Larry G. Ward

VARIATIONS IN SEDIMENTARY ENVIRONMENTS AND ACCRETIONARY PATTERNS IN ESTUARINE MARSHES UNDERGOING RAPID SUBMERGENCE, CHESAPEAKE BAY

Abstract Major processes controlling sedimentological characteristics and accretionary rates and patterns were evaluated in two of the most common tidal marsh settings (estuarine tributaries and estuarine embayments) in Chesapeake Bay, one of the largest microtidal estuaries in the world. Textural analyses of 29 vibracores show spatial and temporal trends in grain size and organic matter which reflect differences in depositional settings within the marshes and changes in the controlling processes, as well as broader events in the surrounding watersheds. Spatially, the surficial sediments normally follow expected patterns, with coarsest sediments and lowest organic matter contents found in bay margin or channel margin locations, while the finest, organic-rich sediments occur in interior and submerged upland marshes. However, temporally, the marshes do not always show the expected increasing dominance of organic input over time (indicated by increasing % loss on ignition with decreasing depth), indicative of the marsh building process. Sea-level rise, tidal channel migrations, and anthropogenic effects likely contribute to decreases in organic matter contents in the upper sediment column. Furthermore, accretion rates and stratigraphic characteristics can be affected by these same processes, illustrating the importance of understanding temporal changes in marsh depositional environments over various time scales before sedimentologic and accretionary patterns are evaluated. Examination of the vibracores reveals that the marsh is composed of a complex stratigraphy with at least four major stratigraphic sequences occurring: (1) emerging or developing marsh sequences characteristic of channel margin and interior marshes; (2) submerging or mineral matter enriched marshes, also characteristic of channel and interior marsh areas; (3) high-energy marsh sequences characteristic of bay margin environments; and (4) submerged upland marsh sequences. The sedimentological properties of the stratigraphic sequences are controlled by overwash processes, tidal channel migrations, duration of tidal flooding, various anthropogenic effects, and sea-level rise. Accretion rates and patterns over the last ∼200 years, determined from pollen histories, differ between the marshes along the estuarine tributary (Nanticoke River) and those found in the tidal embayment (Monie Bay). Marshes in the upper-estuarine tributary have a larger riverine sediment input, and trapping of the high sediment loads in the upper estuary often yields marsh accretion rates (maximum of 0.74 cm/yr) which are greater than relative sea-level rise (∼0.4 cm/yr), resulting in stable marshes. By comparison, estuarine processes limit penetration of high sediment loads to the lower reaches of marshes along the tributary and, as a consequence, accretion rates are generally lower than the rate of relative sea-level rise, and marsh loss is prevalent. Overall, accretion rates in the marshes in the embayment are close to or less than the local sea-level rise and do not have as distinctive spatial patterns as the tributary marshes. Although marsh loss has yet to be identified as a significant process here, decreases in organic content of the upper sediment column may signal the early stages of submergence due to sea-level rise.

The Seafloor: A Key Factor in Lidar Bottom Detection

The environmental factors that determine the ability of airborne lidar bathymetry (ALB) to detect the seafloor are not well understood; however, water clarity is often considered the single factor for detection. A comparison of data from two different ALB systems (LADS-MKII and SHOALS-3000) of a small area offshore Gerrish Island, Maine, USA shows a striking correlation (95% overlap) in areas of no bottom detection that is independent of the tide status, the date of collection and the orientation of the survey flight. The laser measurements from the two ALB systems are compared to acoustic measurements of depth, seafloor slope, and backscatter from a Kongsberg EM3002 echosounder. The comparison shows that in water depths deeper than 7 m, there is a close correlation between the ALB detection patterns and bottom features. The study results indicate that lack of bottom detection by ALB does not necessarily indicate that water depths deeper than the surrounding areas have lidar strong bottom detection. No bottom detection in the study area actually reflects a change in bottom characteristics.

Variations in Physical Properties and Water Quality in the Webhannet River Estuary (Wells National Estuarine Research Reserve, Maine)*

Abstract Time-series observations and point measurements were used to assess seasonal patterns, tidal cycle variations, and the impact of major forcings on salinity, water temperature, turbidity, suspended sediments, and dissolved nutrients in the Webhannet River estuary (Wells National Estuarine Research Reserve). Because of the small size of the Webhannet River watershed, the main tributary of the estuary responds quickly to precipitation events, resulting in rapid increases in freshwater inputs, large decreases in salinity, and increases in suspended sediments and turbidity. Overall, suspended sediment concentrations were typically highest in the upper estuary and lowest in the Webhannet River, averaging 17.2 mg/L and 3.4 mg/L, respectively. The dissolved nutrient concentrations were variable; however, several species showed spatial and temporal patterns. Similar to suspended sediments, ammonium concentrations were typically highest in the upper estuary and lowest in the Webhannet River, averaging 4.60 μM and 1.01 μM, respectively. Average dissolved silica and nitrate plus nitrite concentrations were typically highest in the Webhannet River (32.96 and 5.13 μM, respectively) and the upper estuary (30.09 and 3.66 μM, respectively). Average orthophosphate concentrations decreased up-estuary from 0.56 μM near the mouth to 0.08 μM in the Webhannet River. The impacts of major storms were observed during Hurricane Bob in August 1991 and Tropical Storm Floyd in September 1999. As a result of the passage of Hurricane Bob, approximately 13.49 cm of rain fell in the watershed. The instantaneous peak discharge of the Webhannet River reached 15.9 m3/s (compared with a long-term average of ∼0.5 m3/s), causing the salinity to decrease to 0.1 psu in the upper estuary and to ∼7.0 psu in the lower estuary. The suspended sediment concentrations reached 34.6 mg/L in the upper estuary. The major decrease in salinity in the lower estuary as a result of Hurricane Bob was especially noteworthy because of the significant influence of oceanic inputs. Also notable was how rapid the salinities returned to prehurricane levels at high tide (within a few tidal cycles). Conversely, minimum daily salinities, which occurred at low tide, took over 10 days to rebound to prehurricane levels. Consequently, very large fluctuations in salinity and turbidity occurred over short time scales. Other heavy precipitation events during low-discharge conditions also increased turbidity and suspended sediment concentrations by eroding the adjacent salt marshes and tidal flats.

Observations of backscatter from sand and gravel seafloors between 170 and 250 kHz

Interpreting observations of frequency-dependence in backscatter from the seafloor offers many challenges, either because multiple frequencies are used for different observations that will later be merged or simply because seafloor scattering models are not well-understood above 100 kHz. Hindering the understanding of these observations is the paucity of reported, calibrated acoustic measurements above 100 kHz. This manuscript seeks to help elucidate the linkages between seafloor properties and frequency-dependent seafloor backscatter by describing observations of backscatter collected from sand, gravel, and bedrock seafloors at frequencies between 170 and 250 kHz and at a grazing angle of 45°. Overall, the frequency dependence appeared weak for all seafloor types, with a slight increase in seafloor scattering strength with increasing frequency for an area with unimodal, very poorly to moderately well sorted, slightly granular to granular medium sand with significant amounts of shell debris and a slight decrease in all other locations.

Temporal Variability in Salinity, Temperature and Suspended Sediments in a Gulf of Maine Estuary (Great Bay Estuary, New Hampshire)

Determining temporal and spatial variations of suspended sediments and other water column physical properties (e.g. temperature, salinity, turbidity) in estuarine systems require high-resolution observations over several scales of space and time (Uncles et al., 1988; Dyer, 2000; Grabemann and Krause, 2001; Schmidt and Luther, 2002). Although obtaining these types of measurements can be difficult due to time, equipment and monetary constraints, they are important for developing a fundamental scientific understanding of many estuarine processes, such as primary and secondary productivity, the transport and fate of contaminants, nutrient cycling, or sedimentation (Pritchard and Schubel, 1981; Ward et al., 1984; Fisher et al., 1988; Bilgili et al., 1996; Allen et al., 1998; Lee and Cundy, 2001; Sanford et al., 2001; Johnston et al., 2002; Verity, 2002). Accordingly, numerous studies have been conducted over the last several decades that seek to describe and quantify basic estuarine physics and sedimentological processes (see Kennedy, 1984; Nichols and Biggs, 1985; Eisma, 1993, and Dyer, 2000 for reviews). For instance, it has been long understood that the combination and balance of freshwater input from rivers and tidal energy controls or strongly influences net non-tidal circulation (density driven), water column stratification, and sedimentation (Pritchard, 1952; Schubel and Biggs, 1969; Biggs, 1970; Schubel, 1972; Allen et al., 1980; Biggs and Cronin, 1981; Ward and Twilley, 1986; Dyer, 2000; Sanford et al., 2001; Schmidt and Luther, 2002).

Assessment of Elevation Uncertainty in Salt Marsh Environments using Discrete-Return and Full-Waveform Lidar

ABSTRACT Rogers, J.N.; Parrish, C.E.; Ward, L.G., and Burdick, D.M., 2016. Assessment of elevation uncertainty in salt marsh environments using discrete-return and full-waveform lidar. In: Brock, J.C.; Gesch, D.B.; Parrish, C.E.; Rogers, J.N., and Wright, C.W. (eds.), Advances in Topobathymetric Mapping, Models, and Applications. Journal of Coastal Research, Special Issue, No. 76, pp. 107–122. Coconut Creek (Florida), ISSN 0749-0208. Lidar data can serve as an important source of elevation information for studying, monitoring and managing salt marshes. However, previous studies have shown that lidar data tend to have greater vertical uncertainty in salt marshes than in other environments, hindering the ability to resolve small elevation differences that can be ecologically significant in marshes. For coastal scientists and managers to effectively collect, evaluate, and/or use lidar data in salt marshes, factors affecting elevation uncertainty (e.g., plant species, season, and lidar processing methods) must be well understood. This study addresses this need using discrete-return (DRL) and full-waveform lidar, along with field-surveyed reference data, for four marshes on Cape Cod, Massachusetts (USA). The lidar bias and standard deviation were computed across all four marsh systems and four major taxa using varying interpolation and filtering methods. The effects of seasonality were also investigated using lidar data acquired in the summer and the following spring. Relative uncertainty surfaces (RUS) were computed from lidar waveform-derived metrics and examined for their utility and correlation with individual lidar errors. The results clearly illustrate the importance of seasonality, species, and lidar interpolation and filtering methods on elevation uncertainty in salt marshes. Results also demonstrate that RUS generated from lidar waveform features are useful in qualitative assessments of lidar elevation uncertainty and correlate well with vegetation height (r = 0.85; n = 268). Knowledge of where DRL uncertainty persists within salt marshes and the factors influencing the higher uncertainty should facilitate the development of better correction methods.

Long-term seafloor monitoring at an open ocean aquaculture site in the western Gulf of Maine, USA: development of an adaptive protocol.

The seafloor at an open ocean finfish aquaculture facility in the western Gulf of Maine, USA was monitored from 1999 to 2008 by sampling sites inside a predicted impact area modeled by oceanographic conditions and fecal and food settling characteristics, and nearby reference sites. Univariate and multivariate analyses of benthic community measures from box core samples indicated minimal or no significant differences between impact and reference areas. These findings resulted in development of an adaptive monitoring protocol involving initial low-cost methods that required more intensive and costly efforts only when negative impacts were initially indicated. The continued growth of marine aquaculture is dependent on further development of farming methods that minimize negative environmental impacts, as well as effective monitoring protocols. Adaptive monitoring protocols, such as the one described herein, coupled with mathematical modeling approaches, have the potential to provide effective protection of the environment while minimize monitoring effort and costs.

High-frequency seafloor scattering in a dynamic harbor environment: Observations of change over time scales of seconds to seasons

Predicting sonar performance in a harbor environment can be made challenging by the dynamics of the upper and lower surface boundaries. In this talk we examine measurements of the seabed in Portsmouth Harbor, NH. Tidally influenced currents in the Piscataqua River make Portsmouth Harbor one of the most challenging commercial ports to navigate in the northeastern United States. The level of interaction between these currents and the harbor floor is a function of the substrate type. Stereo-camera observations in a sand-wave field near the harbor entrance show fluctuations in microscale roughness and optical reflectance at time scales of seconds during periods of high current. Despite the observed microscale dynamics in the sand-wave field, acoustic observations of both high-frequency seabed scattering strength and mesoscale topography in the same area appear stationary over time scales up to seasons. The low-level of observed fluctuations in scattering strength from the sand-wave field are commensurate with...

Sedimentology of the New Hampshire Inner Continental Shelf Based on Subbottom Seismics, Side-Scan Sonar, Bathymetry, and Bottom Samples

Typical of glaciated environments, the inner continental shelf of New Hampshire is composed of bedrock outcrops, remnants of glacial deposits (for example, drumlins), sand and gravel deposits, as well as muddier sediments farther offshore. A number of previous studies have defined the general trends of the New Hampshire inner shelf from the coarser deposits nearer the shore to the muddier outer basins. Most recently, a seismic survey (150 km of side-scan sonar and subbottom seismic profiles), as well as bottom sediment sampling (74 stations), has provided a detailed bottom map of the southern New Hampshire shelf area (landward of the 30-m contour). The surficial sediments within this area range from very fine sand to gravel. Bedrock outcrops are common. The seismic survey indicated several large sand deposits exceeding 6-8 m in thickness that occur relatively close to the coast. These sedimentary units, which are within 3 km of the shoreline, are composed of fine to medium sands. Examination of the genera...

Relationships between seafloor substrates, benthic epifauna, and spatial properties of multibeam echosounder bathymetry and backscatter data

The seafloor in the mouth of the Piscataqua River Estuary, near Portsmouth, New Hampshire, contains several geomorphological regions that are evident in bathymetric data from multibeam sonar soundings. Those morphological regions generally are associated with surficial sediment type classes previously identified from sediment samples. We demonstrate that the morphologies are distinguishable using parameters from models of spatial variation for gridded depth soundings and backscatter values. Ground‐truth seafloor‐video transects data suggest that apparent facies and morphological regions are associated with different surficial sediment classes and rates of facies transitions. We show that the spatial variability of depth and backscatter is associated with the substrate transition rate, the number of microhabitats (microfacies), and distribution of certain epifauna identified using video. We show that, in some cases, a single region defined based upon spatial morphological attributes from depth data and app...