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At least 1,603 records · Page 89Linked to original sources
USGS2005

The introduced ribbed mussel (Geukensia demissa) in Estero de Punta Banda, Mexico: Interactions with the native cord grass, Spartina foliosa

Introduced populations of Guekensia demissa occur on the west coast of North America. They have been reported in San Francisco Bay, four southern California wetlands, and in Estero de Punta Banda (EPB), Baja California Norte, Mexico. We randomly sampled benthic invertebrates in four habitat types within EPB: marsh, channel, mudflat and pan. Geukensia demissa was the most abundant bivalve in the wetland at EPB. It was significantly associated with the native cordgrass, Spartina foliosa, and occurred at higher average densities in vegetated marsh sites (24/m2) and Spartina-dominated tidal channels (35/m2), compared to mudflat (0/m2), and pan (0/m 2) sites. We estimated that the total biomass of this invader was over four times that of the next most abundant bivalve, Tagelus spp., in EPB. We examined G. demissa for parasites and found that only a few native parasites colonized this introduced host at very low prevalences and intensities. We performed bird surveys to determine the habitat overlap and potential impact of this mussel on the EPB population of light-footed clapper rails (Rallus longirostrus levipes), an endangered species in the United States. The high abundance of G. demissa in EPB, its presence in clapper rail habitat, and its known effects on salt marsh habitat in it's native range, warrant further investigations of the impact of this invader in EPB and elsewhere. ?? Springer 2005.

Biological Invasions↗
USGS2013

Geographic range and structure of cryptic genetic diversity among Pacific North American populations of the non-native amphipod Grandidierella japonica

Reconstructing the invasion history of aquatic invasive species can enhance understanding of invasion risks by recognizing areas most susceptible to invasion and forecasting future spread based on past patterns of population expansion. Here we reconstruct the invasion history of the Japanese amphipod Grandidierella japonica Stephensen 1938 combining information from historical collection data with molecular genetic data to better understand post-invasion range expansion and anthropogenic connectivity across the Pacific coast of North America. Compilation of collection data from bays and estuaries of the Pacific North American coast show many new localities have been colonized in the last two decades, moving outward from harbors and bays with high commercial traffic into smaller coastal locations dominated by local recreational traffic. DNA barcode sequence data for G. japonica reveals two distinct clades: one found in San Francisco Bay and sites to the north, and one also found in San Francisco Bay and sites to the south. The two clades differ by an average 7.28 % genetic distance, large enough to consider these invasive amphipods two separate species. Both northern and southern clades exhibit low levels of genetic diversity, suggesting a single introduction event for each. The presence of cryptic diversity within this invasive amphipod highlights the need for more extensive study of the invasive and native populations of aquatic invasive invertebrates to address questions of taxonomy, diversity, and invasion history.

California, Oregon, Washington↗
USGS2007

Conservation genetics of snowy plovers (Charadrius alexandrinus) in the Western Hemisphere: Population genetic structure and delineation of subspecies

We examined the genetic structure of snowy plovers (Charadrius alexandrinus) in North America, the Caribbean, and the west coast of South America to quantify variation within and among breeding areas and to test the validity of three previously recognized subspecies. Sequences (676 bp) from domains I and II of the mitochondrial control region were analyzed for 166 snowy plovers from 20 breeding areas. Variation was also examined at 10 microsatellite loci for 144 snowy plovers from 14 breeding areas. The mtDNA and microsatellite data provided strong evidence that the Puerto Rican breeding group is genetically divergent from sites in the continental U.S. (net sequence divergence = 0.38%; F ST for microsatellites = 0.190). Our data also revealed high levels of differentiation between sites from South America and North America (net sequence divergence = 0.81%; F ST for microsatellites = 0.253). In contrast, there was little genetic structure among breeding sites within the continental U.S. Our results suggest that snowy plovers in Florida should be considered part of C. a. nivosus (rather than part of C. a. tenuirostris, where they are currently placed), whereas snowy plovers from Puerto Rico should be considered part of C. a. tenuirostris. Snowy plovers in South America should remain a separate subspecies (C. a. occidentalis). Although U.S. Pacific and Gulf Coast breeding areas were not genetically distinct from other continental U.S. sites, demographic isolation, unique coastal habitats, and recent population declines suggest they warrant special concern. ?? 2007 Springer Science+Business Media, Inc.

Conservation Genetics↗
USGS2010

Uncloaking a cryptic, threatened rail with molecular markers: origins, connectivity and demography of a recently-discovered population

The threatened California Black Rail lives under dense marsh vegetation, is rarely observed, flies weakly and has a highly disjunct distribution. The largest population of rails is found in 8–10 large wetlands in San Francisco Bay (SF Bay), but a population was recently discovered in the foothills of the Sierra Nevada Mountains (Foothills), within a wetland network comprised of over 200 small marshes. Using microsatellite and mitochondrial analyses, our objectives were to determine the origins, connectivity and demography of this recently-discovered population. Analyses of individuals from the Foothills ( n = 31), SF Bay ( n = 31), the Imperial Valley ( n = 6) and the East Coast ( n = 3), combined with rigorous power evaluations, provided valuable insights into past history and current dynamics of the species in Northern California that challenge conventional wisdom about the species. The Foothills and SF Bay populations have diverged strongly from the Imperial Valley population, even more strongly than from individuals of the East Coast subspecies. The data also suggest a historical presence of the species in the Foothills. The SF Bay and Foothills populations had similar estimated effective population size over the areas sampled and appeared linked by a strongly asymmetrical migration pattern, with a greater probability of movement from the Foothills to SF Bay than vice versa. Random mating was inferred in the Foothills, but local substructure among marshes and inbreeding were detected in SF Bay, suggesting different dispersal patterns within each location. The unexpected dimensions of Black Rail demography and population structure suggested by these analyses and their potential importance for management are discussed.

California↗
USGS2014

Genetic structure of Florida green turtle rookeries as indicated by mitochondrial DNA control region sequences

Green turtle ( Chelonia mydas ) nesting has increased dramatically in Florida over the past two decades, ranking the Florida nesting aggregation among the largest in the Greater Caribbean region. Individual beaches that comprise several hundred kilometers of Florida’s east coast and Keys support tens to thousands of nests annually. These beaches encompass natural to highly developed habitats, and the degree of demographic partitioning among rookeries was previously unresolved. We characterized the genetic structure of ten Florida rookeries from Cape Canaveral to the Dry Tortugas through analysis of 817 base pair mitochondrial DNA ( mtDNA ) control region sequences from 485 nesting turtles. Two common haplotypes, CM-A1.1 and CM-A3.1, accounted for 87 % of samples, and the haplotype frequencies were strongly partitioned by latitude along Florida’s Atlantic coast. Most genetic structure occurred between rookeries on either side of an apparent genetic break in the vicinity of the St. Lucie Inlet that separates Hutchinson Island and Jupiter Island, representing the finest scale at which mtDNA structure has been documented in marine turtle rookeries. Florida and Caribbean scale analyses of population structure support recognition of at least two management units: central eastern Florida and southern Florida. More thorough sampling and deeper sequencing are necessary to better characterize connectivity among Florida green turtle rookeries as well as between the Florida nesting aggregation and others in the Greater Caribbean region.

Florida↗
USGS2026

Population structure and genetic stock identification in southeastern United States loggerhead sea turtles (Caretta caretta) using genome-wide SNPs

Characterizing the genetic structure and connectivity between populations of endangered species can be used to inform management actions. In vagile species with high gene flow or recently established populations, such characterizations can be difficult to undertake using traditional genetic markers, and genetic stock identification (GSI) may be confounded by allele-sharing between populations. Loggerhead sea turtles ( Caretta caretta ) in the southeastern United States comprise seven management units (MUs) based on female philopatry inferred via mitochondrial DNA sequences, yet nuclear microsatellite data do not reflect divergence. Further, loci for accurate GSI are not currently known. To address this, we generated genome-wide single nucleotide polymorphism (SNP) data from 146 females nesting at individual sites representative of each southeastern United States MU. We found weak (F ST =0.001–0.003) but significant divergence among all MUs, with more notable divergence between the Gulf Coast and Atlantic Ocean MUs, and amongst the Atlantic Ocean MUs. We then used an iterative leave-one-out approach to identify candidate loci for GSI. This approach identified loci that could assign individuals to natal ocean basins (i.e., to the Gulf Coast or to the Atlantic Ocean), and to individual MUs within the Atlantic Ocean, with high (≥90%) success and accuracy. Analyses of genome-wide SNPs refined our understanding of the magnitude and scale of population connectivity in loggerhead turtles in the southeastern United States, and provided a foundation for the development of SNP panels for accurate, fine-scale GSI in sea turtles.

Alabama, Florida, Georgia↗
USGS2014

Correspondence of biological condition models of California streams at statewide and regional scales

We used boosted regression trees (BRT) to model stream biological condition as measured by benthic macroinvertebrate taxonomic completeness, the ratio of observed to expected (O/E) taxa. Models were developed with and without exclusion of rare taxa at a site. BRT models are robust, requiring few assumptions compared with traditional modeling techniques such as multiple linear regression. The BRT models were constructed to provide baseline support to stressor delineation by identifying natural physiographic and human land use gradients affecting stream biological condition statewide and for eight ecological regions within the state, as part of the development of numerical biological objectives for California’s wadeable streams. Regions were defined on the basis of ecological, hydrologic, and jurisdictional factors and roughly corresponded with ecoregions. Physiographic and land use variables were derived from geographic information system coverages. The model for the entire state ( n  = 1,386) identified a composite measure of anthropogenic disturbance (the sum of urban, agricultural, and unmanaged roadside vegetation land cover) within the local watershed as the most important variable, explaining 56 % of the variance in O/E values. Models for individual regions explained between 51 and 84 % of the variance in O/E values. Measures of human disturbance were important in the three coastal regions. In the South Coast and Coastal Chaparral, local watershed measures of urbanization were the most important variables related to biological condition, while in the North Coast the composite measure of human disturbance at the watershed scale was most important. In the two mountain regions, natural gradients were most important, including slope, precipitation, and temperature. The remaining three regions had relatively small sample sizes ( n  ≤ 75 sites) and had models that gave mixed results. Understanding the spatial scale at which land use and land cover affect taxonomic completeness is imperative for sound management. Our results suggest that invertebrate taxonomic completeness is affected by human disturbance at the statewide and regional levels, with some differences among regions in the importance of natural gradients and types of human disturbance. The construction and application of models similar to the ones presented here could be useful in the planning and prioritization of actions for protection and conservation of biodiversity in California streams.

California↗
USGS2024

Monarch butterfly (Danaus plexippus) roost site-selection criteria and locations east of the Appalachian Mountains, U.S.A.

The monarch butterfly is a flagship species and pollinator whose populations have declined by 85% in the recent two decades. Their largest population overwinters in Mexico, then disperses across eastern North America during March to August. During September-December, they return south using two flyways, one that spans the central United States and another that follows the Atlantic coast. Migrating monarchs fly diurnally and roost in groups nocturnally. We sought to determine the criteria this species uses to select roost sites, and the landscape context where those sites are found. We developed species distribution models of the landscape context of Atlantic flyway roost sites via citizen scientist observations and environmental variables that affect monarchs in the adult stage prior to migration, using two algorithms (Maximum Entropy and Genetic Algorithm for Ruleset Prediction). We developed two model validation methods: a citizen scientist smartphone application and peer-informed comparisons with aerial imagery. Proximity to surface water, elevation, and vegetative cover were the most important criteria for monarch roost site selection. Our model predicted 2.6 million ha (2.9% of the study area) of suitable roosting habitat in the Atlantic flyway, with the greatest availability along the Atlantic coastal plain and Appalachian Mountain ridges. Conservation of this species is difficult, as monarchs range over both large areas and various habitat types, and most current monarch research and conservation efforts are focused on the breeding and overwintering periods. These models can serve to help prioritize surveys of roosting sites and conservation efforts during the monarchs’ fall migration.

Journal of Insect Behavior↗
USGS2026

Local, regional, and coastwide effects of interactions between storms and relative position in tidal frame on Louisiana coastal marshes

Context In coastal wetlands, land area is dynamic in both space and time, and it is crucial to understand the factors that may tip the balance between wetland area loss and gain. Flooding has been shown to be a key regulator of wetland resilience to land loss through effects on vegetation health and productivity. However, the relationship between elevation and land change is complex, likely interacting with lateral processes such as edge erosion to influence land change rates. Objectives We sought to determine the role of storms, major drivers of lateral erosion, to help explain the complex relationships between elevation and land change at multiple spatial scales and improve predictions of wetland loss. Methods We used long-term records of elevation change, water elevation, and surface wind stress together with remotely sensed land-area change datasets to determine the factors that contribute to land loss in Louisiana (USA) coastal wetlands. Results Our results illustrate that wetland elevation alone cannot be used as a predictor of land change. Annual time-integrated wind stress, which we used as an indicator of storminess, is an important predictor of land change and interacts with elevation to impact wetland gain or loss. The data showed high site-level variation, indicating that local factors strongly determine land change. Nonetheless, we were able to identify broad generalities at the larger scales. We found that the interaction of flooding and storminess varied among geographic regions along the coast. In the Delta Plain region of the coast, sites that are frequently flooded (low elevation) were more likely to experience land gain in stormy years and land loss in calm years. Conversely, sites that were frequently drained (high elevation) had greater land loss during stormy years, presumably due to wind-wave erosion. This trend was not observed in the heavily managed Chenier Plain region, where perpetually flooded wetlands gained land during stormy years, likely due to wind-driven drainage of the marsh. Conclusion Together, these results illustrate that models of wetland vulnerability based upon elevation change or storm effects alone are too simple and will not capture observed trends in land change. Incorporating additional factors such as wind stress and management status can improve predictions of coastal wetland loss and our understanding of the mechanisms controlling resilience of these ecosystems.

Louisiana↗
USGS2009

Late Pleistocene to Holocene sedimentation and hydrocarbon seeps on the continental shelf of a steep, tectonically active margin, southern California, USA

Small, steep, uplifting coastal watersheds are prolific sediment producers that contribute significantly to the global marine sediment budget. This study illustrates how sedimentation evolves in one such system where the continental shelf is largely sediment-starved, with most terrestrial sediment bypassing the shelf in favor of deposition in deeper basins. The Santa Barbara-Ventura coast of southern California, USA, is considered a classic area for the study of active tectonics and of Tertiary and Quaternary climatic evolution, interpretations of which depend upon an understanding of sedimentation patterns. High-resolution seismic-reflection data over >570 km 2 of this shelf show that sediment production is concentrated in a few drainage basins, with the Ventura and Santa Clara River deltas containing most of the upper Pleistocene to Holocene sediment on the shelf. Away from those deltas, the major factor controlling shelf sedimentation is the interaction of wave energy with coastline geometry. Depocenters containing sediment 5-20 m thick exist opposite broad coastal embayments, whereas relict material (bedrock below a regional unconformity) is exposed at the sea floor in areas of the shelf opposite coastal headlands. Locally, natural hydrocarbon seeps interact with sediment deposition either to produce elevated tar-and-sediment mounds or as gas plumes that hinder sediment settling. As much as 80% of fluvial sediment delivered by the Ventura and Santa Clara Rivers is transported off the shelf (some into the Santa Barbara Basin and some into the Santa Monica Basin via Hueneme Canyon), leaving a shelf with relatively little recent sediment accumulation. Understanding factors that control large-scale sediment dispersal along a rapidly uplifting coast that produces substantial quantities of sediment has implications for interpreting the ancient stratigraphic record of active and transform continental margins, and for inferring the distribution of hydrocarbon resources in relict shelf deposits.

Marine Geophysical Research↗
USGS2014

A framework for the probabilistic analysis of meteotsunamis

A probabilistic technique is developed to assess the hazard from meteotsunamis. Meteotsunamis are unusual sea-level events, generated when the speed of an atmospheric pressure or wind disturbance is comparable to the phase speed of long waves in the ocean. A general aggregation equation is proposed for the probabilistic analysis, based on previous frameworks established for both tsunamis and storm surges, incorporating different sources and source parameters of meteotsunamis. Parameterization of atmospheric disturbances and numerical modeling is performed for the computation of maximum meteotsunami wave amplitudes near the coast. A historical record of pressure disturbances is used to establish a continuous analytic distribution of each parameter as well as the overall Poisson rate of occurrence. A demonstration study is presented for the northeast U.S. in which only isolated atmospheric pressure disturbances from squall lines and derechos are considered. For this study, Automated Surface Observing System stations are used to determine the historical parameters of squall lines from 2000 to 2013. The probabilistic equations are implemented using a Monte Carlo scheme, where a synthetic catalog of squall lines is compiled by sampling the parameter distributions. For each entry in the catalog, ocean wave amplitudes are computed using a numerical hydrodynamic model. Aggregation of the results from the Monte Carlo scheme results in a meteotsunami hazard curve that plots the annualized rate of exceedance with respect to maximum event amplitude for a particular location along the coast. Results from using multiple synthetic catalogs, resampled from the parent parameter distributions, yield mean and quantile hazard curves. Further refinements and improvements for probabilistic analysis of meteotsunamis are discussed.

Natural Hazards↗
USGS2024

Deterministic, dynamic model forecasts of storm-driven coastal erosion

The U.S. Atlantic and Gulf of Mexico coasts are vulnerable to storms, which can cause significant erosion of beaches and dunes that protect coastal communities. Real-time forecasts of storm-driven erosion are useful for decision support, but they are limited due to demands for computational resources and uncertainties in dynamic coastal systems and storm forcings. Current methods for coastal change forecasts are based on empirical calculations for wave run-up and conceptual models for erosion, which do not represent sediment transport and morphological change during the storm. However, with continued advancements in high-resolution geospatial data and computational efficiencies, there is an opportunity to apply morphodynamic models for forecasts of beach and dune erosion as a storm approaches the coast. In this study, we implement a forecast system based on a deterministic, dynamic model. The morphodynamic model is initialized with digital elevation models of the most up-to-date conditions and forced with hydrodynamics from wave and circulation model forecasts, and its predictions are categorized based on impact to the primary dune, defined in this study as the first ridge of sand landward of the beach. Results are compared spatially to the observed post-storm topography using changes to dune crest elevations and volumes, and temporally to the predicted total water level at the forecasted moment of dune impact.

Florida, Georgia, South Carolina↗
USGS2019

Establishing genome sizes of focal fishery and aquaculture species along Baja California, Mexico

Genome size—the total haploid content of nuclear DNA— is constant in all cells in individuals within a species, but differs among species. Consequently, the genome size is a quantifiable genetic signature that not only characterizes a species, but it can reflect chromatin modifications, which play fundamental roles in most biological processes that are involved in the manipulation and expression of DNA. This characteristic makes the genome size a crucial parameter for genetic research on endemic aquatic species and for genetic manipulations in aquaculture species. Technologies for genetic assessments and improvements applied to fishery and aquaculture species use genome size values as a means by which hybrids, polyploids, and sex can be identified, when sex chromosomes exist. The objectives of this study were to determine genome sizes of aquatic species with economic and biological importance along the Pacific coast of Mexico, as well as to identify the appropriate reference standards for use in this study. Blood, hemolymph or milt were collected from 10 species occurring along the coast of Baja California: Sablefish Anoplopoma fimbria , Black Snapper Lutjanus novemfasciatus , California Halibut Paralichthys californicus , Pacific Sardine Sardinops sagax , Flag Rockfish Sebastes rubrivinctus , Starry Rockfish Sebastes constellatus, Totoaba Totoaba macdonaldi , Whiteleg Shrimp Litopenaeus vannamei and two Yellowtail Seriola lalandi and S. dorsalis . Nuclear DNA was stained with propidium iodide solution and the genome size was determined by flow cytometry, with results ranging from 0.61 pg (1.22 pg/diploid cell) to 2.59 pg (5.18 pg/diploid cell), with the smallest value in Sablefish and the largest in the Whiteleg Shrimp. No significant differences were detected ( P ≤ 0.05) among individuals of the same species; the likely reason behind any dissimilar DNA content values with those from the literature were differences in methodologies or variations in genetics. Red-ear Slider Turtle Trachemys scripta elegans 2.65 pg (5.30 pg/diploid cell) and Red Junglefowl Gallus gallus 1.27 pg (2.54 pg/diploid cell) were chosen as the standards for reference values. These results establish the basis for the Mexican National Aquatic Genetic Resources project supporting genetic improvements for aquaculture and conservation status parameters for fisheries species.

Baja California↗
USGS1980

The human influence on seabird nesting success: Conservation implications

Based on studies of brown pelicans Pelecanus occidentalis californicus and Heermann's gulls Larus heermanni , disturbances by recreationists, educational groups, local fishermen and scientists alike can be seriously disruptive and damaging to breeding seabirds in the Gulf of California and off the west coast of Baja California. Similar instances have been identified throughout the world—the problem is not difficult to document, but it is difficult to eliminate. The increasing human-seabird contacts on islands in the Gulf of California and along the west coast of Baja California raise serious questions and immediate concern about the continued preservation of nesting colonies of marine birds in those areas. Conservation measures must consider the extreme sensitivity of many seabirds to the inter- and intraspecific behavioural imbalances created by human disturbances. In some cases, total exclusion of humans may be required; in others, limited access might be possible under closely managed conditions at certain times of the year. A symbiotic relationship between seabird conservation, legitimate research and tourism should be the desired goal.

Baja California, Baja California Sur, Sinaloa, Son↗
USGS1995

Landscape change and its effects on the wintering range of a lesser snow goose Chen caerulescens caerulescens population: A review

The Texas coast has experienced considerable urban, industrial, and agricultural growth during the 20th Century. The region provides important wintering habitat to many avian species, including lesser snow geese Chen caerulescens caerulescens . This paper draws the biological and ecological fields into an historical perspective by examining available literature on the development of the upper Texas coast and range changes of lesser snow geese. Historically, lesser snow geese wintered in the coastal marshes, but expanded their range into the adjacent prairies in the mid-1900s. Winter range expansion was negatively affected by urban and industrial encroachment in the coastal marshes and positively influenced by agricultural development in the prairies, which increased dramatically during World War II. The lesser snow goose population flourished alongside some human-induced landscape alterations. However, projected declines in agriculture and increased urbanization of prairie and coastal marsh habitats may result in significant negative effects on the lesser snow goose population.

Texas↗
USGS1991

Soil chronosequence studies in temperate to subtropical, low-latitude, low-relief terrain with data from the eastern United States

The Coastal Plain of the eastern United States is a low-latitude, low-altitude, low-relief terrain composed primarily of gently dipping marine and marginal-marine sediments that range in age from Cretaceous to Quaternary. Population density of the area is moderate, and most of the population is concentrated along the coast. Inland of the coast, agriculture, including growing trees for pulp, is the dominant economy. In this region, soils have developed along two different pathways. One pathway is dominated by the dissolution and movement of oxyhydroxides and the accumulation of organic matter; the other by the accumulation of clays and oxyhydroxyides and the adsorption or oxidation of organic matter. The first pathway has resulted in the formation of Spodosols; the second, in the development of Ultisols. No clearly distinguishable age trends have been identified in the Spodosols, but the properties of Ultisols can be measured to quantify surface material alteration through time. Ultisols are, therefore, suited to order-of-magnitude chronostratigraphic interpretations. Potentially, data derived through the study of Ultisols can be used to develop models that predict how surface processes will change due to continued weathering and pedogenesis or as the result of climate change.

Alabama, Delaware, Florida, Georgia, Maryland, New↗
USGS1971

Effects of karst features on circulation of water in carbonate rocks in coastal areas

The normal balance between fresh water in coastal aquifers and sea water applies also to carbonate-rock aquifers that have been karstified, but there are local modifications in the balance that need to be considered. Uneven distribution of permeability, expressed by a network of solution channels bounded by relatively impermeabler rock, causes an uneven distribution of head of the environmental water along the seacoast. Where sinkholes and (or) vertical solution shafts below sea level penetrate the aquifer, the fresh ground water may discharge through these karst features if the fresh-water head is greater than that of the salt water. However, under some conditions the salt-water head may exceed that of the fresh water, and the direction of movement is reversed as sea water flows into the aquifer. This sea-water flow into the aquifer occurs (1) where sinkholes, acting as “cased wells,” penetrate less permeable rock before reaching a lateral solution channel and (2) where (or when) the fresh-water head is less than that required to balance the salt water. On Andros Island, Bahamas, the range in tide (as much as 5 feet) from low tide to high tide is sufficient to cause such a reversal locally. During low tide the salt-water head becomes sufficiently low that the ground-water head exceeds that of the sea water, and the ground water flows through the sinkholes to the ocean floor; during high tide sea water flows in the sinkholes. In the Adriatic Sea along the coast of Yugoslavia, apparently the fresh-water head is sufficient to produce perennial springs in some localities, but in other areas, as in the Bay of Kastela near Split, the fresh-water head becomes low enough during some seasons that the flow is reversed and salt water enters the aquifer through the karst features. The development of sinkholes and other karst features near the present coast and extending below sea level occurred generally during a low stand of the Pleistocene sea when the top of the saturated zone stood lower than the bottom of the deepest sinkholes or natural wells. Integrated evaluations of (1) the distribution of permeability in coastal karst regions and (2) the principles relating to the dynamic balance between fresh aquifer water and sea water are leading to better knowledge of methods that may salvage much karst water which is lost to the sea.

Split↗
USGS1983

The saltwater-freshwater interface in the Tertiary limestone aquifer, southeast Atlantic outer-continental shelf of the U.S.A.

Hydrologic testing in an offshore oil well abandoned by Tenneco, Inc., determined the position of the saltwater-freshwater interface in Tertiary limestones underlying the Florida-Georgia continental shelf of the U.S.A. Previous drilling (JOIDES and U.S.G.S. AMCOR projects) established the existence of freshwater far offshore in this area. At the Tenneco well 55 mi. (∼88 km) east of Fernandina Beach, Florida, drill-stem tests made in the interval 1050–1070 ft. (320–326 m) below sea level in the Ocala Limestone recovered a sample with a chloride concentration of 7000 mg l −1 . Formation water probably is slightly fresher. Pressure-head measurements indicated equivalent freshwater heads of 24–29 ft. (7.3–8.8 m) above sea level. At the coast (Fernandina Beach), a relatively thin transition zone separating freshwater and saltwater occurs at a depth of 2100 ft. (640 m) below sea level. Fifty-five miles (∼88 km) offshore, at the Tenneco well, the base of freshwater is ∼1100 ft. (∼335 m) below sea level. The difference in approximate depth to the freshwater-saltwater transition at these two locations suggests an interface with a very slight landward slope. Assuming the Hubbert interface equation applies here (because the interface and therefore freshwater flow lines are nearly horizontal) the equilibrium depth to the interface should be 40 times the freshwater head above sea level. Using present-day freshwater heads along the coast in the Hubbert equation results in depths to the interface of less than the observed 2100 ft. (640 m). Substituting predevelopment heads in the equation yields depths greater than 2100 ft. (640 m). Thus the interface appears to be in a transient position between the position that would be compatible with present-day heads and the position that would be compatible with predevelopment heads. This implies that some movement of the interface from the predevelopment position has occurred during the past hundred years. The implied movement is incompatible with the hypothesis that the freshwater occurring far offshore in this area is trapped water remaining since the Pleistocene Epoch.

Florida, Georgia↗