ACE Basin NERR/All about Estuaries
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Located at the Dawhoo Boat Landing after crossing the Intracoastal Waterway onto Edisto Island. The intent of this Earthcache is to provide information about the importance of estuarine areas and the need to preserve them in their natural states. It is located where you can view an area that exemplifies the characteristics of the ACE Basin NERR. Cache is most likely wheelchair accessible but there is a short distance to travel along a paved path to GZ and to view the surroundings.
NOTE: You may want to print the cache pages since you will need the information provided to log the cache.
LOGGING REQUIREMENTS:
1. Email me a short paragraph describing the view you are seeing...is it high or low tide??? What do you think is the geologic origin of this area and the type of water circulation you would find here. What do you think the economic and recreational impact would be if this area were to be lost because of pollution, global warming or overdevelopment?? There are NO signs here to tell you this info--use your observations and info provided in the cache pages.
Although not a requirement...post a photo of you or your group.
THE ACE BASIN NERR
The ACE (Ashepoo-Combahee-Edisto River) Basin is a pristine estuarine area wedged between the rapidly growing metropolitan areas of Charleston to the north and Beaufort/Hilton Head to the south. The Basin is recognized as one of the most diverse natural areas of the Atlantic Coast. It is an important wintering area for waterfowl and nesting bald eagles. It provides important nesting beaches for threatened loggerhead sea turtles and supports the largest concentration of wood stork nesting colonies in the nation. The St. Helena Sound is recognized as one of the most pristine estuaries on the East Coast and provides important commercial and recreational finfish and shellfish resources.
The Estuary: where fresh and saltwater mix
Estuaries and their surrounding wetlands are bodies of water usually found where rivers meet the sea. Estuaries are home to unique plant and animal communities that have adapted to brackish water — a mixture of fresh water draining from the land and salty seawater. Estuaries are among the most productive ecosystems in the world. Many animals rely on estuaries for food, places to breed, and migration stopovers. Human communities also rely on estuaries for food, recreation, jobs, and coastal protection. Of the 32 largest cities in the world, 22 are located on estuaries! Estuaries are delicate ecosystems. Congress created the National Estuarine Research Reserve System to protect more than one million acres of estuarine land and water. These estuarine reserves provide essential habitat for wildlife, offer educational opportunities for students, and serve as living laboratories for scientists.
Classifying Estuaries by geology
The features of an estuary are determined by a region's geology, and influenced by physical, chemical, and climatic conditions. For example, movements in the Earth’s crust elevate or lower the coastline, changing the amount of seawater that enters an estuary from the ocean. The coastal elevation also determines the rate of fresh water that flows into an estuary from rivers and streams. The amounts of seawater and fresh water flowing into an estuary are never constant. The quantity of seawater in an estuary changes with the changing tides, and the quantity of fresh water flowing into an estuary increases and decreases with rainfall and snowmelt.
Estuaries are typically classified by their existing geology or their geologic origins (in other words, how they were formed). The five major types of estuaries classified by their geology are: coastal plain, bar-built, deltas,tectonic and fjords. In geologic time, which is often measured on scales of hundreds of thousands to millions of years, estuaries are often fleeting features of the landscape. In fact, most estuaries are less than 10,000 years old.
Coastal plain estuaries, or drowned river valleys, are formed when rising sea levels flood existing river valleys. Bar-built estuaries are characterized by barrier beaches or islands that form parallel to the coastline and separate the estuary from the ocean. Barrier beaches and islands are formed by the accumulation of sand or sediments deposited by ocean waves. A delta, characterized by large, flat, fan-shaped deposits of sediment at the mouth of a river, occurs when sediments accumulate more rapidly than ocean currents can carry them away. When the Earth’s tectonic plates run into or fold up underneath each other, they create depressions that form tectonic estuaries. Fjords are steep-walled river valleys created by advancing glaciers, which later became flooded with seawater as the glaciers retreated.
Classifying Estuaries by Water Circulation
In addition to classifying estuaries based on their geology, scientists also classify estuaries based on their water circulation. The five major types of estuaries classified according to their water circulation include: salt-wedge, fjord, slightly stratified, vertically mixed and freshwater.
Water movements in estuaries transport organisms, circulate nutrients and oxygen, and transport sediments and wastes. Once or twice a day, high tides create saltwater currents that move seawater up into the estuary. Low tides, also once or twice a day, reverse these currents. In some estuaries, the mixing of fresh water from rivers and saltwater from the sea is extensive; in others it is not. The daily mixing of fresh water and saltwater in estuaries leads to variable and dynamic chemical conditions, especially salinity. When fresh water and saltwater meet in an estuary, they do not always mix very readily. Because fresh water flowing into the estuary is less salty and less dense than water from the ocean, it often floats on top of the heavier seawater. The amount of mixing between fresh water and seawater depends on the direction and speed of the wind, the tidal range (the difference between the average low tide and the average high tide), the estuary’s shape, and the volume and flow rate of river water entering the estuary. These factors are different in each estuary, and often change seasonally within the same estuary. For example, a heavy spring rain, or a sustained shift in local winds, can drastically affect the salinity in different parts of an estuary. The degree to which fresh water and saltwater mix in an estuary is measured using isohalines. Isohalines are areas in the water that have equal salt concentrations, or salinities. The shape of the isohalines indicates the amount of mixing that is occurring, and may provide clues about the estuary’s geology. To determine isohalines, scientists measure the water's salinity at various depths in different parts of the estuary. They record these salinity measurements as individual data points. Contour lines are drawn to connect data points that have the same salinity measurements. These contour lines show the bounderies of areas of equal salinity, or isohalines, and are then plotted onto a map of the estuary. The shape of the isohalines tells scientists about the type of water circulation in that estuary.
Estuarine Habitats
A rich array of habitats surround estuaries. The type of habitat is usually determined by the local geology and climate. Habitats associated with estuaries include salt marshes, mangrove forests, mud flats, tidal streams, rocky intertidal shores, reefs, and barrier beaches. Examples of nearly every type of estuarine habitat exist along the coastline of the United States. In New England, salt-tolerant grasses fill salt marshes along the shores of tidal rivers. As one travels further south, the Atlantic Coast becomes much sandier, and barrier beaches enclose huge bays or sounds. In this region, estuarine habitats cover large areas along tidal rivers, and salt marshes reach far inland. Along the southern coast of Florida and lining the Gulf of Mexico are extensive mazes of mangrove forests. From northwestern Florida to the Texas coast are long, narrow, sandy barrier islands and shallow estuaries lined with marshes. Along the Texas coast, barrier islands protect estuaries that have formed narrow lagoons with small openings to the Gulf of Mexico. In these areas, estuaries with very little freshwater input often become hypersaline or super salty. Along the Pacific Coast of the United States, from northern California to Alaska, coastal rivers flow quickly out of the mountains and into very small estuaries. San Francisco Bay is one of the largest estuaries on the U.S. West Coast, and one of only a few that is similar in size to those found on the East Coast. Here we are primarily interested in the Salt Marsh Habitats.
Salt marshes are a mosaic of snaking channels called tidal creeks that fill with seawater during high tides and drain during low tides. Fish species including flounder and mullet live most of their lives in marsh creeks. Levees are areas of higher ground that border the marsh creeks. Between the levees and tidal creeks are marsh flats, which contain pools and salt pannes. Salt pannes are shallow depressions that contain very high concentrations of salt. Pannes retain seawater for very short periods of time. When the seawater evaporates, the salts remain and accumulate over many tidal cycles. Glasswort, a plant tolerant to very high salt concentrations, is one of the only organisms able to survive in salt pannes. Pools are generally deeper than pannes, and retain water all year long. Salt-marsh snails and green crabs are some of the creatures found in pools scattered across the marsh.
Low-lying areas of the marsh are often covered with large, flat expanses of mud called mud flats. Composed of fine silts and clays, mud flats harbor burrowing creatures including clams, mussels, oysters, fiddler crabs, sand shrimp, and bloodworms. Salt marshes are salty because they are flooded by seawater every day. They are marshy because their ground is composed of peat. Peat is made of decomposing plant matter that is often several feet thick. Peat is waterlogged, root-filled, and very spongy. Because salt marshes are waterlogged and contain lots of decomposing plant material, oxygen levels in the peat are extremely low—a condition called hypoxia. Hypoxia promotes the growth of bacteria which produce the rotten-egg smell that is attributed to marshes and mud flats. Salt marshes are covered with salt-tolerant plants, or halophytes, like salt hay, black rush, and smooth cordgrass. However, these plants do not grow together in the same area. Marshes are divided into distinct zones, the high marsh and the low marsh. The difference in elevation between these two areas is usually only a few centimeters, but for the plants that inhabit each of these zones, a few centimeters makes a world of difference. The low marsh floods daily at high tide. The high marsh usually floods about twice a month during very high tides associated with new and full moons. The more often an area is flooded, the more saline it is. Plants living in salt marshes have different tolerances to salt. Those with higher tolerances are found in the low marsh, and those with lower tolerances to salt are found in the high marsh zones. Plants from one marsh zone are never found in the other. Smooth cordgrass (Spartina alterniflora) dominates the low marsh all the way down to the estuary’s edge. It is tall, sturdy, broad-leaved, and one of the main components of peat. As one moves toward the high marsh, salt hay (Spartina patens), a very fine-leaved grass about 1-2 feet tall, and spike grass (Distichlis spicata) dominate the area. The highest parts of the marsh are characterized by black rush (Juncus gerardii), which grows in dense swaths. Surrounding the high marsh are the upland habitats. Uplands are rarely, if ever, flooded with saltwater.
Adaptations to life in the Estuary
In almost all estuaries the salinity of the water changes constantly over the tidal cycle. To survive in these conditions, plants and animals living in estuaries must be able to respond quickly to drastic changes in salinity. Plants and animals that can tolerate only slight changes in salinity are called stenohaline. These organisms usually live in either freshwater or saltwater environments. Most stenohaline organisms cannot tolerate the rapid changes in salinity that occur during each tidal cycle in an estuary. Plants and animals that can tolerate a wide range of salinities are called euryhaline. These are the plants and animals most often found in the brackish waters of estuaries. There are far fewer euryhaline than stenohaline organisms because it requires a lot of energy to adapt to constantly changing salinities. Organisms that can do this are rare and special. Some organisms have evolved special physical structures to cope with changing salinity. The smooth cordgrass (Spartina alterniflora) found in salt marshes, for example, has special filters on its roots to remove salts from the water it absorbs. This plant also expels excess salt through its leaves.
Unlike plants, which typically live their whole lives rooted to one spot, many animals that live in estuaries must change their behavior according to the surrounding waters' salinity in order to survive. Oysters and blue crabs are good examples of animals that do this. Blue crabs live in estuaries along the United States' Atlantic and Gulf coasts. They are mobile predators whose salinity requirements change at different stages in their lives. Adult male crabs live in the low-salinity waters upstream, while adult female crabs live in the higher-salinity waters near the mouth of the estuary. During the crabs' mating season (May to October), the high-salinity preference of the female overlaps with the lower-salinity preference of the male . After mating, female crabs migrate offshore, sometimes up to 200 km, to high-salinity waters to incubate their eggs. The females release their larvae, called zoeae, during spring high tides. The zoeae, resembling tiny shrimp, develop in the coastal waters. Zoeae require water with a salinity over 30 ppt (parts per thousand) for optimal development, which is only found in the ocean. Winds and coastal currents keep the larvae near the ocean shore, until they return to the estuary as young crabs, called megalops. When the megalops return to the estuary, they swim up and down in the water in response to light and tides. This is called vertical migration. The young crabs use nighttime flood tides to move upriver into the shallow parts of the estuary. Eventually, the young crabs take up life on the bottom of the estuary, seeking out shallow-water habitats like seagrass beds and submerged aquatic vegetation (SAV), where they feed and gain protection from predators.
Natural Disturbances to an Estuary
Estuaries are fragile ecosystems that are very susceptible to disturbances. Natural disturbances are caused by the forces of nature, while anthropogenic disturbances are caused by people. Natural disturbances include winds, tidal currents, waves, and ice. Anthropogenic disturbances include pollution, coastal development, and the introduction of non-native species to an area. We like to think of natural places as being stable over time, but, in fact, they are not. Natural habitats are continually disturbed by natural processes, followed by periods of recovery. When a natural disturbance is followed by an anthropogenic disturbance or vice versa, a habitat may become so damaged that it never recovers. One type of natural disturbance is the continual pounding of ocean waves. In many estuaries, barrier beaches protect inland habitats from wave erosion. If these beaches are destroyed, salt marshes and inland habitats adjacent to the estuary may become permanently damaged. Waves can also dislodge plants and animals, or bury them with sediments, while objects carried by the water can crush them. Large storms are especially destructive to estuaries, particularly in areas like Florida and the Carolinas, where barrier beaches are common. Another natural disturbance in salt marshes is the burial of vegetation by rafts of dead floating plant material, called wrack. Wracks can be quite large—up to hundreds of square meters, and up to 30 centimeters thick. The spring high tides often move these wracks into the high marsh, where they become stranded.
Human Disturbances to an Estuary
Because they are transitional areas between the land and the sea, and between freshwater and saltwater environments, estuaries can be seriously impacted by any number of human, or anthropogenic, activities. The greatest threat to estuaries is, by far, their large-scale conversion by draining, filling, damming or dredging. These activities result in the immediate destruction and loss of estuarine habitats. Until the last few decades, many estuary habitats in North America were drained and converted into agricultural areas; others were filled to create shipping ports and expand urban areas. In the United States, 38 percent of the wetlands associated with coastal areas have been lost to these types of activities. In some areas, the estuarine habitat loss is as high as 60 percent.
Of the remaining estuaries around the world, many are seriously degraded by pollution. People have historically viewed estuaries and waterways as places to discard the unwanted by-products of civilization. Pollution is probably the most important threat to water quality in estuaries. Poor water quality affects most estuarine organisms, including commercially important fish and shellfish. The pollutants that have the greatest impact on the health of estuaries include toxic substances like chemicals and heavy metals, nutrient pollution (or eutrophication), and pathogens such as bacteria or viruses. Another, less widely discussed human-caused disturbance is the introduction of non-native or invasive species into estuarine environments. Toxic substances are chemicals and metals that can cause serious illness or death. They may be poisonous, carcinogenic (cancer-causing) or harmful in other ways to living things. Pesticides, automobile fluids like antifreeze, oil or grease, and metals such as mercury or lead have all been found to pollute estuaries. These substances can enter an estuary through industrial discharges, yard runoff, streets, agricultural lands, and storm drains. Once consumed by plants and animals, some toxic substances can accumulate in these organisms' tissues. The insecticide DDT, and the metal mercury, are known to progressively accumulate or build up in the tissues of organisms as they make their way from the bottom of the food web (algae, shrimp, oysters, fish) to the top (osprey, eagles, bears, people). Sometimes, toxic substances become attached to sediments (sand or mud) that flow down rivers and get deposited in estuaries. Toxic substances that enter the estuary this way often contaminate bottom-dwelling animals like oysters or clams, making them a serious health risk to people who eat them. Nutrient pollution is the single largest pollution problem affecting coastal waters of the United States. Most excess nutrients come from discharges of sewage treatment plants and septic tanks, stormwater runoff from overfertilized lawns, golf courses and agricultural fields. Over 60 percent of the coastal rivers and bays in the United States are moderately to severely affected by nutrient pollution.
Monitoring an Estuary
The National Estuarine Research Reserve System or NERRS is a partnership program between NOAA and U.S. coastal states that protects more than one million acres of estuarine land and water. These estuarine reserves provide essential habitat for wildlife; offer educational opportunities for students, teachers and the public; and serve as living laboratories for scientists. The health of every reserve is continuously monitored by the NERRS System-wide Monitoring Program or SWMP (pronounced “swamp”). SWMP measures changes in estuarine waters to record how human activities and natural events affect coastal habitats. The NERRS SWMP uses automated data loggers to monitor the temperature, depth, salinity, dissolved oxygen, turbidity and pH of the estuary’s water. These variables are recorded every 30 minutes at four stations in each of the 26 NERRS sites. They are key indicators of water quality and environmental conditions for the plants and animals that live in or use the estuary. The reserves also sample the water for nutrients (nitrogen and phosphorus) and chlorophyll on a monthly basis. Weather can have a major impact on water quality in estuaries. For example, rainfall can increase sediment runoff, which, in turn, influences dissolved oxygen, turbidity, pH and temperature. As part of SWMP, every reserve has a weather station that collects data every 15 minutes on temperature, relative humidity, atmospheric pressure, rainfall, wind speed and direction. Several reserves are able to send real-time data as they are collected directly to Web sites on the Internet. These data have already helped scientists gain a better understanding of how environmental conditions fluctuate in estuaries. The SWMP data have been used to detect conditions related to oyster diseases, measure the recovery of estuaries after hurricanes, and evaluate restoration projects in estuaries.
The Future: Managing, Protecting and Restoring Estuaries
Estuaries are biologically and economically invaluable natural resources. Assaulted by natural and man-made disturbances, estuaries, and the plants and animals that call them home, are in danger of disappearing if actions are not taken to protect them. During the last century, millions of acres of estuarine habitats have been destroyed; many more are in poor health and in danger of being lost. In 1996, 62% of estuaries had good water quality. By 2000, only 49% of estuaries had good water quality. How we choose to treat our estuaries today will have an enormous impact on their existence in the future. Recognizing the value and importance of estuaries and the dangers facing them, Congress created the National Estuarine Research Reserve System (NERRS) in 1972. NERRS is dedicated to protecting a system of estuaries that represent the range of coastal estuarine habitats in the United States and its territories. The system protects more than one million acres of estuarine land and water in 17 states and Puerto Rico. NERRS sites serve as laboratories and classrooms where the effects of natural and human activities on estuaries can be monitored and studied by scientists and students. In addition, all estuaries, whether or not they are in the National Estuarine Research Reserve System, are protected under every U.S. state’s coastal zone management program. Many states have designated estuaries as areas to preserve or restore for their conservation, recreational, ecological, historical, and aesthetic values. When we have failed to protect estuaries, another course of action is to restore them. Restoring habitats involves removing pollutants and invasive species from the water and surrounding lands, reestablishing natural ecosystem processes, and reintroducing native plants and animals. The goal is to rebuild the estuary to a healthy, natural ecosystem that works like it did before it was polluted or destroyed. In November 2000, the Estuary Restoration Act (ERA) was signed into law making restoration of our nation’s estuaries a national priority.
CREDITS: NOAA: National Ocean Service Education.
Additional hands on information about the ACE Basin NERR can be obtained at the Interpretivie(Educational Resource) Center in Edisto Beach State Park located off Palmetto Road, Edisto Island SC.
Please enjoy your visit to Edisto!!
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