Chapter 12

Estuaries: Where Rivers Meet the Sea

Estuaries are scattered along the shores of all the oceans and vary widely in origin, type, and size.

They may be called lagoons, sloughs, or even bays, but all share the mixing of freshwater with the sea in a partially enclosed section of the coast.

Origins and Types of Estuaries

A coastal estuary off of North Carolina

Types of Estuaries:1. Drowned River

Valleys or Coastal Plain Estuaries

2. Bar-Built Estuary3. Tectonic Estuaries4. Fjords

Many estuaries were formed when sea level rose because of the melting of ice. The sea invaded lowlands and river mouths in this process.

These estuaries are called drowned river valleys or coastal plain estuaries.

They are the most common.

Drowned River or Coastal Plain Estuaries

When the accumulation of sediments along the coast builds up sand bars and barrier islands that act as a wall between the ocean and fresh water from rivers, we call this a bar-built estuary.

They are found along the Texas coast of the Gulf of Mexico, along the Outer Banks of North Carolina, and along the North Sea coast of the Netherlands and Germany.

Bar-Built Estuaries

At times, estuaries have been created not because the rising of the sea level, but because of the sinking of the land as result of movements of the lithospheric crust.

An example is the San

Francisco Bay in California.

Tectonic Estuaries

Another type of estuary was created when retreating glaciers cut deep, spectacular valleys along the coast. The valleys were partially submerged when sea level rose, and rivers now flow into them.

Common in Southeastern Alaska, British Columbia, Norway, Southwestern Chile, and the south island of New Zealand.

Fjords

Influenced by the tides and the mixing of fresh and salt water, estuaries have a unique combination of physical and chemical characteristics, such as:SalinitySubstrateWater temperatureWater clarity / light penetration

These characteristics govern the lives of the organisms that live there.

Physical Characteristics of Estuaries

As noted previously, the average salinity of sea water is around 35%. When this mixes with freshwater (0% salinity) the salinity level falls somewhere in between.

The more freshwater being mixed in, the lower the salinity. Salinity will therefore decrease as one moves upstream.

Salinity also varies with depth as salty seawater is more dense and will stay at the bottom (salt wedge) while the fresher, less dense water will flow from the river onto the surface of the estuary.

Organisms that stay in one place within the estuary are faced with dramatic fluctuations in salinity. They are submerged under the salt wedge at high tide and experience low salinity levels at low tide.

Salinity

Rivers carry large amounts of sediment and other materials, including pollutants, into most estuaries.

When the river meets the sea the waters drastically slow, causing deposition (dropping) of sand or other coarse sediments while the finer particles continue to be carried out.

As result, the substrate of most estuaries is sand or soft mud.

Substrate

Water Temperature: varies drastically because of shallow depths and large surface area. Organisms exposed at low tide may have to face even more drastic temperature extremes and fluctuations.

Water Clarity: Large amounts of suspended sediments are typical in estuaries, which ultimately reduces water clarity. Very little amounts of light are able to penetrate through this as well, leaving waters darker. Also, the sediments “clog” the feeding surfaces of some filter feeders causing death to those sensitive to sediments.

Other Physical Factors

Estuaries are productive and home to large numbers of organisms, many of which are of commercial importance.

Estuaries provide vital breeding and feeding grounds for many birds, fishes, shrimp, and other animals.

Estuarine ecosystems consist of several distinct communities, each with its own characteristic assembly of organisms.

Estuaries as an Ecosystem

Life in an estuary revolves largely around the need to adapt to extremes in salinity, temperature, and other physical factors.

Though other marine environments may be more extreme, none change as rapidly or in as many different ways as an estuary.

Because of this, few species have successfully adapted to estuarine conditions.

Living in an Estuary

Maintaining the proper salt and water balance of cells and body fluids is one of the greatest challenges facing estuarine organisms. Most estuarine organisms are marine species that have developed the ability to tolerate low salinities. How far they move up the estuary depends on how tolerant they are.

Euryhaline species: tolerate a wide range of salinityStenohaline species: tolerate a narrow range of salinities

Coping with Salinity Fluctuations

In a muddy region, there is nothing to hold onto so animals must either burrow or live in permanent tubes beneath the sediment surface.

Because it is difficult to move through mud, the inhabitants tend to be stationary or slow-moving. Living in mud, however, has a benefit in that salinity fluctuations are less dramatic.

Adapting to the Mud

There are several distinctive communities that are associated with estuaries. One consists of the plankton, fishes, and other open-water organisms that come in and leave with the tides. Others are permanent parts of the ecosystem.

Estuarine communities consist of relatively few species. Despite this, the communities are represented by a large number of individuals with a surprising number being widely distributed around the world.

Types:1. Open Water2. Mudflats3. Primary Producers4. Salt Marshes5. Mangrove Forests6. Other Communities

Estuarine Communities

Many commercially important fish use estuaries as nurseries for their young, taking advantage of the abundant food and safety from predators. An example of this type of fish is the Atlantic Menhaden.

A rich variety of fishes live in most estuaries. Many are the juveniles of marine species that breed at sea but use estuaries as nurseries. (Ex) menhaden, anchovies, mullets, croakers, and flat fishes

Relatively few fish spend their entire lives in estuaries. Some move through estuaries during their migrations. These fish are either anadromous (from sea to spawn in freshwater) or catadromous (from freshwater to spawn at sea).

Open Water

The life cycle of the

Atlantic Menhaden

shows that adults

spawn in the coastal

ocean, where eggs

and larvae are

found.

Late-stage larvae

then enter into

estuaries, such as

the Chesapeake

Bay, and transform

to juveniles. Most

of the juveniles

leave the Bay in

their first winter.

Mudflat communities in estuaries are similar to those on muddy shores.

Low tides expose organisms to desiccationWide variations in temperatures and predation

In estuaries however, mudflat organisms must also withstand regular variations in salinity.

Mudflats

Primary producers are not usually evident on mudflats.

Seaweeds (green algae, sea lettuce, red algae) manage to grow minimally

Benthic diatoms grow and undergo extensive blooms

Bacteria are extremely abundant, decomposing huge amounts of organic matter brought in by rivers / tides

Diatoms and bacteria, including photosynthetic bacteria, account for most of the primary production on mudflats.

Primary Producers on Mudflats

By far, the most important predators in the mudflat community are fishes and birds.

Fishes invade mudflats at high tideBirds congregate at low tide to feed

Estuaries are important stopover and wintering areas for many species of migratory birds. The often open spaces offer them safety from natural enemies, and food is plentiful.

The most significant predator on mudflats are the wading shorebirds, including the willet, godwits, dowitcheers and plovers and sandpipers.

Primary Predators of the Mudflats

Marine birds do not exploit the same type of prey, varying lengths of their bills represent the specialization due to the different types of

prey living within different depths of the mud

Estuaries in temperate and subarctic regions are usually bordered by extensive grassy areas that extend from the mudflats. These are partially flooded at high tide and are known as salt, or tidal, marshes.

Though mostly associated with estuaries, salt marshes can also develop along sheltered open coasts. They develop as long as disturbance from wave action is minimal to allow the accumulation of muddy sediments.

Tidal creeks, freshwater streams, and shallow pools frequently cut through the marsh.

In North America, salt marshes are extensive along the Atlantic and Gulf coasts.

Salt Marshes

Salt marshes are prevalent along coastal estuaries in temperate regions and are often referred to as “wetlands”

Many marine and land animals live in salt marshes:Plant life is the dominant life form in salt marshes, with

hardy grasses and salt-tolerant land plants being most abundant (cordgrasses)

Small crustaceans – Fiddler crabs – burrow along the mudflat edges, increasing the oxygenation of soils

Coffee bean snails and marsh periwinkles are air breathing snails that feed on algae and fungi growing on marsh grasses and plants

Ribbed, or horse, mussel lives half buried in the mud along the cordgrass

Killifishes and juvenile silversides are small fish living in tidal creeks and pools within the salt marshes to escape predators

Marine birds (Rails and American coots) feed and nest within salt marshes

Dominant Life in Salt Marshes

Mangroves are flowering land plants adapted to live in the intertidal. These trees and shrubs often form dense forests called mangals to distinguish them from mangroves, the actual plants themselves.

Mangroves are typically of tropical and subtropical regions, replacing temperate salt marshes.

Mangroves are rapidly being destroyed by humans to make marinas, harbors and seaports.

Mangrove Forests

The Indo-West Pacific Mangrove Forests are the most extensive in the world, serving as the natural habitat for the largest number of

mangrove species

Crabs, mudskippers, mangrove sponges, and mudshrimp call the Mangrove Forest home

The muddy bottom below tide level are sometimes covered by beds, or meadows, of grass-like flowering plants known as seagrasses. These include eel grass in temperate waters and turtle grass in tropical waters surrounding mangrove forests.

The roots of these grasses stabilize sediments and leaves provide shelter to many organisms.

Oysters form extensive beds (oyster reefs) on the muddy bottoms of estuaries in temperate waters, providing a complex surface for many organisms.

Other Communities

OYSTER REEF

Oyster reef

communities

can include

seaweeds,

sponges,

tubeworms,

barnacles, and

other organisms

that are able to

attach to the

oysters hard

shells

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Human Impact on Estuarine Communities