Atlantic Coast Barrier Island Research Articles

The contraction of freshwater lenses in barrier island: A combined geophysical and numerical analysis

Coastal aquifers, vital for supplying freshwater to over one billion people worldwide, often face saltwater intrusion, with barrier island aquifers playing a crucial role in this coastal system. Despite the proliferation of studies exploring the impacts of sea-level rise, storm surges, and over-pumping, the effect of droughts on barrier island aquifers salinization remains largely under-investigated. This lack of attention is particularly concerning given the heightened vulnerability of barrier island aquifers; most ultimately rely solely on aerial recharge compared to continental coastal aquifers. An in-depth understanding of recharge and salinization processes is important for sustainably managing these island’s marginal freshwater resources, especially considering climate change impacts. This study presents an evaluation approach for the response of a freshwater lens (FWL) to drought conditions, which incorporates in-situ observations, geophysical measurements, and numerical modeling. The study examines the response of a shallow unconfined aquifer on an Atlantic coast barrier island to the 2020 Northeast extreme hydrological drought. It encompasses a density-driven flow model informed by time-lapse electrical resistivity imaging and in-situ measurements, including groundwater head and salinity collected from monitoring wells. Results from our approach indicate that FWL volume is reduced by 11% during the drought. It returns to its previous volume over the following spring season, indicating that the net volume of the FWL remained unchanged on an annual scale. The mean groundwater residence time on the island is approximately a year, indicating the barrier island aquifer is a hydrodynamically active coastal zone. These findings highlight the vulnerability and resilience of shallow unconfined barrier island aquifers to droughts and climate change.

Relations entre le niveau de risque psychosocial et le niveau de stress professionnel perçu chez les collaborateurs des agences commerciales d’un groupe industriel de fabrication de matériel électrique en France

Barrier islands shrub thickets, the dominant woody community of many Atlantic coast barrier islands, are very sensitive to changes in the freshwater lens and thus, constitute a strong indicator of summer drought. NDVI was computed from airborne images and multispectral images on Hog Island (VA, USA) to evaluate summer growing season changes in woody communities for better predictions of climate change effects. Patterns of NDVI were compared year to year and monthly relative to precipitation and water table depth at the appropriate temporal scale. The highest absolute values of NDVI as well as the larger surface covered by woody vegetation (NDVI > 0.5) occurred in the wet year (2004) with a bimodal distribution of NDVI values (around 0.65 and 0.9) while both dry years (2007 and 2008) showed similar values in maximum, mean and standard deviation and unimodal distributions (around 0.75) of NDVI values. Positive linear adjustments were obtained between maximum (r2 > 0.9) and mean NDVI (r2 > 0.87) and the accumulated rainfall in the hydrological year and the mean water table depth from the last rainfall event till the date of the image acquisition. The spatial variations revealed that water table depth behaved different in wet and dry years. In dry years there was a remarkable increase in mean and maximum values linearly related to water table depth. The highest slope of the adjustment in 2007 indicated a sharp response of vegetation in the driest year. Monthly series of NDVI showed the major role of lack of precipitation through July and August in 2007 with missing classes of NDVI above 0.8 and unimodal distributions in mid-late summer. Best linear fits (r2 close to 1) were obtained with rainfall at different temporal scales: accumulated rainfall in the hydrological year 2004 and accumulated rainfall in the last month previous to the date of 2007 image. Thus, in dry years productivity is closely related to water available from recent past as opposed to over the year for wet years. Good fits (r2 values higher than 0.88) were obtained between monthly decrease in water table depth and NDVI variables just in the dry year. These results demonstrate the important feedback between woody vegetation response to changes in the freshwater lens using empirical data and could apply to other systems with strong directional gradients in resources.

Unraveling natural versus anthropogenic effects on genetic diversity within the southeastern beach mouse (Peromyscus polionotus niveiventris)

Anthropogenic habitat loss is among the greatest threats to biodiversity. Populations undergoing fragmentation and loss of habitat are also threatened by erosion of genetic diversity. However, contemporary genetic diversity may be the legacy of natural processes acting prior to anthropogenic influences. Measurements of genetic diversity from contemporary and historical samples can evaluate the relative impact of natural and anthropogenic effects on its status. We investigated the genetic diversity of a threatened subspecies occupying Atlantic Coast barrier islands of Florida, Peromyscus polionotus niveiventris (southeastern beach mouse). To test for recent loss of genetic diversity, we compared cyt b data from museum samples (historical—prior to human impact) with contemporary samples throughout their range. Ten microsatellite loci were genotyped for samples from the contemporary range, to determine current population interconnectedness and structure. The results using cyt b data revealed no statistically significant loss of genetic diversity between historical and contemporary populations of P. p. niveiventris. Both nuclear and mitochondrial data support our conclusion that the observed capture and conservation of historical genetic diversity is explained by the large federally protected region of continuous habitat that remains with minimal human impact. Whereas, the two disjunct populations isolated by anthropogenic habitat destruction, exhibit significant losses of genetic diversity. Collectively, these findings offer a sound basis from which to formulate a conservation strategy to maintain the genetic diversity of P. p. niveiventris. Furthermore, our study underscores the importance of large expanses of continuous habitat within the geographic range of species to facilitate the maintenance of genetic integrity.

NDVI as an indicator for changes in water availability to woody vegetation

Barrier islands shrub thickets, the dominant woody community of many Atlantic coast barrier islands, are very sensitive to changes in the freshwater lens and thus, constitute a strong indicator of summer drought. NDVI was computed from airborne images and multispectral images on Hog Island (VA, USA) to evaluate summer growing season changes in woody communities for better predictions of climate change effects. Patterns of NDVI were compared year to year and monthly relative to precipitation and water table depth at the appropriate temporal scale. The highest absolute values of NDVI as well as the larger surface covered by woody vegetation (NDVI>0.5) occurred in the wet year (2004) with a bimodal distribution of NDVI values (around 0.65 and 0.9) while both dry years (2007 and 2008) showed similar values in maximum, mean and standard deviation and unimodal distributions (around 0.75) of NDVI values. Positive linear adjustments were obtained between maximum (r2>0.9) and mean NDVI (r2>0.87) and the accumulated rainfall in the hydrological year and the mean water table depth from the last rainfall event till the date of the image acquisition. The spatial variations revealed that water table depth behaved different in wet and dry years. In dry years there was a remarkable increase in mean and maximum values linearly related to water table depth. The highest slope of the adjustment in 2007 indicated a sharp response of vegetation in the driest year. Monthly series of NDVI showed the major role of lack of precipitation through July and August in 2007 with missing classes of NDVI above 0.8 and unimodal distributions in mid-late summer. Best linear fits (r2 close to 1) were obtained with rainfall at different temporal scales: accumulated rainfall in the hydrological year 2004 and accumulated rainfall in the last month previous to the date of 2007 image. Thus, in dry years productivity is closely related to water available from recent past as opposed to over the year for wet years. Good fits (r2 values higher than 0.88) were obtained between monthly decrease in water table depth and NDVI variables just in the dry year. These results demonstrate the important feedback between woody vegetation response to changes in the freshwater lens using empirical data and could apply to other systems with strong directional gradients in resources.

Woody vegetative cover dynamics in response to recent climate change on an Atlantic coast barrier island: a remote sensing approach

Considering impacts of predicted increases in sea-level, storms, and alterations in precipitation patterns on geomorphological and associated ecological processes, woody vegetation dynamics may serve as sentinels to climate change on barrier islands. We examined island-scale conversion of land (i.e. sand to grassland to woody cover) while relating the importance of climate variables on rate of woody expansion. Light Detection and Ranging (LiDAR) was used to evaluate potential distribution of woody species based on distance to shoreline and elevation. Using Landsat TM imagery, we monitored changes in island size and vegetation classes (1984–2010). These comparisons revealed conversion of grassland to woody cover (285% increase) was closely linked to air temperature, precipitation and atmospheric [CO2]. LiDAR data indicated that woody species have not expanded completely into the potential range. Our results suggest that woody species are responsive to climate change, thus serving as sentinels on Virginia barrier islands.

Simulating a natural fire regime on an Atlantic coast barrier island complex in Florida, USA

The HFire fire regime model was used to simulate the natural fire regime (prior to European settlement) on Kennedy Space Center, Merritt Island National Wildlife Refuge, Canaveral National Seashore, and Cape Canaveral Air Force Station, Florida. Model simulations were run for 500 years and the model was parameterized using information generated from previously published empirical studies on these properties (e.g., lightning fire ignition frequencies and ignition seasonality). A mosaic pattern of frequent small fires dominated this fire regime with rare but extremely large fires occurring during dry La Niña periods. This simulated fire size distribution very closely matched the previously published fire size distribution for lightning ignitions on these properties. A sensitivity analysis was performed to establish which parameters were most influential and the range of variation surrounding empirically parameterized model output. Dead fuel moisture and wind speed had the largest influence on model outcome. A wide range of variance was observed surrounding the composite simulation with the least being 6% in total burn frequency and the greatest being 49% in total area burned. Because simulation modeling is the best option for fire regime reconstruction in many rapidly growing shrub dominated systems, these results will be of interest to scientists and fire managers for delineating the natural fire regime on these properties, the southeastern United States and other fire adapted shrub systems worldwide.

Shifts in litterfall and dominant nitrogen sources after expansion of shrub thickets

Woody encroachment into herbaceous ecosystems is emerging as an important ecological response to global change. A primary concern is alterations in C and N cycling and associated variations across a variety of ecosystems. We quantified seasonal variation in litterfall and litter N concentration in Morella cerifera shrub thickets to assess changes in litterfall and associated N input after shrub expansion on an Atlantic coast barrier island. We also used the natural abundance of (15)N to estimate the proportion of litterfall N originating from symbiotic N fixation. Litterfall for shrub thickets ranged from 8,991 +/- 247 to 3,810 +/- 399 kg ha(-1) year(-1) and generally declined with increasing thicket age. Litterfall in three of the four thickets exceeded previous estimates of aboveground annual net primary production in adjacent grasslands by 300-400%. Leaf N concentration was also higher after shrub expansion and, coupled with low N resorption efficiency and high litterfall, resulted in a return of as much as 169 kg N ha(-1) year(-1) to the soil. We estimated that approximately 70% of N returned to the soil was from symbiotic N fixation resulting in an ecosystem input of between 37 and 118 kg ha(-1) year(-1) of atmospheric N depending on site. Considering the extensive cover of shrub thickets on Virginia barrier islands, N fixation by shrubs is likely the largest single source of N to the system. The shift from grassland to shrub thicket on barrier islands results in a substantial increase in litterfall and foliar N concentration that will likely have a major impact on the size and cycling of ecosystem C and N pools. Increasing C and N availability in these nutrient-poor soils is likely to permanently reduce cover of native grasses and alter community structure by favoring species with greater N requirements.

Orchids of Atlantic Coast Barrier Islands from North Carolina to New York

During the past 25 years, we have documented the occurrence of 17 orchid species from Atlantic coast barrier islands from North Carolina to New York, including Calopogon tuberosus, Corallorhiza wisteriana, Cypripedium acaule, Epipactis helleborine, Goodyera pubescens, Habenaria repens, Listera australis, Malaxis spicata, Platanthera cristata, Pogonia ophioglossoides, Spiranthes cernua, S. lacera var. gracilis, S. laciniata, S. odorata, S. praecox, S. vernalis, and Tipularia discolor. For each species, we present data on localities of extant orchid populations, fluctuations in population size, flowering dates, habitat preferences, threats to some populations, results of herbarium and literature searches, and our opinion on the status of orchid species reported by others but not observed by us in the field. Spiranthes vernalis is the most common orchid on mid-Atlantic coast barrier islands. Other species occurring in large (> 1000 individuals) populations include Platanthera cristata, Spiranthes cernua, S. praecox, and Tipularia discolor. Four orchid species occur in moderately large (50 to 100 individuals) populations but have limited distributions, including Calopogon tuberosus, Cypripedium acaule, Epipactus helleborine, and Pogonia ophioglossoides. When analyzed by locality, several regions of high orchid diversity can be identified, including False Cape, Virginia, and the region between and including Nags Head Woods and Kitty Hawk Woods on Bodie Island, North Carolina. These orchid “hot spots” occur in regions that provide a high diversity of habitats.

Erosional and depositional characteristics of regional overwash deposits caused by multiple hurricanes

AbstractRegional‐scale washover deposits along the Florida Gulf and Atlantic coasts induced by multiple hurricanes in 2004 and 2005 were studied through coring, trenching, ground‐penetrating radar imaging, aerial photography, and prestorm and poststorm beach‐profile surveys. Erosional and depositional characteristics in different barrier‐island sub‐environments, including dune field, interior wetland and back‐barrier bay were examined. Over the eroded dune fields, the washover deposits are characterized by an extensive horizontal basal erosional surface truncating the old dune deposits and horizontal to slightly landward‐dipping stratification. Over the marshes in the barrier‐island interior, the washover deposits are characterized by steep tabular bedding, with no erosion at the bottom. Overwash into the back‐barrier bay produced the thickest deposits characterized by steep, prograding sigmoidal bedding. No significant erosional feature was observed at the bottom. Washover deposits within the dense interior mangrove swamp demonstrate both normal and reversed graded bedding. The washover deposits caused by hurricanes Frances (2004) and Jeanne (2004) along the southern Florida Atlantic coast barrier islands are substantially different from those along the northern Florida barrier islands caused by Ivan (2004) and Dennis (2005) in terms of regional extension, erosional features and sedimentary structures. These differences are controlled by different overall barrier‐island morphology, vegetation type and density, and sediment properties. The homogeneity of sediment along the northern Florida coast makes distinguishing between washover deposits from Ivan and Dennis difficult. In contrast, along the Atlantic coast barrier islands, the two overwash events, as demonstrated by two phases of graded bedding of the bimodal sediments, are easily distinguishable.

Spatial/temporal variations in shrub thicket soil seed banks on an Atlantic Coast barrier island

Potential species replacement within low-diversity shrub thicket communities was investigated for a Virginia barrier island. Seed bank species composition was quantified in a glasshouse study using soil samples collected beneath closed Myrica cerifera thickets, as well as from thicket gaps. Samples were collected from productive and aging thickets, corresponding to differences in soil age. These data were compared to species presently occurring within the thickets and gaps. Seedbank species composition was not indicative of current community composition for either the intact thickets or the gaps. Seed banks resembled a more pioneer community. Thirteen families, 23 genera, and 25 species were identified from the seed bank beneath the M. cerifera thickets. Four species were woody. The within-gap seed bank included 19 families, 30 genera, and 34 species. Eight species were woody. The current community included 21 families, 33 genera, and 36 species beneath the intact thickets as well as within the thicket gaps. Eighteen species were woody. The species richness of gaps was more than three times that of intact thickets. For low-diversity shrub thickets, gaps enhance species richness.

Comparison of Gaps and Intact Shrub Thickets on an Atlantic Coast Barrier Island

Shrub thicket (Myrica cerifera) gaps were sampled on a Virginia barrier island to elucidate successional mechanisms in the development of maritime forests. Microclimate, edaphic characteristics and spatial heterogeneity within gaps, as well as within intact thicket understories, were compared for young and aging thickets. Midday photosynthetic photon flux density in gaps ranged from 5–1000 µmol m−2 s−1, but at the gap center photon flux density was up to 15% higher for the aging thicket. Soil temperatures at the surface were 9 C warmer in aging thicket gaps compared to young gaps, and 6 to 14 C warmer when comparing gaps to the understories for the young and aging thickets, respectively. Edaphic characteristics differed little with thicket age; however, gaps in the aging thicket had greater structural diversity. Species richness was 2.5 to 3.7 times greater within gaps than in intact thicket understories, with highest richness within aging thicket gaps. Relative to intact shrub thickets, gaps enhance environmental variability through greater structural diversity. Gaps may facilitate the establishment of later seral species.

Small-scale distribution and salinity response of Juniperus virginiana on an Atlantic Coast barrier island

A field and laboratory study examined the hypothesis that the small-scale distribution pattern of Juniperus virginiana on barrier islands is related to salinity patterns and plant responses to salinity. Temporal (May – October) and spatial variability in ground water availability, ground water salinity, and total soil chlorides were quantified across a Virginia barrier island. Groundwater depth and salinity increased throughout the summer; microtopographic position and location on the island also affected soil salinities. Highest salinities occurred near the ocean side beach and bay side marsh, as well as in low lying swales that flood during extreme high tides or storms. Median rooting zone chloride level for J. virginiana was 54 μg/g. In contrast, laboratory germination and growth studies indicated that J. virginiana was significantly affected only at high salinity levels (1000 and 1400 μg/g), suggesting that salinity is not the only factor regulating small-scale distribution patterns. The broad tolerance to salinity may account for the abundance of J. virginiana in coastal environments. Key words: barrier island, eastern red cedar, Juniperus virginiana, salinity response, water relations.

Shoreline Periodicities and Edge Waves

Spectral analysis of shoreline rate change data spanning 40 years and 122.6 km between Cape Hatteras and Cape Lookout reveals a hierarchy of longshore periodicities. The periodicities closely match wavelengths of hypothetical standing waves trapped between the offshore shoals of the two large capes. In addition, active and relic inlets and site-level beach characteristics coincide with the periodicities. We conclude that edge waves may play a significant role in determining the intensity of regional-scale processes and geomorphological responses along the Atlantic coast barrier islands.

Herpetofaunal Colonization Patterns of Atlantic Coast Barrier Islands

This study presents information on the population and general ecology of the herpetofauna of Kiawah Island, South Carolina, and compares species numbers and composition with other Atlantic Coast barrier islands. Lizards are highly successful island colonizers, whereas salamanders are exceptionally poor. Explanation for the unexpected absence from any barrier islands of certain common mainland species of reptiles and amphibians is inconclusive, although winter drought result- ing in higher salinity in freshwater habitats is suggested as an important deterrent to colonization for some species. The amount of woodland habitat, rather than island size per se, is the island feature most highly correlated with numbers of reptile or amphibian species.