Estuaries, Salt Marshes & Mangroves
A salt marsh is a marshy area found near estuaries and sounds. The water in salt marshes varies from completely saturated with salt to freshwater. Estuaries are partly sheltered areas found near river mouths where freshwater mixes with seawater. Both salt marshes and estuaries are affected by high and low tides. Mangrove forests are found in the intertidal zone of tropical coastlines and estuaries, commonly in the tropical coastal areas of Australia, Africa, North and South America between 32° N and 38° S. Mangrove forests are made up out different types of mangrove trees and a wide variety of plants. The mangrove tree is a tree with roots and leaves that filter salt and other materials. Different mangrove species are adapted to serve different functions depending on their location. Mangroves are so good at expelling salt, that in some species the water in the roots is fit to drink.
The Salt Marsh: A Self-sustaining Cycle
Many salt marshes are located in the southern United States, particularly in South Carolina with more than 344,500 acres, which is more marshland than any other state on the Atlantic coast. Marine life in salt marshes is incredibly diverse and abundant. Salt marsh species rely on the decay of marsh plants to supply a steady source of food in the form organic material, or detritus, resulting from the decomposition of plants and animals. Most marsh plants flourish in the spring and summer, growing taller and more abundant. In the fall, they begin to decay and are distributed within the same marsh or into other marshes and mudflats where they become the first level of the food chain. Microscopic organisms like bacteria, small algae, and fungi help decompose the detritus resulting from salt marsh plants. These microorganisms and the remaining decomposing plant material become an ideal source of food for bottom-dwellers in salt marshes like worms, fishes, crabs, and shrimps. The cycle continues when the feces of the bottom-dwellers is cleaned up by microorganisms. Anything left over is great fertilizer for the next spring, when the marsh plants fill the marsh with green lush leaves.
As with many food webs, microorganisms at the most primary level on the food chain are responsible for more than one role. The same microorganisms feeding on detritus cover the mud surface, stabilize sediments, feed larger animals, and add nutrients to the sediments.
South Carolina marshes are home to many species of birds like the red-winged black bird, herons, and egrets.
They feed on a variety of food sources in the marsh environment such as insects, seeds, fishes, fiddler crabs, and shrimps. It is common to see these birds guarding the tide pools for any splash indicating a fiddler crab or other marsh delicacy. As with all of the marsh residents, birds contribute to the cycle by breaking down detritus and discarding organic material (feces) to fertilize marsh grass and be used by microorganisms.
The leaves, stems, and roots of salt marsh plants provide a vital shelter from predators and nourishment for young fish, shrimps, and crabs. Without this environment, only a handful would survive. Among young salt marsh species are blue crab, spot tail bass, and white shrimp. Larger predators live in creeks waiting for the fish to come out of the marsh when the tide changes. Some marsh shrimps and fishes, including the mummichogs and grass shrimp, stay in potholes or standing pools of marsh water after the tide goes out. Several reptiles reside in the salt marsh habitat, including the most commonly found diamondback terrapin, a turtle that searches for food and lays its eggs when the tide comes into the marsh. Occasionally, American alligators can be found in the less salty waters of brackish salt marshes.
The largest estuary in the United States is the Chesapeake Bay, located off of the Atlantic Ocean bordered by Virginia and Maryland, although the watershed covers 165,800 km in the District of Columbia and New York, Pennsylvania, Delaware, Maryland, West Virginia, and Virgina. Over 150 streams and rivers drain into the 304 km long Chesapeake Bay. Like many other estuaries, the Bay was once a valley with a river running through it, until the sea level rose or the Chesapeake Bay impact crater was formed by the bolide impact event towards the end of the Eocene period about 35.5 million years ago. At its narrowest section, the Chesapeake Bay estuary is only 6.9 km wide. The Bay is extremely shallow. A person of average height could probably walk across the 2,800 km of the bay. The average depth of the Bay is less than 9 m.
Another large estuary is Galveston Bay, formed by the Trinity and San Jacinto Rivers flowing together and combining with tides from the Gulf of Mexico. This estuary is located along the coast of Texas and covers about 1,500 km with a length of 50 km and a width of 27 km. Although large, the Bay is only 3 m deep on average and flows into the channel between Galveston Island and the Bolivar Peninsula. The world’s largest estuary is the Gulf of Saint Lawrence, a place where all the great lakes can flow into and out of the Atlantic Ocean through the Saint Lawrence River.
The Amazing Mangrove Forest
A crucial component of the coastal ecosystem and a powerful form of erosion control, mangrove trees provide shelter and nutrients to their ecosystems. Like salt marshes, these shallow, nutrient rich areas provide shelter to young fish, shrimps, crabs and mollusks where they can live safely and develop. Hundreds of bird species migrate and nest in mangrove forests such as those found in Belize that provide a home to over 500 species of birds. Other animals that inhabit mangrove forests include manatees, sea turtles, fishing cats, monitor lizards and mud-skipper fish. Not only do mangrove trees directly support countless food webs, they are also indirectly responsible for the survival of the most primary planktonic and epiphytic algal food chains, which in turn provide carbon for the mangrove tree. Mangroves protect coastlines from storm damage, wave effects, and erosion. Erosion is avoided when mangroves take on the force of the waves and help replace lost sediment by catching suspended particles in their root system while simultaneously keeping that same silt from covering (and damaging) coral reefs and sea grass beds.
Mangroves in Detail
The highly adaptable mangrove tree is classified into 16-24 families and 54-75 species, with only four of those living on the southern coasts of the United States and 12 in the western hemisphere. Most mangrove species are found in Southeast Asia. Mangroves are highly adaptable depending on their environment in size, ability to spread seeds, and their niche in the ecosystem. Mangroves range in size from a small shrub up to 40 m tall. The red mangrove and several other species of mangroves have lenticels, or small pores in the prop roots through which oxygen can be brought into the aerenchyma, or air space tissue in the cortex of the plant, during low tide.
Reproductive strategies including viviparity and long-living propagules allow the mangrove to spread over large distances. Viviparity is the reproductive strategy where the embryo is safely nourished and germinated on the parent tree (rather than in the ground), allowing the developing tree to avoid the severe saline environment. The embryos drop to the soil from the protection of the parent tree after early development has already occurred and they have stored enough carbohydrates to survive. Some species of mangroves distribute what are known as propagules, seedlings that fall from the branches and float long distances. These propagules can establish roots up to 1 year after they fall from the parent plant. The mangrove can take root on the edge of islands, in sheltered bays and estuaries and further inland. Mangroves must be able to adjust to the changing of tides, temperature, ocean currents, steep sloping shores and a variety of soil types. They can thrive in mud, sand, coral, peat and rock.
Environmental Concerns – Salt Marshes
Although they are now protected by federal and state laws and regulations, between 1950-1970 countless salt marshes were lost forever when they were filled due to land use, ditched for mosquito control, and diked to collect water. The value of salt marshes to juvenile species was not realized at this time. Now however these areas are recognized for their ability to filter out and break down toxins and sediments from incoming water.
In areas where the salt marshes have been filled and there is no basin to absorb the extra water, flooding from storm surges develops into a major problem, leading to erosion of the coastal soils and saturation of coral reefs and grasses by silt. Although protected by laws, salt marshes can still suffer in quality and function when the population fails to respect the environment near the marsh area. Detrimental effects include pollution and modification of water flow by ditching to control the mosquito population or the building of canals for flood control. Runoff containing petroleum products, industrial waste, pesticides and fertilizers continue to pollute these ecosystems, leading to loss of species and the increase of others upsetting the balance and damaging the beauty of the marshes. When ditching alters water flow, the majority of nutrients pass right by the marshes affecting everything higher up on the food chain including the birds. When canals are built, water levels in the marshes increase, which stresses the marsh grass.
Environmental Concerns – Mangrove Forests
The mangrove ecosystem is a sustainable resource that provides huge numbers of people with food, tannins, fuel wood, construction materials and even medicines. When a mangrove forest is protected, it will support an entire population of coastal residents. Mangroves offer protection of property and life from hurricanes and storms, as well as reduction in erosion and siltation. Plants in mangrove forests can absorb nitrates and phosphates, cleaning up and restoring water near the shore in a natural and completely cost-free manner. Unfortunately, as with many of our natural resources, mangrove forests are quickly being lost to pollution and development. The lenticels in mangrove roots are extremely sensitive to parasite attack, clogging by crude oil and unnatural prolonged flooding. The most severe problem is the clearing of thousands of hectares of forest to create man-made shrimp ponds for the shrimp aquaculture industry. Along with the impact from the charcoal and timber industries, the mangrove forest will eventually be lost to environmental stress if these trends continue. Another contributing factor to the devastation of mangrove forests is the governmental and industrial classification of these areas as useless swampland. Areas most severely affected by the devastation are Thailand (50% loss of mangrove forests since 1960), the Philippines (338,000 hectares lost between the 1920s and 1990), and Ecuador (20% loss of its mangrove coastline). In the Muisne region of Ecuador, approximately 90% of the mangrove forests have been lost. Overall, up to 50% of the world’s mangrove destruction can be attributed to the shrimp farm activity.
Increased conservation efforts for mangrove protection are needed to address clearing of these areas for shrimp farming and land development.