediment and nutrient retention, it is not well understood how SAV on the Flats modulate sediment input from the Susquehanna River into the Upper Chesapeake Bay over different timescales. This study evaluates sedimentation on the Flats over seasonal to decadal timescales, using naturally occurring radioisotopes (⁷Be, ²¹⁰Pb) within the context of SAV biomass and Flats geomorphology. Results indicate that sedimentation on the Flats is both spatially and temporally variable. Although this variability cannot be explained by relationships with grain size and SAV biomass, river discharge, sediment supply, and geometry over the SAV bed likely control sedimentation in this system. Decadal-scale sedimentation is influenced by both flood events and changes in SAV biomass abundance. Average annual sediment accumulation was higher when SAV were present than when SAV were absent. SAV bed area was strongly correlated with average annual accumulation rate. These results suggest that a positive feedback between SAV abundance and accumulation rate exists; however, sediment supply and transport pathways are also important factors.     

现代黄河三角洲南部潮间带及附近海域沉积特征认识与分析

基于潮间带和浅海12个柱状样粒度、地球化学元素、210Pb同位素实测数据,结合河流年代特征、岸线历史变化等资料,参照前人关于沉积相和沉积速率研究成果,对现代黄河三角洲南部潮间带及莱州湾西部海域沉积特征进行了分析。根据剩余210Pb比活度信息,综合运用历史地理学方法、沉积环境判别公式和粒度曲线变化特征,初步认识了现代黄河三角洲南部潮间带地区具体沉积年代。对柱状样的数据进行分析后,运用相关特征值定量区分了沉积物的物源。结果表明:在潮间带这样不稳定的沉积环境下,210Pb_ex比活度阶段性倒置的分布模式虽然无法对年代进行连续标定,但可以在很大程度上指示沉积环境变化。现代黄河三角洲南部潮间带沉积物形成时间要远早于莱州湾西部,210Pb结合沉积环境判别函数、历史地理资料以及粒度特征可以将深度102 cm、80 cm、32 cm分别标定为1929年、1934年和1947年。广利河水系在深度20 cm范围内影响沉积环境。地球化学元素相关特征值分析显示:现代黄河三角洲南部潮间带沉积物风化程度介于黄河和莱州湾南岸之间,个别微量元素富集程度与莱州湾沉积物差异明显,就沉积环境而言,与现代黄河沉积物已有所不同。     

The Effect of Submerged Aquatic Vegetation Expansion on a Declining Turbidity Trend in the Sacramento-San Joaquin River Delta

Submerged aquatic vegetation (SAV) has well-documented effects on water clarity. SAV beds can slow water movement and reduce bed shear stress, promoting sedimentation and reducing suspension. However, estuaries have multiple controls on turbidity that make it difficult to determine the effect of SAV on water clarity. In this study, we investigated the effect of primarily invasive SAV expansion on a concomitant decline in turbidity in the Sacramento-San Joaquin River Delta. The objective of this study was to separate the effects of decreasing sediment supply from the watershed from increasing SAV cover to determine the effect of SAV on the declining turbidity trend. SAV cover was determined by airborne hyperspectral remote sensing and turbidity data from long-term monitoring records. The turbidity trends were corrected for the declining sediment supply using suspended-sediment concentration data from a station immediately upstream of the Delta. We found a significant negative trend in turbidity from 1975 to 2008, and when we removed the sediment supply signal from the trend it was still significant and negative, indicating that a factor other than sediment supply was responsible for part of the turbidity decline. Turbidity monitoring stations with high rates of SAV expansion had steeper and more significant turbidity trends than those with low SAV cover. Our findings suggest that SAV is an important (but not sole) factor in the turbidity decline, and we estimate that 21–70 % of the total declining turbidity trend is due to SAV expansion.     

Impacts of Coastal Land Use and Shoreline Armoring on Estuarine Ecosystems: an Introduction to a Special Issue

The nearshore land-water interface is an important ecological zone that faces anthropogenic pressure from development in coastal regions throughout the world. Coastal waters and estuaries like Chesapeake Bay receive and process land discharges loaded with anthropogenic nutrients and other pollutants that cause eutrophication, hypoxia, and other damage to shallow-water ecosystems. In addition, shorelines are increasingly armored with bulkhead (seawall), riprap, and other structures to protect human infrastructure against the threats of sea-level rise, storm surge, and erosion. Armoring can further influence estuarine and nearshore marine ecosystem functions by degrading water quality, spreading invasive species, and destroying ecologically valuable habitat. These detrimental effects on ecosystem function have ramifications for ecologically and economically important flora and fauna. This special issue of Estuaries and Coasts explores the interacting effects of coastal land use and shoreline armoring on estuarine and coastal marine ecosystems. The majority of papers focus on the Chesapeake Bay region, USA, where 50 major tributaries and an extensive watershed (~ 167,000 km²), provide an ideal model to examine the impacts of human activities at scales ranging from the local shoreline to the entire watershed. The papers consider the influence of watershed land use and natural versus armored shorelines on ecosystem properties and processes as well as on key natural resources.     

Mechanisms of Storm-Related Loss and Resilience in a Large Submersed Plant Bed

There is a growing emphasis on preserving ecological resilience, or a system’s capacity to absorb or recover quickly from perturbations, particularly in vulnerable coastal regions. However, the factors that affect resilience to a given disturbance are not always clear and may be system-specific. We analyzed and synthesized time series datasets to explore how extreme events impacted a large system of submersed aquatic vegetation (SAV) in upper Chesapeake Bay and to identify and understand associated mechanisms of resilience. We found that physical removal of plants around the edge of the bed by high flows during a major flood event as well as subsequent wind-driven resuspension of newly deposited sediment and attendant light-limiting conditions were detrimental to the SAV bed. Conversely, it appears that the bed attenuated high flows sufficiently to prevent plant erosion at its inner core. The bed also attenuated wind-driven wave amplitude during seasonal peaks in plant biomass, thereby decreasing sediment resuspension and increasing water clarity. In addition, clear water appeared to “spill over” into adjacent regions during ebb tide, improving the bed’s capacity for renewal by creating more favorable growing conditions in areas where plant loss had occurred. These analyses demonstrate that positive feedback processes, whereby an SAV bed modifies its environment in ways that improve its own growth, likely serve as mechanisms of SAV resilience to flood events. Although this work focuses on a specific system, the synthetic approach used here can be applied to any system for which routine monitoring data are available.     

Sea level rise and impacts on nearshore sedimentation: an overview

 Impacts on nearshore sedimentation arising from potential sea level change of the magnitude predicted in Intergovernmental Panel on Climatic Change scenarios associated with global warming are reviewed. For sandy duned coasts, the obvious sedimentation impacts include potential erosion of coastal dunes with implied deposition of the eroded material in the nearshore, possible deepening of embayments, and flooding of wetlands. For the sandy coasts a number of two-dimensional models are available for predicting shoreline change, but there are significant difficulties in applying Bruun-type models for dune erosion and assessment of sediment redistribution over the inner shelf, and for predicting the amount of shoreline retreat for a given rate of sea level rise. If the beach profile contains excessive sand relative to its equilibrium profile, sensu Dean (1991), then shoreline retreat may not occur upon sea level rise. From the evidence of Kiel Bay, at least in these semi-enclosed basin types, it is during major transgressions that maximum deposition in adjacent basins occurs, due to the sea eroding weakly consolidated and weathered surface regolith. But at the same time climatic patterns were re-adjusting and probably contributed to maximum deposition in adjacent shelf and basins below wave base. Received: 16 June 1995 / Accepted: 29 January 1996     

Streambank fish-shelter structures help stabilize tributary streams in Wisconsin

 A significant proportion of stream sediment yield in North America comes from stream channel and bank erosion. One method used for stream stabilization is the bank installation of timber and stone fish-shelter structures, but there is little evidence for their potential effectiveness. Nine to nineteen years of precise survey data from Coon Creek, Wisconsin, however, show that fish structures enhance sediment deposition along the stream and may retard lateral migration of channels. Such structures have greater utility for sediment control when streams are eroding away a high bank and replacing it with a lower bank. Received: 18 October 1996 · Accepted: 4 February 1997     

Chemical oceanographic influences on sediment accumulations of a carbonate ramp: Holocene Yucatan Shelf,Mexico

Global controls on the oceanographic influences on the nature of carbonate factories are broadly understood. The details of the influences of changes in temperature and nutrients across individual carbonate shelves are less well constrained, however. This study explores spatial and temporal variations in chemical oceanography along and across the Yucatan Shelf, a modern carbonate ramp, and how these factors relate to variable bottom character, sediment and sediment geochemistry. In‐situ sensors and remote‐sensing data indicate the sporadic presence of cool, upwelled water with low dissolved oxygen and elevated Chlorophyll‐a. This current‐driven, westward flow of upwelled water is most evident in a zone just offshore of the northern peninsular shoreline, but its influence wanes ca 75 km offshore and as the shore turns southward. The impacts of this water mass include a transitional photozoan–heterozoan assemblage with biosiliceous components, relict grains and common thin Holocene sediment accumulations nearshore; further offshore are coralgal reefs and expansive sand plains. Geochemical proxies of bulk sediment, including high δ¹⁸O and elevated HREE/LREE (heavy rare‐earth element/light rare‐earth element) ratios near, and downcurrent of, the upwelling source, are interpreted to represent the signal of nearshore, westward movement of the cool and nutrient‐rich, upwelled water. Collectively, these data emphasize how local processes such as upwelling and longshore transport can variably influence carbonate sediment accumulations and their geochemical signatures, both along and across individual shelves. These data and insights provide an analogue for the influences of spatial variability of water masses in the geological record, and for accurate interpretation of stratigraphic changes of sedimentary and geochemical proxy data in carbonate archives.     

Geological Responses to Urbanization of the Naples Bay Estuarine System,Southwestern Florida,USA

The Naples–Dollar Bay Estuarine System (NDBES), situated in southwestern Florida, has undergone extensive modifications caused directly and indirectly by anthropogenic influences. These alterations include: (1) the substitution of mangrove-forested shorelines with concrete bulkheads and installation of residential canals; (2) installation of a regionally extensive navigational channel; and (3) canalization of the watershed, resulting in annexation of a heavily altered drainage basin ten times the size of the pre-alteration basin and with a significantly different soil and bedrock. The NDBES consists of northern Naples Bay, southern Naples Bay, and Dollar Bay, whose shorelines range from highly developed to undeveloped, respectively. This project explored the geological response of the system to these alterations using data from side-scan sonar, sediment grab samples, and vibracores. In highly urbanized northern Naples Bay, benthic substrates consist primarily of muddy sand with few oyster reefs. Southern Naples Bay and Dollar Bay, however, consist of coarser sediment, and are characterized by extensive mangrove shorelines and numerous fringing oyster reefs. The impact of anthropogenic alterations has significantly shifted sediment distributions in northern Naples Bay from a relatively coarser to a relatively fine grained substrate; to a lesser degree in southern Naples Bay, and Dollar Bay, this transition has not taken place due to the general lack of anthropogenic modifications made to this part of the system.     

Temporal changes in subglacial meltwater activity: field evidence from the late Devensian in the north of Ireland

Temporal changes in meltwater abundance, distribution and characteristics (controlling subglacial processes and ice sheet dynamics) can be inferred from subglacial sediment successions. Field evidence for changes in subglacial meltwater characteristics over time is presented from two sites (Doonan, Drummee) near a former late Weichselian (Devensian) ice centre in the north of Ireland. On a macroscale, both sites investigated show subglacial diamicton overlying glacially planated bedrock platforms. In more detail, primary sedimentary structures and facies variability show a complex relationship between depositional processes and meltwater characteristics at the ice/bed interface (IBI). Sedimentary evidence suggests sediment transport and deposition took place by low-viscosity subglacial slurries (mobile sediment–meltwater admixtures), which are part of a continuum between the processes of subglacial sediment deformation and subglacial meltwater flooding. Subtle changes in meltwater abundance and distribution at the IBI controlled slurry rheology, mechanisms of particle support and detailed sediment depositional processes.     

成因层序地层学的回顾与展望

回顾了以成因地层层序为基础的成因层序地层学的形成、发展与研究现状,对成因地层层序及其内部构成、高分辨率成因地层层序、成因地层层序的旋回性、非海相成因地层层序、成因地层层序与沉积物堆积速率等主要观点作了简略评述,并结合我国陆相沉积特征对成因层序地层学未来研究前景作了初步展望。     

古地理和古地形三维再造的发展趋势

“模拟技术”是当今科学研究的主要手段之一，把它应用于地质学的某些方面，能取得很好的效果，如盆地模拟已经发展到了比较成熟的阶段。但在古地理和古地形再造方面，国内外在这方面的研究甚少，尤其是在三维模拟方面。根据Hay等（1989）提出的物质平衡理论，在给定时间间隔内，作用在研究区表面的构造、侵蚀和沉积过程所赞成的沉积物的侵蚀总量与沉积总量之间物质守恒，与古地形再行喀古地理重建相结合，用三维数值模型来模拟研究区域的变化过程，用GIS技术把这个变化过程的动态显示出来，是一项具有理论意义和实践意义的工作。     

Assessment of sedimentation processes determined from side-scan sonar surveys in the Buffalo River, New York, USA

An assessment of sedimentation processes was made using side-scan sonar records collected over a 15-year period in the Buffalo River, New York, USA. Spatial and temporal changes in natural and man-made bedforms documented sediment erosion, slumping, and sediment depositional sites. Bedforms and sediment texture were used to divide the mapped portion of the river (9 km) into four sedimentary environments, with Section 1 at the upper end and Section 4 near the mouth where the river discharges into the east end of Lake Erie. Based on annual changes in side-scan sonar records, depositional rates decrease in the downriver direction. Section 2, located 5 km from the river’s mouth, contains sedimentary furrows. The persistence of furrows in this portion of the river requires both sediment erosion and deposition under a bidirectional flow regime. Results from this study confirmed that side-scan sonar is a valuable tool in understanding sediment dynamics and can be useful in river restoration decision making.     

Effect of Deepwater Horizon Oil on Growth Rates of Juvenile Penaeid Shrimps

Marsh shoreline, an important habitat for juvenile penaeid shrimps, was extensively oiled in coastal Louisiana by the Deepwater Horizon oil spill of 2010. The effect of this spill on growth was examined for brown shrimp Farfantepenaeus aztecus and white shrimp Litopenaeus setiferus held for 7 days in field mesocosms in Barataria Bay during May and August 2011, respectively. The experiments each had 10 treatment combinations, five apparent oil levels, each one with and without added food. Mesocosms were placed in northern Barataria Bay along shorelines that varied in oiling (designated as heavy, moderate, light, very light, or none based on NOAA surveys), and shrimp in half the mesocosms received additional food. Polycyclic aromatic hydrocarbon (PAH) concentrations determined from sediment cores collected at each mesocosm were significantly higher at heavy and moderate than very light shorelines and also higher at moderate than light and none shorelines. Brown shrimp grew more slowly at heavy than very light or none shorelines, and a statistically significant negative relationship was detected between brown shrimp growth rates and sediment PAH concentrations. In August, PAH sediment concentrations had decreased significantly from the values measured in May, no significant difference in white shrimp growth rates was detected among oiling levels, and no relationship was detected between white shrimp growth and sediment PAH concentrations. Both brown shrimp and white shrimp grew more rapidly in mesocosms where food was added. Our study shows that exposure to nonlethal concentrations of petroleum hydrocarbons can reduce growth rates of juvenile penaeid shrimps.