Estuaries, bays, lagoons, and coastal rivers

Estuaries are partially enclosed bodies of water with a connection to open ocean in which seawater enters according to the rhythm of the tides. Seawater entering the estuary is diluted with freshwater flowing from rivers and streams, and the pattern of dilution depends on the volume of freshwater, tidal amplitude range, and evaporation rate. These ecosystems are known to thrive where the inflow of both seawater and freshwater provide high levels of nutrients in both the water column and sediments. The high concentration of nutrients supports phytoplankton, which is the primary producer in the ecosystem. Each estuary is unique with respect to its physical, chemical, and biological characteristics. Classification into general groups is made according to the geological processes that formed their embayment or, alternatively, according to their water circulation and mixing characteristics. Dynamics inside an estuary depend on its geographic location, the shape of the coastline and ocean floor, the depth of the water, tidal patterns and currents, local winds, freshwater input, and any restrictions to water flow. Additionally, there is a constant fluctuation in dissolved oxygen, salinity, temperature, and sediment load. At any point, salinity and temperature can vary considerably over time and season. Sediment often settles in intertidal mudflats, and dissolved oxygen variations can limit livability for many organisms. The wide range of salinity, temperature, and other chemical conditions, coupled with high primary productivity, creates environments that support many species of fishes and invertebrates.

Estuary Classification Based on Geomorphology

Four types of estuaries are recognized: coastal-plain estuaries, bar-built estuaries, tectonic estuaries, and fjords. In coastal-plain estuaries, formation occurred at the end of the last ice age, between 10,000 and 18,000 years ago as sea level rose to flood river valleys. These estuaries, often called drowned river valleys, are especially abundant on passive margins, such as the east coast of the United States. Their width-to-depth ratio is typically large, appearing wedge-shaped in the inner part, broadening and deepening seaward. Water depths rarely exceed 98 feet (30 meters). Chesapeake Bay, Delaware Bay, and New York Harbor are examples of coastal-plain estuaries. The Thames River in England, the Ems River in Germany, the Seine River in France, the Si-Kiang River in Hong Kong, and the Murray River in Australia are other examples. The Mississippi River delta is an example of a former coastal-plain estuary that has been filled with river-borne sediment.

Bar-built estuaries are formed when a sandbar is constructed parallel to the coastline by wave action and long-shore drift, and the bar separates the ocean from a shallow lagoon. Bar-built estuaries typically develop on gently sloping plains located along tectonically stable edges of continents and marginal sea coasts. They are extensive along the Atlantic and Gulf coasts of the United States in areas with active coastal deposition of sediments and where tidal ranges are less than 13 feet (4 meters).

The average water depth is usually less than 16 feet (5 meters) and rarely exceeds 33 feet (10 meters). Examples include Albemarle Sound and Pamlico Sound in North Carolina, Barnegat Bay in New Jersey, and Laguna Madre in Texas. Some estuaries have mixed characteristics. For example, the Hudson River estuary that passes through New York Harbor and Raritan Bay is primarily a drowned river valley, but the Sandy Hoop spit gives Raritan Bay certain bar-built estuary characteristics.

The formation of tectonic estuaries occurs when a section of land drops or tilts below sea level as a result of vertical movement along a fault, volcanoes, or landslides. Such estuaries are found on coasts along a subduction zone or on a transform fault plate boundary. There are only a small number of tectonically produced estuaries in the world. One example is the San Francisco Bay, which was formed by the movements of the San Andreas Fault system, causing the inundation of the lower reaches of the Sacramento and San Joaquin Rivers.

Fjord-type estuaries are found in drowned valleys that were cut by glaciers when sea level was lower. The U-shaped estuaries are generally steep-sided, both above and below sea level, and are often deep. Depth can exceed 980 feet (300 meters). Many fjords have a shallow sill near the mouth that was formed by sediment deposited at the lower end of the glacier when it flowed through the valley. Fjords are common along the coast of Alaska, the Puget Sound region of western Washington State, eastern Canada, Greenland, Iceland, New Zealand, and Norway.

Estuary Classification Based on Water Circulation

Movement and mixing of freshwater and seawater in estuaries are affected by many factors, including tidal currents and mixing; wind-driven wave mixing; shape and depth of the estuary; rate of freshwater discharge; friction between moving freshwater and seawater layers, and between the water and the seafloor; and the Coriolis effect. For this, estuary circulation is extremely important because major cities are located on estuaries. These cities discharge large quantities of waste, particularly sewage treatment plant effluents and storm water runoff, into the estuaries. Additionally, circulation in many estuaries is altered by piers and other port structures, dredging, filling of wetlands, and construction of levees and other structures. These alterations can affect the life cycles of marine species that inhabit and transit estuaries. Based on the major characteristics in water circulation, five types of estuaries are recognized: salt wedge estuaries, partially mixed estuaries, well-mixed estuaries, fjord estuaries, and inverse estuaries.

In salt wedge estuaries, river output exceeds marine input, and freshwater flows down the estuary as a surface layer separated by a steep density interface (halocline) from seawater flowing up the estuary. The differences in density form a wedge-shaped intrusion. Vertical mixing is slow, but a velocity difference between seaward-flowing river water and the underlying seawater generates internal waves mixing the seawater upward with the freshwater. Almost no freshwater is transferred into the lower seawater layer. Examples of salt wedge estuaries are the Mississippi, Columbia, and Hudson Rivers.

In partially mixed estuaries, freshwater and seawater layers are separated by a relatively weak halocline, and therefore the density gradient between the two layers is much less pronounced. Partially mixed estuaries are most common where tidal currents are relatively fast, river flow rate is moderate, and river current speed does not greatly exceed tidal current speed. Examples include the Chesapeake Bay and Narragansett Bay, the Puget Sound, and San Francisco Bay.

In well-mixed estuaries, tidal mixing forces exceed river output, and no halocline is present. Salinity decreases progressively toward the head of the estuary, although it is uniform from surface to bottom. Some variation may occur during the tidal cycle, increasing as the tide floods and decreases as the tide ebbs. This type of estuary tends to be wide and relatively shallow, with limited freshwater inputs and strong tidal currents. Some examples include the Delaware Bay and the Raritan River in New Jersey.

Most fjords have a deep, elongated basin where vertical mixing does not reach the bottom waters, even if tidal currents are strong. Consequently, almost all fjords have a halocline separating high-salinity bottom water from lower-salinity surface water. Fjords are almost never well-mixed. Fjord-type estuaries are found along glaciated coasts such as British Columbia, Alaska, Chile, New Zealand, and the Scandinavian countries.

In arid regions, shallow estuaries can sustain high evaporation rates where salinity is higher than the ocean water outside. High-salinity estuarine water flows seaward in the bottom layer, and ocean water flows landward as a relatively lower-salinity surface layer. Thus, estuarine circulation is inverted. Inverse estuaries generally occur at in subtropical regions (30 degrees north and south), where evaporation is strong and rainfall is low. Some examples include the Red Sea, San Diego Bay in California, and Laguna Madre in Texas.

Estuaries are stressful environments for organisms because of variable temperature, salinity, turbidity, currents, and other environmental factors. Salinity can induce stressful changes and organisms adapted to live in an estuary must be able to cope with widely varying osmotic pressures. Because of the high-stress environment, estuaries support fewer species than the adjacent ocean or freshwater environment. However, there is an abundant supply of nutrients and sunlight, resulting in large biomass.

Despite being an ecosystem with a high stress environment, estuaries are ideal habitats for many larvae and juvenile stages of marine animals because they provide abundant food and substantial protection. Many marine fish and invertebrates use estuaries as nursery rounds and other species briefly visit the estuary to spawn. Along the southeast coast of the United States, more than half of the commercially important marine fish species are known to use estuarine wetlands as nursery areas or for breeding. Because of the abundant supply of detritus, estuaries also support huge populations of commercially important shellfish, including clams, oysters, mussels, and crabs.

Estuaries are also among the most heavily populated areas throughout the world, with about 60 percent of the world's population living along estuaries and the coast. As a result, these ecosystems are suffering degradation by many factors, including sedimentation from soil erosion from deforestation, overgrazing, and poor farming practices; overfishing; drainage and filling of wetlands; eutrophication; and pollution with heavy metals and polychlorinated biphenyls (PCBs). These impacts in turn negatively affect not only the people living around the estuaries, but the degraded environment reduces habitable space for animal and plant species and can make those populations sick.

Bays

A bay is a small area of water or broad inlet set aside from a larger body of water where the land curves inward. Bays have calmer waters than the surrounding sea, since land is blocking waves and often reducing winds. A large bay may be called a gulf, a sea, a sound, or a bight. A cove is a circular or oval coastal inlet with a narrow entrance, and some coves may be referred to as bays. Bays are essential for many bird, fish, and marine mammal species. For example, the Bay of Fundy located on the Atlantic (east) coast of North America, on the northeast end of the Gulf of Maine, has been compared in marine biodiversity to the Amazon Rainforest. Other examples include San Francisco Bay, off the coast in northern California. Other bays include the Bay of Pigs in Cuba, Hudson Bay in Canada, Chesapeake Bay in Maryland and Virginia, and the Bay of Bengal, near India. When large and deep enough, bays become natural harbors that are often of great economic and strategic importance. Many of the great cities around the world are located on natural harbors.

Shark Bay is located in Gascoyne, the most western point of Australia, in the federal territory of Western Australia, 497 miles (800 kilometers) north of Perth. Shark Bay is currently composed of two bays, several peninsulas, and islands with a coastline that extends around 932 miles (1,500 kilometers) and covers an area of 3,861 square miles (10,000 square kilometers). Shark Bay gathers three of the most important climatic regions of the world, and has great ecological importance. This bay is home to more than 10,000 dugongs, a large community of bottlenose dolphins (Turciops truncatus), and is an important reproduction zone for hundreds of species of fishes, cnidarios, and crustacean, including sharks and rays that give its name to the bay. Additionally, Shark Bay has also the largest and most varied area of seagrass in the world, with a cover of about 1,544 square miles (4,000 square kilometers). The bay also hosts a unique community of microbes (located in Hamelin Pool) that are building colonies of algae around 3,000 years old (stromatolites).

The Bay of Fundy is located on the Atlantic coast of North America, on the northeast end of the Gulf of Maine between the Canadian provinces of New Brunswick and Nova Scotia, with a small portion in the state of Maine. It extends 170 miles (270 meters) and it is known for having the highest tides in the world (54 feet, or 16 meters), with the potential of becoming one of the world's greatest producers of tidal energy. The marine biodiversity in the Bay of Fundy is vast; at least 12 species of whales can be found here. An abundant number of dolphins, porpoises, fish, seals, and seabirds populate its waters.

The Bay of Islands is one of the most popular holiday destinations in New Zealand, located in the Northland Region of the North Island, 37 miles (60 kilometers) northwest of Whangarei. The bay is an irregular, 10-mile (16-kilometer) wide inlet and contains 144 islands, many secluded bays, and sandy beaches. This bay has abundant marine life including whales, penguins, dolphins, and marlins. It is also a popular gathering place for sailing yachts and international sport fishermen. The bay is also historically significant because it was the first part of New Zealand settled by Europeans.

The Bay of Kotor is a winding bay on the Adriatic Sea, located in southwestern Montenegro. This bay is often mistaken for a fjord, but is in fact a submerged river canyon of the disintegrated Bokelj River. Because of its location and peculiar topography, this bay is the second-wettest place after Japan's Kii Peninsula, north of the Himalayas. The Bay of Kotor is considered a picturesque place to visit, with numerous beaches.

Phang Nga Bay is located over 60 miles (95 kilometers) from the island of Phuket and the Malay Peninsula in the Andaman Sea. It is an extensive bay, and since 1981 an extensive section was protected as the Ao Phang Nga National Park. In 2002, this bay was declared a Ramsar Site of international ecological significance, comprising shallow marine waters and intertidal forested wetlands, with 28 species of mangroves, seagrass beds, and coral reefs. Species like the dugong and the black finless porpoise (Neophocaena phocaenoides) are also found in the bay.

San Francisco Bay is situated on the Californian coast, and it includes an entire group of interconnected bays. The area around it is considered the second-largest urban area, with approximately 8 million residents. Water drains from the Sacramento and San Joaquin Rivers from the Sierra Nevada Mountains, where both rivers then flow into the Suisun Bay. This water then flows through the Carquinez Strait, which meets with the Napa River at the entrance of the San Pablo Bay. This group of rivers finally drains into the Pacific Ocean through the San Francisco Bay. The bay occupies between 400 and 1,600 square miles (1,040 to 4,160 square kilometers), and it is considered the largest estuary in the Americas. The San Francisco Bay and the Sacramento–San Joaquin Delta are considered California's most important ecological habitats, where organisms like the Dungeness crab, California halibut, and the Pacific salmon fisheries rely on the bay as a nursery. This bay is protected by the California Bays and Estuarine Policy.

Lagoons

Coastal lagoons are among the most common coastal environments, occupying around 13 percent of the world's coast. A lagoon can be defined as a shallow body of water, usually oriented parallel to the coast and separated from the ocean by a barrier, connected to the ocean by one or more restricted inlets. These ecosystems are found on all continents, from arctic to equatorial, and from arid to humid, but are less common on emergent high-latitude coasts. They may have one or more natural entrances from the ocean, which can be permanent or intermittent gaps through the enclosing barriers. Lagoons are characteristically shallow, typically 3–10 feet (1–3 meters), and almost always less than 16 feet (5 meters, with the exception of inlet channels and isolated relict holes or channels), and are subject to tidal mixing. They are strongly influenced by precipitation and evaporation, which results in fluctuating water temperature and salinity.

Salinity can range from completely fresh to hypersaline conditions. Size varies substantially, with surface areas ranging up to 3,938 square miles (10,200 square kilometers), as in the case of Lagoa dos Patos in Brazil. Coastal lagoons serve as material sinks or marine filters because they trap inorganic sediment and organic matter. They often exhibit high primary and secondary productivity, and are valuable for fisheries and aquaculture, and sometimes for salt extraction. These are fragile ecosystems that experience forcing from river input, wind stress, tides, precipitation to evaporation balance, and surface heat balance. Lagoons are classified into three geomorphic types, according to water exchange with the coastal ocean. They are classified as: choked lagoons, restricted lagoons, and leaky lagoons.

Choked lagoons occur along high-energy coastlines, and have one or more long narrow channels that restrict water exchange with the ocean. Tidal oscillations are often reduced to 5 percent or less, compared to adjacent coastal tide. Circulation within choked coastal lagoons is characterized by long flushing times, dominated by wind patterns, and intermittent stratification events from intense solar radiation or runoff events. Examples of chocked coastal lagoons include Lagoa dos Patos and Lagoa de Araruama in Brazil, Lake St. Lucia in South Africa, the Coorong in Australia, and Lake Songkla in Thailand.

Restricted lagoons consist of a large and wide water body with two or more channels or inlets, and have a well-defined exchange with the ocean. Wind patterns influence circulation, and lagoons exhibit a variety of salinities, ranging from brackish to hypersaline. Examples include the Indian River Lagoon and the Pontchartrain in the United States, and Laguna de Terminos in Mexico.

Leaky lagoons are elongated shore-parallel water bodies with many ocean channels along the coast. Leaky lagoons are characterized by numerous wide tidal passes; unimpaired exchange of water with the ocean on wave, tidal, and longer time scales; strong tidal currents; and salinities close to that of the coastal ocean. Examples include the Mississippi Sound in the United States and the Wadden Zee in the Netherlands.

Coastal lagoons are extensive in low-lying coasts and are poorly developed on coasts dominated by high retreating cliffs, such as the Great Australian Bight, and the steep and rocky fjord coasts of Norway, Chile, and southern New Zealand. They are also rare on macro-tidal coasts such as the Bay of Fundy between Canada and the United States, and the Baie de Mont Saint Michel in France. Other examples include the Albemarle Sound in North Carolina; Great Sound Bay between Long Island and the barrier beaches of Fire Island in New York; Isle of Wight Bay, which separates Ocean City, Maryland, from the rest of Worcester County, Maryland; Banana River in Florida; and Lake Illawarra in New South Wales. In India, the Chilika Lake in Orissa, near Puri, and the Vembanad Lake in Kerala are also considered lagoons. Some other well known lagoons include the Lagos Lagoon and the Keta Lagoon in Ghana, Africa.

Coastal Rivers

Rivers are the result of precipitation falling across the land, coalescing into even larger streams and rivers. These ecosystems are one of the most dramatic features of a continent, delivering sediment and nutrients downstream, eroding valleys, and eventually reaching the sea or an inland lake. This water movement shapes terrain, creates a variety of freshwater environments, and allows the evolution of several species of plants, animals, and microbes. Rivers vary in latitude, topography, and size, contributing to a great variation in biodiversity and ecological characteristics. Climate, including temperature and precipitation patterns, further characterizes the landscape and is influenced greatly by global air circulation patterns, ocean currents, shape of the continent, and mountain ranges.

Coriolis forces have a predominant effect in river discharge patterns, and significantly influence the living marine resources of coastal areas. Gravity causes the higher-density oceanic water to sink beneath the less-dense surface water, and mixing processes produce water of intermediate density. Buoyancy-driven flows that are supplied by this mixed water formation cause positively buoyant materials and vertically migrating planktonic organisms to accumulate in the slowly sinking waters of the convergent frontal zone. In this scenario, the distribution of food particles becomes highly concentrated, and feeding organisms may dramatically increase caloric intake per unit. This process tends to work its way upward through the trophic levels.

For this reason, coastal rivers are extremely important to biological population dynamics. Major factors that threaten river biodiversity, and consequently the marine environment, include agriculture, deforestation, freshwater extraction, and human and industrial waste discharge, plus the effects of artificial changes to river hydraulic characteristics. Nutrient pollution, also known as overenrichment of nitrogen and phosphorous from coastal rivers, can lead to phytoplankton blooms, causing estuaries to receive more nutrients per surface area than any other ecosystem. In the United States, for example, more than 60 percent of coastal rivers and bays are moderately to severely degraded by nutrient pollution. Although such problems occur in all coastal states, the situation is particularly acute in the mid-Atlantic states, southeast, and Gulf of Mexico.

High sediment loads and stagnant nutrient-rich organic matter flows, plus excessive extraction of freshwater in coastal rivers, can also lead to high turbidity rates and bottom anoxia in many coastal waters. These changes in nutrients, light, and oxygen can favor some species over others and cause shifts in the structure of phytoplankton, zooplankton, and bottom-dwelling (benthic) communities. For example, blooms of harmful algae such as red and brown tide organisms can become more frequent and extensive, sometimes resulting in human shellfish poisonings and even marine mammal deaths. Other factors, such as how often a bay or estuary is flushed and its nutrients diluted by open ocean water, make some coastal ecosystems more susceptible to nutrient overenrichment than others. Ecosystems like coral reefs and seagrass beds are particularly susceptible to environmental changes in coastal river flows.

The largest coastal system affected by eutrophication in the United States is the dead zone in the Gulf of Mexico, an extensive area of reduced oxygen levels. Other severely impacted coastal systems in the United States include Chesapeake Bay, Long Island Sound, and the Florida Keys. In Europe, the Baltic, North, Adriatic, and Black Seas have all experienced problems from nutrient overenrichment.

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