Southeastern Atlantic Coastal Plain Research Articles

Hydrometeorological Trends in a Low-Gradient Forested Watershed on the Southeastern Atlantic Coastal Plain in the USA

Hydrology and meteorological data from relatively undisturbed watersheds aid in identifying effects on ecosystem services, tracking hydroclimatic trends, and reducing model uncertainties. Sustainable forest, water, and infrastructure management depends on assessing the impacts of extreme events and land use change on flooding, droughts, and biogeochemical processes. For example, global climate models predict more frequent high-intensity storms and longer dry periods for the southeastern USA. We summarized 17 years (2005–2021) of hydrometeorological data recorded in the 52 km2, third-order Turkey Creek watershed at the Santee Experimental Forest (SEF), Southeastern Coastal Plain, USA. This is a non-tidal headwater system of the Charleston Harbor estuary. The study period included a wide range of weather conditions; annual precipitation (P) and potential evapotranspiration (PET) ranged from 994 mm and 1212 mm in 2007 to 2243 mm and 1063 in 2015, respectively. The annual runoff coefficient (ROC) varied from 0.09 in 2007 (with water table (WT) as deep as 2.4 m below surface) to 0.52 in 2015 (with frequently ponded WT conditions), with an average of 0.22. Although the average P (1470 mm) was 11% higher than the historic 1964–1976 average (1320 mm), no significant (α= 0.05) trend was found in the annual P (p = 0.11), ROC (p = 0.17) or runoff (p = 0.27). Runoff occurred on 76.4% of all days in the study period, exceeding 20 mm/day for 1.25% of all days, mostly due to intense storms in the summer and lower ET demand in the winter. No-flow conditions were common during most of the summer growing season. WT recharge occurred during water-surplus conditions, and storm-event base flow contributed 23–47% of the total runoff as estimated using a hydrograph separation method. Storm-event peak discharge in the Turkey Creek was dominated by shallow subsurface runoff and was correlated with 48 h precipitation totals. Estimated precipitation intensity–duration–frequency and flood frequency relationships were found to be larger than those found by NOAA for the 1893–2002 period (for durations ≥ 3 h), and by USGS regional frequencies (for ≥10-year return intervals), respectively, for the same location. We recommend an integrated analysis of these data together with available water quality data to (1) assess the impacts of rising tides on the hydroperiod and biogeochemical processes in riparian forests of the estuary headwaters, (2) validate rainfall–runoff models including watershed scale models to assess land use and climate change on hydrology and water quality, and (3) inform watershed restoration goals, strategies, and infrastructure design in coastal watersheds.

Long‐term hydro‐meteorology and water quality data from low‐gradient catchments of varying scales on the Santee experimental Forest, South Carolina

AbstractThe forest landscape of the Southeastern Atlantic coastal plain of the United States is experiencing increased threats from extreme precipitation events, tropical storms, flooding, droughts, and growing urbanization impacting on vegetation, road infrastructure, water extraction and other related ecosystem processes (e.g., storm runoff, nutrient and carbon cycling) and functions (e.g., flood protection, aquatic habitat). We describe ongoing hydro‐meteorologic monitoring on two first—(1.6 km2 each), the second—(5 km2), and the third—(52.5 km2) order catchments. The catchments are located on the Santee Experimental Forest at the headwaters of the Cooper River that drains into Charleston Harbour, South Carolina. Monitoring started in 1946, with a gap between 1982 and 1989, includes precipitation, weather, streamflow, water table, and water quality (anions, cations, and physical parameters) at varying temporal scales. The catchments are forest ecosystems dominated by loblolly (Pinus taeda) and longleaf pine (Pinus palustris) (LLP) mixed hardwood stands on the uplands and bottomland hardwoods on well‐drained to poorly‐drained soils characteristic of the low‐gradient Atlantic Coastal Plain. These data support multiple field and modelling studies, ranging from water and carbon budgets to hydrologic and biogeochemical processes to effects of microtopography and extreme climate on hydrology and water quality of these coastal catchments withstands regenerated since Hurricane Hugo (1989). One significant finding from the long‐term data was a reversal of the pre‐Hugo flow pattern between the paired watersheds 3 years after Hugo. This reversal persisted for 10 years and was attributed to change in forest vegetation due to hurricane damage to older pine and larger hardwood stands followed by delayed regeneration before full forest recovery. These publicly available data serve as a reference within a rapidly developing landscape and also for addressing potential impacts of extreme climate and forest disturbance on ecohydrology, biogeochemistry, habitat, water supply and infrastructure using validated models and tools.

Using Convolutional Neural Networks for Detection and Morphometric Analysis of Carolina Bays from Publicly Available Digital Elevation Models

Carolina Bays are oriented and sandy-rimmed depressions that are ubiquitous throughout the Atlantic Coastal Plain (ACP). Their origin has been a highly debated topic since the 1800s and remains unsolved. Past population estimates of Carolina Bays have varied vastly, ranging between as few as 10,000 to as many as 500,000. With such a large uncertainty around the actual population size, mapping these enigmatic features is a problem that requires an automated detection scheme. Using publicly available LiDAR-derived digital elevation models (DEMs) of the ACP as training images, various types of convolutional neural networks (CNNs) were trained to detect Carolina bays. The detection results were assessed for accuracy and scalability, as well as analyzed for various morphologic, land-use and land cover, and hydrologic characteristics. Overall, the detector found over 23,000 Carolina Bays from southern New Jersey to northern Florida, with highest densities along interfluves. Carolina Bays in Delmarva were found to be smaller and shallower than Bays in the southeastern ACP. At least a third of Carolina Bays have been converted to agricultural lands and almost half of all Carolina Bays are forested. Few Carolina Bays are classified as open water basins, yet almost all of the detected Bays were within 2 km of a water body. In addition, field investigations based upon detection results were performed to describe the sedimentology of Carolina Bays. Sedimentological investigations showed that Bays typically have 1.5 m to 2.5 m thick sand rims that show a gradient in texture, with coarser sand at the bottom and finer sand and silt towards the top. Their basins were found to be 0.5 m to 2 m thick and showed a mix of clayey, silty, and sandy deposits. Last, the results compiled during this study were compared to similar depressional features (i.e., playa-lunette systems) to pinpoint any similarities in origin processes. Altogether, this study shows that CNNs are valuable tools for automated geomorphic feature detection and can lead to new insights when coupled with various forms of remotely sensed and field-based datasets.

Southeastern Pocket Gopher (Geomys pinetis) Tunnels Provide Stable Thermal Refugia

Animals living underground deal with multiple physiological challenges, such as hypoxia and hypercarbia, but may have reduced thermoregulation demands because of the more stable underground microclimate. Southeastern pocket gophers (Geomys pinetis Rafinesque) occur in the fire-adapted, open-pine forests of the southeastern Atlantic Coastal Plain where prescribed fire is commonly used to manage understory vegetation. They are almost exclusively fossorial, and their tunnels provide ecological services, including shelter, for a suite of commensal vertebrates and invertebrates. To quantify potential thermoregulation benefits of southeastern pocket gopher tunnels, we compared temperatures in active tunnels (n = 31) to aboveground temperatures during winter (December 2018–February 2019), and to aboveground temperatures during prescribed fire events (n = 16) occurring in spring (March–May 2019). During winter, tunnels provided a more stable thermal environment (average range = 6.5 ± 0.8 C; mean ± se) relative to aboveground (average range = 24.8 ± 1.8 C) temperatures. Similarly, mean tunnel temperature range (2.05 ± 0.5 C) was significantly narrower than aboveground temperature range associated with fire events (497.0 ± 101.4 C). Clearly, tunnels provide a stable thermal environment for pocket gophers and commensals that use their tunnel systems.

Depositional sequence stratigraphy of Turonian to Santonian sediments, Cape Fear arch, North Carolina Coastal Plain, USA

ABSTRACT: A new sequence stratigraphic framework for Turonian to Santonian (94-84 Ma) sediments is established using data from the USGS Kure Beach and Elizabethtown cores collected from the Atlantic Coastal Plain of North Carolina (NC). These sediments represent some of the oldest marine units deposited on the southeastern Atlantic Coastal Plain and record the early development of a clastic wedge atop crystalline basement. Sediments were deposited as transitional marginal-marine to marine units in a complex interplay of fluvial, estuarine, and shelf environments. Repetitive lithologies and minimal biostratigraphic control requires an integrated analysis of grain-size data, geophysical logs, biostratigraphy, and 87Sr/86Sr isotopic data to identify systems tracts and establish a sequence stratigraphic framework. From this integrated approach, three Turonian to Santonian sequences in the Elizabethtown core and six in the Kure Beach core are identified. The new sequences from oldest to youngest are Clubhouse II, Fort Fisher I, Fort Fisher II, Collins Creek I, Collins Creek II, Pleasant Creek I, and Pleasant Creek II. Sequences from North Carolina document significant shifts of global and regional sea-level during greenhouse conditions in the early Late Cretaceous. Maximum sea-level rise occurred globally during the early Turonian and is documented from the marine sediments of the Clubhouse II sequence. This sequence is unconformably overlain by terrestrial sediments deposited during a major fall in sea level and maximum progradation of the shoreline, as evidenced by the Fort Fisher I sequence. Global sea-level rise in the Coniacian resulted in the deposition of the Fort Fisher II sequence, which is present only in the Kure Beach core. Local marine circulation and erosion on the shelf is suggested by the absence of the Collins Creek I sequence at Kure Beach; this sequence is present only in the up-dip Elizabethtown core. Activation of a possible buried fault structure along the Cape Fear arch resulted in the formation of a regional depocenter during the late Coniacian to early Santonian and is reflected in the unusual thickness of the Collins Creek II and Pleasant Creek I sequences. The return to a more global sea-level influence occurred in the late Santonian with the deposition of the Pleasant Creek II sequence. A comparison of temporal distribution of sequences in the Elizabethtown and Kure Beach cores to corresponding sequences in New Jersey indicates significant differences in erosional and tectonic processes in the Cape Fear region during the Turonian and Santonian.

Long-Term Water Table Dynamics of Forested Wetlands: Drivers and their Effects on Wetland Hydrology in The Southeastern Atlantic Coastal Plain

Sustainable management of forested wetlands requires an understanding of water Table (WT) dynamics affected by rainfall and evapotranspiration (ET) and management practices, designed to sustain water quality, quantity, and ecosystem functions and productivity. In this paper analysis of long-term data from four drained and six undrained sites on coastal forested wetlands showed that their growing season WT dynamics depends upon ET as affected by vegetation stand age, with undrained sites yielding deeper WT than the drained young sites but shallower than the mature ones. Wetland hydrology criterion was not met on any of the drained sites including with controlled drainage and one unditched site with moderately well-drained sandy soil, where the WT was within 30 cm of the surface for <8% of the time compared to 31% or greater on undrained sites. WT response was similar on both drained and undrained watersheds soon after vegetation removal compared to the baseline level. Similarly, the WT dynamics on all soil types and vegetation behaved similarly during extreme storms. No trend was found in limited annual mean WT data. These results may have implications for coastal wetland forest restoration and modeling studies relating wetland hydrology as affected by anthropogenic and natural disturbances.

Mega-meander paleochannels of the southeastern Atlantic Coastal Plain, USA

Paleodischarge estimates based on the slope-area method and channel boundaries determined from stratigraphic cross-sections indicate that large, terminal Pleistocene meandering paleochannels (“mega-meanders”) in river valleys of the southeastern Atlantic Coastal Plain of the United States represent bankfull flows that were at least double the magnitude of those on modern rivers. Correlation of radiocarbon- and luminescence-dated paleomeanders with previously reported pollen and eolian sedimentary records suggests that greater discharge was driven by seasonally wetter conditions resulting from dynamic changes to regional precipitation and runoff that occurred in association with global warming at the end of the Pleistocene. While reflecting larger channel-forming flows, the exceptionally large widths and radii of curvature of mega-meanders were nonetheless maintained by a relatively modest discharge magnitude that was between two and four times larger than modern bankfull flow and within the size range of the present two- to five-year flood. Despite their large planform, the paleochannels conveyed relatively modest bankfull discharges because a wide, shallow shape limited their cross-sectional area and hydraulic radius. Within the late Quaternary evolution of fluvial systems in the region, scrolled mega-meanders constitute a transitional meandering planform that remained influenced by large volumes of sandy bedload following the sand-bed braided channels of the late Wisconsin interval, circa 30–17 ka. In addition to greater discharge, the large planimetric dimensions of paleochannels reflect a lack of cohesive vertical accretion facies on paleomeander floodplains, a sediment regime that transported large quantities of bedload sand, and the influence of these factors on channel boundary composition, bank stability, and channel shape. Findings underscore the importance of reconstructing channel cross-sectional dimensions and slope when estimating discharge for infilled paleomeanders. This approach reduces uncertainties surrounding channel cross-sectional area, gradient, and boundary composition inherent to studies lacking subsurface data that rely upon meander geometry to retrodict discharge.

Soil Organic Carbon Stocks in a Large Eutrophic Floodplain Forest of the Southeastern Atlantic Coastal Plain, USA

Anthropogenic land use has significantly altered sediment and nutrient dynamics at watershed-scales, resulting in significant redeposition within large floodplain ecosystems. Some upland land uses have had documented negative effects on soil carbon (C) stocks, although the specific impacts of these disturbances on soil organic carbon (SOC) dynamics in depositional environments are poorly understood. Assessment of SOC stocks in floodplain environments will allow for more precise estimates of C distribution at watershed and regional scales. In this study, we measured SOC pools to depths of 100 and 200 cm in four distinct floodplain landscapes (natural levee, flats, mineral wetlands, organic wetlands) in a large bottomland forest within Congaree National Park, South Carolina, USA. Mean SOC stocks to a depth of 100 cm were 108–109 Mg C ha−1 in flats and levees, 193 Mg C ha−1 in mineral wetlands, and 533 Mg C ha−1 in organic wetlands. In addition, hydric soils contained significantly more SOC in deep horizons (100–200 cm depths). At a regional scale, similar alluvial soils within large floodplains were estimated to store approximately 0.l Pg of SOC. These results highlight the importance of inclusion of deep SOC storage in alluvial settings when estimating watershed C budgets.

Geochronologic evidence for a possible MIS-11 emergent barrier/beach-ridge in southeastern Georgia, USA

Predominantly clastic, off-lapping, transgressive, near-shore marine sediment packages that are morphologically expressed as subparallel NE-trending barriers, beach ridges, and associated back-barrier areas, characterize the near-surface stratigraphic section between the Savannah and the Ogeechee Rivers in Effingham County, southeastern Georgia. Each barrier/back-barrier (shoreline) complex is lower than and cut into a higher/older complex. Each barrier or shoreline complex overlies Miocene strata. No direct age data are available for these deposits. Previous researchers have disagreed on their age and provenance. Using luminescence and meteoric beryllium-10 (10Be) inventory analyses, we estimated a minimum age for the largest, westernmost, morphologically identifiable, and topographically-highest, barrier/beach-ridge (the Wicomico shoreline barrier) and constrained the age of a suite of younger barrier/beach-ridges that lie adjacent and seaward of the Wicomico shoreline barrier.At the study site, the near-shore marine/estuarine deposits underlying the Wicomico shoreline barrier are overlain by eolian sand and an intervening zone-of-mixing. Optically stimulated luminescence (OSL) data indicate ages of ≤43 ka for the eolian sand and 116 ka for the zone-of-mixing. Meteoric 10Be and pedostratigraphic data indicate minimum residence times of 33.4 ka for the eolian sand, 80.6 ka for the zone-of-mixing, and 247 ka for the paleosol. The combined OSL and 10Be age data indicate that, at this locality, the barrier/beach ridge has a minimum age of about 360 ka. This age for the Wicomico shoreline-barrier deposit is the first for any Pleistocene near-shore marine/estuarine deposit in southeast Georgia that is conclusively older than 80 ka. The 360-ka minimum age is in agreement with other geochronologic data for near-coastline deposits in Georgia and South Carolina. The geomorphic position of this barrier/beach-ridge is similar to deposits in South Carolina considered to be ∼450 ka to >1 Ma. The age and geomorphic data for Georgia and South Carolina possibly suggest the presence of MIS-11 (∼420−360 ka) shoreline deposits between 15 m and 28 m above present sea level in the Southeastern Atlantic Coastal Plain.

Results of rainfall simulation to estimate sediment-bound carbon and nitrogen loss from an Atlantic Coastal Plain (USA) ultisol

Assessment of erosion impact on soil carbon and nitrogen loss and redistribution within landscapes is needed to develop estimates of soil carbon sequestration potential, soil quality management plans, and to evaluate potential for transport of sediment bound agrochemicals. We used variable intensity rainfall simulations to quantify the effects of tillage, conventional and strip, and antecedent soil water content on sediment-bound carbon and nitrogen loss from a Tifton loamy sand located in the southeastern Atlantic Coastal Plain (USA). Carbon and nitrogen loss via erosion of silt+clay sized versus sand sized sediment particles were quantified. Antecedent water content had no effect on mean sediment loss within tillage treatments, but losses from conventional till treatments were significantly greater than from strip till. Sediment lost as silt+clay was from 58 to 78% of the total under conventional and from 30 to 39% under strip tillage. The fraction of sediment lost as silt+clay versus sand was greater under conventional tillage than under strip tillage. Within-event sediment carbon enrichment compared to the top 2cm of soil was 0.9–7.2 for conventional and 0.6–3.7 for strip tillage. The strip till silt+clay fraction had significantly higher carbon content than the strip till sand fraction and the conventional till silt+clay fraction. Carbon loss from treatments was directly proportional to sediment loss. However, the conventional till treatments lost 4.6–6 times more carbon from the silt+clay sized fraction and 1.9–4.8 times more carbon from the sand sized fraction than strip till treatments. Results suggest that the higher proportion of silt+clay fraction sediment loss from conventional till may deplete nitrogen enriched organic matter while decreased erosion from strip till may serve to increase retention of organic nitrogen. Findings also indicate that an approach that adjusts loss estimates of organic carbon and nitrogen by using wet-sieved subsamples for analysis followed by standardization against total bulk sediment loss accounts for introduced errors from both sub-sampling efficiency and disturbance.

Late Pleistocene marginal marine ostracodes from the southeastern Atlantic coastal plain and their paleoenvironmental implications

Upper Pleistocene deposits from 21 localities in Maryland, Virginia, South Carolina, North Carolina, and northern Florida yielded 77 ostracode species; virtually all are living today in brackish and marine water. Five late Pleistocene ostracode biofacies signifying lagoonal, oyster bank, estuarine, open sound, and inner sublittoral environments were delineated using Principal Coordinate Analysis. During the late Pleistocene, the Lagoonal and Oyster Bank Biofacies predominated in the Chesapeake Bay area, whereas east-central North Carolina was characterized by an Open Sound Biofacies similar to that in Pamlico Sound today. The Inner Sublittoral Biofacies was present in southeastern Virginia and along the South Carolina coast. The Estuarine Biofacies was found only in the Chesapeake Bay region. Paleoclimates were inferred by a comparison of Holocene and late Pleistocene ostracode zoogeography; apparently the climate during the late Pleistocene was as warm as, and in some areas warmer than at the same latitudes today. Ostracode species are illustrated by scanning electron photomicrographs Cyprideis margarita, Neocaudites atlan-tica, and Microcytherura norfolkensis are described as new species.

Sensitivity of Stream flow and Water Table Depth to Potential Climatic Variability in a Coastal Forested Watershed1

Dai, Zhaohua, Carl C. Trettin, Changsheng Li, Devendra M. Amatya, Ge Sun, and Harbin Li, 2010. Sensitivity of Streamflow and Water Table Depth to Potential Climatic Variability in a Coastal Forested Watershed. Journal of the American Water Resources Association (JAWRA) 1–13. DOI: 10.1111/j.1752‐1688.2010.00474.xAbstract: A physically based distributed hydrological model, MIKE SHE, was used to evaluate the effects of altered temperature and precipitation regimes on the streamflow and water table in a forested watershed on the southeastern Atlantic coastal plain. The model calibration and validation against both streamflow and water table depth showed that the MIKE SHE was applicable for predicting the streamflow and water table dynamics for this watershed with an acceptable model efficiency (E &gt; 0.5 for daily streamflow and &gt;0.75 for monthly streamflow). The simulation results from changing temperature and precipitation scenarios indicate that climate change influences both streamflow and water table in the forested watershed. Compared to current climate conditions, the annual average streamflow increased or decreased by 2.4% with one percentage increase or decrease in precipitation; a quadratic polynomial relationship between changes in water table depth (cm) and precipitation (%) was found. The annual average water table depth and annual average streamflow linearly decreased with an increase in temperature within the range of temperature change scenarios (0‐6°C). The simulation results from the potential climate change scenarios indicate that future climate change will substantially impact the hydrological regime of upland and wetland forests on the coastal plain with corresponding implications to altered ecosystem functions that are dependent on water.

Life-history Aspects of Stereochilus marginatus, with a Comparison of Larval Development in Syntopic S. marginatus and Pseudotriton montanus (Amphibia: Plethodontidae)

Abstract The plethodontid salamanders Stereochilus marginatus (Many-lined Salamander) and Pseudotriton montanus (Mud Salamander) have overlapping distributions in the southeastern Atlantic Coastal Plain, where they often co-occur in low, swampy habitats. The main objective of this study was to document life-history traits of S. marginatus at Cool Springs in eastern North Carolina, and compare these findings with life-history parameters of a population surveyed in the late 1960s at a nearby locality (Croatan Forest). A second objective was to compare larval development of S. marginatus and P. montanus at Cool Springs. I found that S. marginatus has a larval period of 13–14 months, hatching in early spring and undergoing metamorphosis late in the second spring. Males may breed initially as early as the autumn following metamorphosis, at 19–21 months of age; females probably require an additional year to attain maturity, ovipositing initially at 3 years. Clutch sizes, based on counts of yolked ovarian follic...

Rapid dissolution of calcareous nannofossils: a case study from freshly cored sediments of the south-eastern Atlantic Coastal Plain.

Rapid dissolution of calcareous nannofossils: a case study from freshly cored sediments of the south-eastern Atlantic Coastal Plain.

Establishment of wetlands on the southeastern Atlantic Coastal Plain: Paleolimnological evidence of a mid-Holocene hydrologic threshold from a South Carolina pond

The series of climatic events affecting landscapes on the southeastern Atlantic Coastal Plain over the past several thousand years is poorly understood. This is due, in part, to the paucity of depositional basins in this region containing adequately preserved microfossils for paleoecological analyses. This research reconstructs past hydrologic environments from the microfossil record of Peat Bay, a small, peat-filled pond in south-central South Carolina. We cored the deepest location in the pond and retrieved 120 cm of sediment representing 5200 yr of deposition. Fine clays at the base of the core resembled sediments deposited in abandoned channels of the nearby Savannah River, suggesting the basin may have functioned as an oxbow wetland prone to periodic flooding and drying. Overlying peats containing planktonic diatom assemblages mark the onset of persistent open-water conditions ca. 4600 B.P. These data, in conjunction with a small body of paleoecological evidence from other southeastern wetlands, suggest that a hydrologic 'threshold' was surpassed during the mid-Holocene that resulted in flooding and expansion of low-lying depressions that characterize the regional landscape. This is likely the culmination of long-term hydrologic trends related to glacial retreat, including increased precipitation and a rise in ground water levels corresponding to a global rise in sea level. In Peat Bay, lake-like conditions persisted until about 3800 B.P., after which time aerophilic diatoms and remains of other temporary pond organisms dominated the fossil record. Episodic drying of pond sediments during the late-Holocene may have been a response to decreased precipitation, stabilization of the water table and infilling of the pond by decaying vegetation and terrigenous clastics. The microfossil record indicates that temporary pond conditions persisted until recent years (ca. 1985) when the basin returned to a permanently flooded state due to groundwater connection to a large reservoir constructed nearby.

Estuaries of the South Atlantic Coast of North America: Their Geographical Signatures

Estuaries of the southeastern Atlantic coastal plain are dominated by shallow meso-tidal bar-built systems interspersed with shallow sounds and both low flow coastal plain and high flow piedmont riverine systems. Three general geographical areas can be discriminated: the sounds of North Carolina; the alternating series of riverine and ocean dominated bar-built systems of South Carolina, Georgia, and northeast Florida, and the subtropical bar-built estuaries of the Florida southeast coast. The regional climate ranges from temperate to subtropical with sea level rise and hurri- canes having a major impact on the region's estuaries because of its low and relatively flat geomorphology. Primary production is highest in the central region. Seagrasses are common in the northern and southern most systems, while intertidal salt marshes composed of Spartina alterniflora reach their greatest extent and productivity in South Carolina and Georgia. Nuisance blooms (cyanobacteria, dinoflagellates, and cryptomonads) occur more frequently in the northern and extreme southern parts of the region. Fishery catches are highest in the North Carolina and Florida areas. Human population growth with its associated urbanization reaches a maximum in Florida and it is thought that the long-term sustainability of the Florida coast for human habitation will be lost within the next 25 years. Tidal flushing appears to play an important role in mitigating anthropogenic inputs in systems of moderate to high tidal range, i.e., the South Carolina and Georgia coasts. The most pressing environmental problems for the estuaries of the southeastern Atlantic coast seem to be nutrient loading and poor land use in North Carolina and high human population density and growth in Florida. The future utilization of these estuarine systems and their services will depend on the development of improved management strategies based on improved data quality.

Are Small, Isolated Wetlands Expendable?

What is most evident in the recent debate concerning new wetland regulations drafted by the U.S. Army Corps of Engineers is that small, isolated wetlands will likely continue to be lost. The critical biological question is whether small wetlands are expendable, and the fundamental issue is the lack of biologically relevant data on the value of wetlands, especially so‐called “isolated” wetlands of small size. We used data from a geographic information system for natural‐depression wetlands on the southeastern Atlantic coastal plain (U.S.A.) to examine the frequency distribution of wetland sizes and their nearest‐wetland distances. Our results indicate that the majority of natural wetlands are small and that these small wetlands are rich in amphibian species and serve as an important source of juvenile recruits. Analyses simulating the loss of small wetlands indicate a large increase in the nearest‐wetland distance that could impede “rescue” effects at the metapopulation level. We argue that small wetlands are extremely valuable for maintaining biodiversity, that the loss of small wetlands will cause a direct reduction in the connectance among remaining species populations, and that both existing and recently proposed legislation are inadequate for maintaining the biodiversity of wetland flora and fauna. Small wetlands are not expendable if our goal is to maintain present levels of species biodiversity. At the very least, based on these data, regulations should protect wetlands as small as 0.2 ha until additional data are available to compare diversity directly across a range of wetland sizes. Furthermore, we strongly advocate that wetland legislation focus not only on size but also on local and regional wetland distribution in order to protect ecological connectance and the source‐sink dynamics of species populations. Son los Humedales Pequeños Prescindibles? Algo muy evidente en el reciente debate sobre las nuevas regulaciones de humedales elaboradas por el cuerpo de ingenieros de la armada de los Estados Unidos es que los humedales aislados pequeños seguramente se continuarán perdiendo. La pregunta biológica crítica es si los humedales pequeños son prescindibles y e asunto fundamental es la falta de datos biológicos relevantes sobre el valor de los humedales, especialmente los llamados humedales “aislados” de tamaño pequeño. Utilizamos datos de GIS para humedales de depresiones naturales en la planicie del sureste de la costa Atlántica (U.S.A.) para examinar la distribución de frecuencias de los tamaños de humedales y las distancias a los humedales mas cercanos. Nuestros resultados indican que la mayoría de los humedales naturales son pequeños y que estos humedales pequeños son ricos en especies de anfibios y sirven como una fuente importante de reclutas juveniles. Análisis simulando la pérdida de humedales pequeños indican un gran incremento en la distancia al humedal mas cercano lo cual impediría efectos de “rescate” a nivel de metapoblación. Argumentamos que los humedales pequeños son extremadamente valiosos para el mantenimiento de la biodiversidad, que la pérdida de humedales pequeños causará una reducción directa en la conexión entre poblaciones de especies remanentes y que tanto la legislación propuesta como la existente son inadecuadas para mantener la biodiversidad de la flora y fauna de los humedales. Si nuestra meta es mantener los niveles actuales de biodiversidad de especies, los humedales pequeños no son prescindibles. En base en estos datos, las regulaciones deberían por lo menos proteger humedales tan pequeños como 0.2 ha hasta que se tengan a la mano datos adicionales para comparar directamente la diversidad a lo largo de un rango de humedales de diferentes tamaños. Mas aún, abogamos fuertemente por que la regulación de los pantanos se enfoque no solo en el tamaño, sino también en la distribución local y regional de los humedales para poder proteger la conexión ecológica y las dinámicas fuente y sumidero de poblaciones de especies.

Regional Shallow Soil-Gas Hydrocarbon Results in the Southeastern Atlantic Coastal Plain

Abstract A regional soil-gas study centered on the U.S. Department of Energy Savannah River Site (SRS), located in the southeast Atlantic Coastal Plain, was conducted between 1993 and 1996. The study involved 2870 samples in three separate sampling events and covered an area of ~3200 km2. The primary purpose of this study was to determine background values for various chemical constituents to aid in waste-unit screening at SRS. In addition, the study was conducted to support the evaluation of regional geology and possibly provide input from methane data into climatological models. Both free soil-air and soil-plug samples were obtained from the near-surface environment. Data distribution of soil-air and soil-plug values were generally analogous but differed in magnitude. Soil-plug C2–C4 and toluene showed sparse but clustering patterns most likely related to geological sources of hydrocarbon associated with the Riddleville and Dunbarton Triassic basins. Oxygen data suggest that the near-surface soils (to a depth of 1 m) were in full contact with the near-surface atmosphere. For all analytes, except methane and ethane/ethylene, modal values approached zero or were less than instrument detection limits. The presence of anomalous soil-gas constituents occurring naturally within the region, notably toluene and the light saturates ethane through butane, suggests that regional values must be considered when evaluating localized geochemical characterization. Additionally, this study confirms that there is a variation in soil-gas results as affected by different soil-air and soil-plug techniques. Regional values for C1 through C10 and halogenated hydrocarbons are presented.

When is a Depression Wetland a Carolina Bay?

90Southeastern Geographer Lowrance, R., Sharpe, J. K., and Sheridan, J. M. 1986. "Long-Term Sediment Deposition in the Riparian Zone ofa Coastal Plain Watershed," Journal ofSoil and Water Conservation , Vol. 41, pp. 266-271. Lowrance, R., Mclntire, S., and Lance, C. 1988. "Erosion and Sediment Deposition in a Field-Forest System Estimated using Cesium- 137," Journal ofSoil and Water Conservation , Vol. 43, pp. 195-199. Markewich, H. W., Pavich, M. J., and Buell, G. R. 1990. "Contrasting Soils and Landscapes of the Piedmont and Coastal Plain, Eastern United States," Geomorphology, Vol. 3, pp. 417-448. NCDEM (North Carolina Division of Environmental Management). 1994 et seq. TarPamlico River Basinwide Water Quality Management Plan (Raleigh, NC: Division of Environmental Management). Phillips, J. D. 1993. "Pre- and Post-Colonial Sediment Sources and Storage in the lower Neuse River basin, North Carolina," Physical Geography, Vol. 14, pp. 272-284. Phillips, J. D., Wyrick, M. J., Robbins, J. G., and Flynn, M. 1993. "Accelerated Erosion on the North Carolina Coastal Plain," Physical Geography, Vol. 14, pp. 1 14-130. SCS (Soil Conservation Service). 1985. Haw River Erosion Report. Deep River Erosion Report (Raleigh, NC: United States Department of Agriculture). SCS (Soil Conservation Service). 1989. Soil Survey of Craven County, North Carolina (Raleigh, NC: United States Department of Agriculture). Slattery, M. C, Burt, T. P., and Boardman, J. B. 1 994. "Rill Erosion along the Thalweg ofa Hillslope Hollow: A Case Study from the Cotswold Hills, Central England," Earth Surface Processes and Landforms, Vol. 19, pp. 377-385. Trimble, S. W. 1974. Man-Induced Soil Erosion in the Southern Piedmont, 1700-1970 (Ankeny, IA: Soil Conservation Society of America). Southeastern Geographer Vol. XXXVII, No. I, May 1997, pp. 90-98 WHEN IS A DEPRESSION WETLAND A CAROLINA BAY?1 Robert F. Lide Are sites such as those shown in Figures 1 and 2 the same type of geomorphic feature? Does the term "Carolina bay" have a specific geomorphological definition , or is it simply a colloquial term that may be used to describe any depression wetland found on the southeastern Atlantic Coastal Plain? The purpose ofthis research note is: (1) to illustrate that, despite a seemingly precise definition, the term "Carolina bay" often is loosely applied in the scientific literature and (2) to Mr. Lide is a research technician at the Savannah River Ecology Laboratory, Drawer E, Aiken, SC 29802. VOL. XXXVII, NO. 1 91 argue that the term should be limited to geomorphic features that have a more or less elliptical, ovate, or circular outline. Carolina bays are a common type of depression wetland found on the Atlantic Coastal Plain of the southeastern United States (Fig. 3). Their size is highly variable , ranging from 50 meters in length to 8 kilometers (Sharitz and Gibbons, 1982). Although generally characterized by internal drainage, many have natural overflow outlets that may allow the establishment of temporary connections to stream and drainage networks during wet periods. Some Carolina bays form the headwaters of perennial streams, and a few have small creeks flowing into them. These wetlands increasingly are recognized as critical habitat for the maintenance of biodiversity. In recent years, many have been the subject of wetland preservation and restoration efforts. Historically, Carolina bays have been defined in the scientific literature by several specific geomorphological criteria: (1) being elliptical, ovate, or circular; (2) having a long-axis orientation that usually is northwest-southeast; and (3) often having a sand rim that is best developed along the eastern and southeastern perimeter (e.g., Melton and Schriever, 1933; Johnson, 1942; Prouty, 1952; Price, 1968; Savage, 1982). Many sites meet these geomorphological criteria (Fig. 1). In recent years, however, some published reports have used the term "Carolina bay" loosely to include coastal plain wetlands that lack some or all ofthese specific criteria (e.g., Norris, 1963; Pechmann et al., 1989; Schalles et al., 1989; Gibbons, 1 990; Mahoney, Mort, and Taylor, 1 990; Newman and Schalles, 1 990; Pechmann et al., 1991). Furthermore, wetlands with highly irregular outlines sometimes are identified as "Carolina bays" (Fig. 2). Although a few notes about circular or elliptical "bays" were recorded by early scientists (e.g., Tourney, 1848; Glenn, 1895), modern scientific investigation of Carolina bays began...

Sedimentology and depositional environments of middle Eocene terrigenous-carbonate strata, southeastern atlantic coastal plain, USA

Basin-margin sediments of middle Eocene age in the Savannah River Site area consist of five terrigenous-carbonate lithofacies: quartz sand; calcareous quartz sand; sandy carbonate; muddy carbonate; and a transitional lithofacies that ranges from sandy, muddy carbonate to calcareous mud. The middle Eocene sediment package, which includes calcareous parts of the upper Congaree, Warley Hill, and Santee Formations, dips southeast at 4.7 m/km and thickens from 2 m at its updip edge to 95 m downdip. The presence of glauconite and a diverse faunal assemblage in all lithofacies suggests deposition in clear, well-oxygenated, open-marine waters of normal salinity on the inner to middle shelf with periods of marginal marine, nearshore, and deltaic influence. Coarse-grained terrigenous sand and calcareous sand, deposited in higher-energy, nearshore environments, occur near the updip limit. Fine-grained terrigenous mud, calcareous mud, and sandy and muddy carbonate are located downdip and accumulated in quieter water conditions on the inner and middle shelf. The transition from terrigenous to carbonate sediment occurs near the updip limit in a narrow zone less than 5 km wide. Three depositional sequences, which contain transgressive and highstand system tracts, are recognized within the middle Eocene calcareous interval. One is assigned to the upper Congaree Formation ( TA 3.5 3.6 cycles ). The main control on areal distribution Hill Formation (TA3.4 cycle), and one to the Santee Formation ( TA 3.5 3.6 cycles ). The main control on areal distribution of facies was depositional environment, which was controlled primarily by sea-level eustasy and the amount, rate and locus of terrigenous influx. In updip areas, however, sediment distribution and thickness were also influenced by middle Eocene growth faulting. Diagenetic pathways vary with facies type, but generally include: (1) marine phreatic — boring of skeletal fragments by algae and fungi, grain micritization, and radially fibrous cementation; and (2) freshwater phreatic — inversion of high-Mg calcite to low-Mg calcite, dissolution of aragonitic allochems, formation of moldic porosity, precipitation of syntaxial calcite overgrowths on echinoderms, formation of calcite spar and isopachous calcite cement, neomorphism of micrite to microspar and pseudospar, precipitation of opal-CT lepispheres, replacement of mollusk shells by chalcedony, and precipitation of zeolites within secondary moldic pores. Quartz-rich facies have high interparticle porosity and excellent permeability. Mud-rich facies have low to moderate porosity and permeability owing to the isolation of moldic and vug pores.