Lynn A. Leonard

Hydrodynamics and Sediment Transport Through Tidal Marsh Canopies

Flow dynamics on the vegetated surfaces of coastal wetlands may impact a wide range of processes including geochemical exchanges at the sediment water interface, larval recruitment and dispersion, and sediment deposition and retention. Nevertheless, little field data exist which describe flow behavior through emergent vegetated wetlands and its control over sediment transport and deposition. The goal of this paper is to describe canopy flow dynamics and suspended particulate transport for a variety of marshes that differ with respect to vegetation type and tidal regime. In situ measurements of tidal currents were collected in micro-, meso-, and macrotidal marshes of the Pacific, Gulf of Mexico and Atlantic coasts of the US and in a UK marsh on the North Sea. Mean flow speeds, vertical velocity profiles, and turbulence intensities were evaluated as were canopy characteristics and total suspended solid (TSS) levels. Broad scale flow characteristics exhibited little variation among sites. Mean flow speeds were almost always less than 10 cm s-1 regardless of tidal regime. The presence of vegetation (regardless of type) significantly reduced both flow speed and turbulence intensity relative to adjacent open water areas. Variations in canopy morphology and the physical structure of individual plants control fine scale hydrodynamics, and influence particle advection, and particle settling. Flow speed magnitude and the importance of creek channel processes, however, appear to be most strongly influenced by the tidal regime in each of the marsh types examined. ADDITIONALINDEXWORDS: salt marsh, flow dynamics Journal of Coastal Research SI 36 459-469 (ICS 2002 Proceedings) Northern Ireland ISSN 0749-0208

Controls of sediment transport and deposition in an incised mainland marsh basin, southeastern North Carolina

Sedimentation on the surface of tidal marshes is a process that is controlled by the interactions of a complex set of variables. To adequately describe the patterns of sediment transport and deposition in any particular system requires extensive sampling of biological, physical, sedimentological, and geomorphological parameters. In this study, measurements of sediment deposition, marsh elevation, water level, total suspended solid (TSS) loads, overmarsh current speeds, and vegetative cover were used to determine which of these factors control sediment distribution patterns in a small marsh basin in southeastern North Carolina. The results of this study suggest that marsh elevation exerted significant control over deposition but that its effects were largely muted by other processes. Creek hydrology, sediment instability associated with areas of new creek development, plant/flow interactions, and tidal creek TSS concentrations also influenced sediment deposition in the study area. Flow patterns, resulting from the interaction between tidal stage and marsh topography, controlled the dispersal of particulate matter across the marsh surface and contributed to significant depositional differences among sampling sites as a function of tidal stage. The results of this study indicate that tidal creek geometry, creek channel position, and tidal stage interact synergistically to control sediment/particulate delivery on the surfaces of tidal wetlands.

Using transplanted oyster (Crassostrea virginica) beds to improve water quality in small tidal creeks: a pilot study

Abstract 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.

FLOW DYNAMICS AND SEDIMENTATION IN SPARTINA ALTERNIFLORA AND PHRAGMITES AUSTRALIS MARSHES OF THE CHESAPEAKE BAY

The introduction of invasive species such as Phragmites australis in the Chesapeake Bay has been viewed to be deleterious to habitat quality. Little is known, however, on the extent to which the replacement of Spartina alterniflora by Phragmites affects hydrodynamics and sediment trapping on the surface of impacted marshes. This study examined sediment deposition, sediment mobility, and flow conditions in adjacent Phragmites australis and Spartina alternifora marshes in Prospect Bay, Maryland, USA in order to determine if differences in plant morphology affect surficial flow properties and particle dispersion patterns. Measures of fine-scale flow dynamics, total suspended sediment (TSS) concentration, and particulate deposition were obtained at various distances from open water across the marsh surface over four sequential tidal cycles in Fall 1999. The hydrodynamic data indicate that both the gross and fine-scale properties of tidal flows were similar in both types of vegetation and that flow conditions were conductive to particle deposition. TSS concentrations did not differ between canopy types and decreased over time in both systems. There was no difference in TSS reduction over distance between Spartina and Phragmites. The sediment trap data indicate that maximum deposition occurs closer to open water in both Spartina and Phragmites and that the organic content of deposited matter increased with distance into the marsh interior. This study provides the first in situ, high resolution, over-marsh flow data for marshes dominated by Phragmites. The data provided herein suggest that differences in vegetative cover do not significantly affect flow regime, sediment transport, and sediment deposition patterns in the marsh systems examined.

Recent and historic drivers of landscape change in the everglades ridge, Slough, and Tree Island Mosaic

More than half of the original Everglades extent formed a patterned peat mosaic of elevated ridges, lower and more open sloughs, and tree islands aligned parallel to the dominant flow direction. This ecologically important landscape structure remained in a dynamic equilibrium for millennia prior to rapid degradation over the past century in response to human manipulation of the hydrologic system. Restoration of the patterned landscape structure is one of the primary objectives of the Everglades restoration effort. Recent research has revealed that three main drivers regulated feedbacks that initiated and maintained landscape structure: the spatial and temporal distribution of surface water depths, surface and subsurface flow, and phosphorus supply. Causes of recent degradation include but are not limited to perturbations to these historically important controls; shifts in mineral and sulfate supply may have also contributed to degradation. Restoring predrainage hydrologic conditions will likely preserve remaining landscape pattern structure, provided a sufficient supply of surface water with low nutrient and low total dissolved solids content exists to maintain a rainfall-driven water chemistry. However, because of hysteresis in landscape evolution trajectories, restoration of areas with a fully degraded landscape could require additional human intervention.

Characteristics of surface-water flows in the ridge and slough landscape of Everglades National Park: Implications for particulate transport

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.

The Effects of Thin Layer Sand Renourishment on Tidal Marsh Processes: Masonboro Island, North Carolina

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 nos-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.

Hydrologic Versus Biogeochemical Controls of Denitrification in Tidal Freshwater Wetlands

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.

Integrating Environmental Monitoring and Observing Systems in Support of Science to Inform Decision-Making: Case Studies for the Southeast

Abstract IOOS ® is a network of coastal and ocean observing systems providing data, data management, models and other products in support of science-based decision-addressing issues of marine safety, coastal hazards, climate variability, ecosystem management, and water quality. IOOS Regional Associations (RAs) work at federal, state, and local levels assuring that systems provide information at appropriate spatial and temporal scales addressing local to regional concerns and informing decision-making. Stakeholder engagement is the primary way for RAs to make positive impacts within their region. The Southeast Coastal Ocean Observing Regional Association (SECOORA) strives to identify stakeholders, determine observing needs, and provide data and products to support science-informed decision-making. In this chapter, we review the state of observing system efforts in the Southeast; present case studies demonstrating the value of integrating data from these systems to support marine safety, water quality, and ecosystem management decision-making; and present recommendations for the path forward.

Observing system depiction of circulation on the SE US coastal ocean

A series of observing platforms deployed between Cape Henry, VA and Cape Canaveral, FL provide multi-year records of winds, surface currents, current profiles and vertical stratification at a number of locations on the shallow southeast US continental shelf and are used to construct a seasonal circulation pattern. Upwelling-favorable winds and circulation in summer promote poleward shelf transport and stratification in summertime, assisted near the shelfbreak by the influence of the Gulf Stream. Downwelling-favorable winds and circulation dominate in fall and winter, but mean alongshore currents are weak and cross-shelf exchange is strong relative to the alongshore flow. Convection driven by thermal cooling over the shallowly sloping shelf is explored as a potentially important component of the observed wintertime circulation. This observing system-based Eulerian depiction of the circulation described above is compared with previous published seasonal circulation patterns and Lagrangian representations from drifter and modeling studies. The consistency of the results over the domain is considered to assess the adequacy of the observing system to resolve subregional-scale circulation features.