Estuaries can take on many different forms, but all share two key aspects: the mixing of two distinct water bodies and a partially enclosed environment. Most estuaries are formed at the land-sea interface by the meeting and mixing of fresh and salt water, creating a unique brackish environment. Water constantly moves in and out of an estuary, with rivers supplying the main source of freshwater and tides bringing in the largest amount of saltwater. However, some estuaries occur at the meeting of two freshwater bodies, such as in the Great Lakes Region, where rivers flow into large freshwater lakes.
Estuarine circulation involves the outflow of freshwater along the surface and the inflow of denser saltwater along the bottom of the estuary. This two-layer flow creates a dynamic exchange where deeper saltwater rises to replace freshwater lost to the ocean, facilitating the distribution of salt and nutrients throughout the estuary. One of the most influential factors in estuarine circulation is the volume of freshwater input. Seasonal variations, such as droughts or heavy rainfall, can significantly alter salinity and circulation patterns. For instance, low input of freshwater during prolonged dry periods reduces water levels and increases salinity, while heavy rainfall can flood estuaries and decrease salinity.
Although greatly influenced by tides, estuaries are protected from the full force of ocean waves, wind, and storms by natural land masses such as peninsulas, barrier islands, or surrounding salt marshes. This sheltered setting, combined with nutrient rich brackish waters, makes estuaries one of the most productive ecosystems on earth. They support a wide range of habitats and serve as vital nursery grounds for countless marine species, earning them the title “nurseries of the sea.”
Coastal Habitats
Coastal habitats can vary depending on geographic region. The three main types of estuary habitats include salt marshes, seagrass, and mangroves. RAE works to restore all of these habitats in order to preserve an ecosystem that has brought tranquility, education, and jobs to millions of people all over the country.
Salt marshes
Salt marshes are coastal wetlands dominated by grass and shrub plants. Salt marshes are often characterized by having very high salinity (salt content) and sporadic small islands. Organisms that live here must withstand harsh environments due to rapidly changing tides and salinity. In addition to providing key marine habitat, salt marshes help to filter pollutants, stabilize the shoreline, serve as a buffer to storms and flooding, and sequester and store large amounts of carbon.
Seagrass
Seagrass (or submerged aquatic vegetation) habitats are constantly submerged underwater and consist of aquatic grasses, providing food and shelter to many aquatic species. These species include, but are not limited to, fish, crabs, manatees, and sea turtles. Sea grasses are very sensitive to water pollution and changes in pH.
Mangroves
Mangroves are a type of tropical forest found in coastal areas that are regularly flooded by tidal water. They are among the most carbon-rich forests in the tropics and provide many ecosystem services. They provide spawning grounds for fish species, filter pollutants from coastal waters, and protect coastal development and communities against storms, floods, and erosion. These unique evergreen plants can withstand high salinity and harsh environments.
Tidal Flats
Tidal flats are coastal wetlands characterized by fine-grained sediment due to tidal forces outweighing other hydrodynamic forces. They are primarily found within the intertidal zone and are therefore submerged underwater and exposed to air twice a day. Tidal flats have three zones: the supratidal zone (area above high tide), the intertidal zone (area covered high tide and barren during low tide), and the subtidal zone (area below low tide). Vegetation is sparse, however, these ecosystems are essential habitats for a large number of organisms including single-celled organisms such as diatoms and cyanobacteria, and larger organisms such as crabs, mollusks, and worms.
Oyster Reefs
Oyster reefs are large underwater structures made up of living and dead oyster shells. They form when oysters cluster on hard submerged surfaces and fuse together to form a reef. These reefs provide habitat to countless organisms including mussels, crabs, fish, and other invertebrates. Additionally, they offer essential ecosystem services. The filter feeding nature of oysters results in these reefs functioning as a natural filter, drawing down excess nutrients from the water column. They also stabilize shorelines and act as a storm surge barrier, mitigating coastal erosion and reducing storm damage.
Freshwater Estuary
Estuaries are characterized by the mixing of two distinct bodies of water. This mixing, however, is not limited to occurring between saltwater and freshwater. Freshwater estuaries can form where a river or creek empties into a lake. Although both are freshwater, these two water bodies are distinct in chemical composition, and their mixing can create an estuary. Instead of being influenced by tidal forces, freshwater estuaries are affected by seiches, standing waves that oscillate within a lake, and by storm surges.
Most freshwater estuaries in the US are found in the Great Lakes region. For more information on the work that RAE is supporting within the Great Lakes region check out the Great Lakes Restoration Grant Program.
Why are estuaries important?
Support the Economy
Coastal and estuarine areas are vital to the U.S. economy, supporting a large portion of the population, jobs, and GDP through industries like tourism, fishing, shipping, and recreation. Healthy estuaries are especially productive, providing critical habitats for fish and shellfish and generating billions in economic activity. Coastal restoration also drives economic growth, creating jobs and boosting property values with strong returns on investment.
Protect Against Climate Change
Estuary ecosystems protect coastlines by acting as a buffer against storm surge and wave energy, fight against pollution by filtering pollutants and excess nutrients, and sequester substantial amounts of carbon (blue carbon). As we continue to see more drastic weather shifts, estuary ecosystems will become even more critical in protecting both coastal and inland communities.
Provide Critical Habitat
Beyond providing food for our consumption, estuaries are the home of thousands of species of fish, birds, plants, and animals that depend on healthy habitat for their survival.
Preserve Culture and Education
Healthy estuaries support centuries-old cultures, traditions, and ways of life, especially for Indigenous coastal communities that have relied on them for thousands of years. Today, they remain vital for livelihoods, cultural identity, and education, serving as living laboratories of biodiversity that must be protected.
To learn more about the importance of estuaries check out our Why are estuaries important page.
Threats to Estuaries
Climate Change
Climate change threatens estuaries through rising sea levels, more frequent and intense storms, and accelerating coastal land loss, all of which disrupt habitats and the balance of these vital ecosystems.
Pollution
Harmful chemicals and excess nutrients from stormwater runoff, agricultural activities, and industrial sources degrade habitats, and threaten the health of all species that utilize estuaries, including humans.
Human Impact
Human activity threatens estuaries through population growth in coastal areas, dredging, the spread of invasive species, overharvesting of resources, and oil and gas drilling.
To explore these threats further as well as solutions check out our Coastal Threats page.
