Estuaries are incredibly diverse ecosystems, with many habitats available. Different bird, fish, mammal, and invertebrate species inhabit these delta-like structures.
Estuaries are a section of water where a river meets the ocean. Several rivers will converge to one large opening to the sea. The water found inland will be brackish and typically has a marshy appearance.
The wetlands habitat formed by an estuary does not require saltwater. The Great Lakes water systems have many rivers flowing into them, creating a similar environment. Chemically and biologically, the two ecosystems differ. But, biologists have shown they serve similar purposes.
The article below will teach you how these unique habitats develop and the variety of animals that call them home.
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What is an Estuary?
Estuaries are where a freshwater river meets the ocean. The water near the mouth of the river and the surrounding pools is brackish. The mix of fresh and seawater allows a wide variety of species to inhabit and explore the estuaries.
The tides significantly impact the salinity of the estuaries. During high tide, salt levels will rise with the increased ocean water. If the river levels are normal, low tide will drop the salt levels of the estuary.
Salt levels indicate what species can live there under the current conditions. Only a few species, like the mangrove tree, have adapted to constantly changing salinity. Some species of mangrove trees have roots that filter out salt, while others have glands that secrete salt crystals over time.
Blue crabs are another species well adapted to live in estuaries. The crabs require different levels of salinity based on their life stage. Males prefer the fresher water upstream. Meanwhile, females prefer the salty water of the estuary’s mouth. During the mating season, their ranges overlap, and mating occurs.
There are a few freshwater estuaries in the world. In the Great Lakes, many large rivers open into the lakes. When the warmer river water meets the colder lake water, it changes the chemical composition of the surrounding area.
Where Are Estuaries Found?
Estuaries exist around the world. Almost anywhere a bay or river directly flows into the ocean is deemed an estuary.
The ocean service classifies estuaries either by water composition or geological structure. Different levels of salinity and certain chemicals define the estuary’s water composition. The four geological structures differ based on where they are in the world and help shape surrounding estuaries.
Estuaries Around the World
The Chesapeake Bay is the largest estuary in the United States. It is only second to St. Lawrence Bay, the biggest in North America.
The Chesapeake Bay stretches around 200 miles (321.9 kilometers) from the ocean to the northernmost river.
Dozens of rivers feed into the bay to form one of the largest bays of brackish water found in the world. Here marshes, tidal pools, and rivers meet the ocean water to provide a diverse habitat.
Crustaceans make us of the river systems to breed and seek refuge from the open water. Different species of fish, like the striped bass, manage the brackish water well throughout the bay. Some apex predators like the bull shark will turn up in the bay to feed on the healthy bony fish populations.
The Eastern Neck Wildlife Refuge is a popular destination for birders and wildlife enthusiasts. Here visitors are exposed to all the different fresh and saltwater ecosystems that feed one of the world’s largest estuaries.
San Francisco Bay
The San Francisco Bay estuary has five distinct parts. The river delta, Suisun Bay, North Bay, Central Bay, and South Bay.
San Francisco Bay is the largest estuary on the west coast of North America. Its range extends from the rivers in the Sierra Nevada Mountains to the Golden Gate Strait. The marshes at the delta are only a few feet deep, while the bays are hundreds of feet deep.
The estuary provides water to millions of California residents and farms. The habitat provided is one of the most diverse on the west coast. It is necessary habitat for millions of wildlife.
The Outer Banks is a bar-built estuary in North Carolina. The barrier island is a popular vacation, fishing, and birdwatching spot. The large lagoon between the island and the mainland forms a protected marsh for many species.
Herons and shorebirds will comb the shallows for crustaceans and small fish. Ospreys will nest on tall posts near the lagoon, and many waterfowl use the lagoon as a stopover for winter migration.
Redfish, speckled trout, and drum live in the shallows and out to the ocean. Many crustaceans use the shelter of the lagoon to make their nests. The abundance of crabs and crayfish draws in the predators hoping to earn an easy meal.
The barrier island’s primary function is to protect the estuary from large ocean tides. The highest level typically reaches the first row of homes on the beach but is still over a mile from the lagoon.
If sea levels rise and a large storm blows through, the lagoon risks permanent damage.
Glacier Bay is a network of U-shaped valleys that once housed glaciers. When the glaciers left, the ocean and surrounding rivers filled the space. Today the fjord estuary is one of the most complicated ocean ecosystems.
Food is so abundant in Glacier Bay that it attracts a wide offering of visitors. The phytoplankton blooms here are so massive that any species that consume them will visit. Nutrients in the water from glacial runoff support giant kelp forests.
As one of America’s national parks, Glacier Bay is protected and maintained by the federal government. The stunning scenery and wildlife attract visitors annually and make it a popular cruise destination.
Types of Estuaries
Coastal Plain/Drowned River Estuaries
Like Fjord estuaries, the coastal plains were formed millions of years ago by glaciers retreating to the ocean. The resulting movement carved out river valleys, and ocean water rushed to fill the lowlands.
The Chesapeake Bay is the most recognizable example of a coastal plain estuary. Most of the water is brackish and gets supplied with fresh water from dozens of rivers around the bay.
Coastal plain estuaries are typically the most shallow type of estuary. The rivers that flow into the bays are typically faster-flowing than in a Fjord or Bar-built. The force of the current pushes more sediment into the river valleys and keeps them shallow.
Coastal plain estuaries will have a good deal of marshland among the shallows. Marshes draw in a vast number of migratory birds, and it makes them among the best birdwatching destinations.
With such a large amount of fresh and saltwater converging in one area, there’s habitat for a broader range of species.
Tectonic estuaries get formed by the shifting plates of the earth’s crust. Along the fault lines, earthquakes cause depressions that shift the elevation. In newly formed depressions, the ocean water can rush in and form a tectonic estuary.
San Francisco Bay is a famous example of a tectonic estuary. The San Andreas fault line has caused numerous earthquakes over time. One shift in the plates depressed the land enough to allow the San Francisco bay to fill with saltwater.
The river water can flow to the new lowest point and helps form a brackish bay of water. The enclosed bay may seem vast but is much more variable in temperature and salinity compared to the ocean. The earth shifted to form a bowl and now acts as an enclosed ecosystem.
Bar-built estuaries are lagoons and marshes formed by a barrier island. The Outer Banks in North Carolina is the most famous example in the United States. The long sand bar is a few miles from shore and is large enough for humans to build on.
The water trapped between the barrier and the mainland forms a giant, brackish bay. Islands and sand bars act as a protective wall for the delicate estuaries behind them.
Without the bar, tidal waves would drive up the salt water content and push out other species.
The rivers that flow into a bar-built estuary are typically low-level rivers. If the rivers flood, the flow may reopen the mouth of the bay into the ocean. Bar-built estuaries are among the most delicate estuaries and require some harmony between the river and the barrier island.
Fjord estuaries maintain the highest salt levels of any estuary. These narrow, deep ravines were carved out by glaciers millions of years ago and leave a sharp valley of brackish water behind.
The deep trenches carved out by the glaciers can run for miles inland. The farther the fjord runs inland, the more freshwater fill the fjord. Deep into the fjord, there is little flow, and the water lacks oxygen. The stagnant water that builds up prevents much saltwater from mixing in.
Fjords occur at more northern latitudes across the globe. Scandinavia, Russia, Alaska, and Canada have had many fjords carved out by glaciers. Puget Sound is the most well-known example of a fjord in the continental United States.
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Why Are Estuaries Important?
Estuaries are a necessary ecosystem for so many species. The valleys, pools, and ravines formed are delicate, nutrient-rich environments.
Without the marshland, migratory birds would need to adjust routes, salmon runs would be disrupted, and crustacean populations would suffer.
Any of these factors going wrong trickles down to the surrounding environment and eventually to us. With fewer crustaceans, fish populations will decline or move. If there are no protected bays for species to breed and nurse in, their numbers will strain.
Surviving estuaries are hugely important ecosystems that indicate a healthy environment. If an estuary suffers, it’s proof the surrounding environment is under some strain.
Threats to Estuaries
Like any delicate ecosystem, natural disasters are the primary threat to its health. Large influxes of ocean water may flood the lagoons, alter the salt levels, and push out native species.
The primary threat to them is human interference. Developing near estuaries introduces potential contaminants that could kill sensitive species. Runoff from farmland can affect the water quality and affect reproductive numbers.
The rising sea levels present a unique problem for shallower estuaries. Coastal-plain and bar-built estuaries risk total elimination if sea levels rise beyond a certain point. Higher sea levels would permanently alter the ecosystem from its salt levels to those who live there.
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Do sharks live in estuaries?
It depends on the type of estuary, but sharks will visit and live in estuaries. Some sharks, like the bull shark, prefer to breed and nurse their young in brackish estuaries.
The bull shark’s tolerance of freshwater allows them to raise young farther up the river, farther away from predators. People once spotted a bull shark over 1,000 kilometers up the Mississippi River.
Bottom-feeding sharks like the lemon and nurse shark enjoy living in estuaries. Their coloration camouflages well with the sandy marsh bottom, and the loose sediment deposited by the river is easily combed over for food.
How many estuaries are in North America?
There are hundreds of estuaries throughout North America. The highest concentration of estuaries is along the coastlines of North America, where all four types of estuaries occur.
The barrier islands along the east coast and bays of the west coast provide habitat for thousands of different species. While abundant, these habitats and their residents are sensitive to environmental changes.
How deep is an estuary?
The depth of an estuary depends on its formation. While many kinds form by glacial movement, some deposits left are steeper than others.
The Gulf of St. Lawrence is one of the deepest estuaries in the world. The deepest part of the bay is over 1700 feet (meters) down.
Fjords formed from glacial movement leave drastic, deep trenches in their wake. Coastal plain estuaries and bar-barrier estuaries are usually the shallowest kinds of estuaries.