1. Estuaries: Where Rivers Meet the Sea
Chapter 12

2. Origins and Types of Estuaries
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.
Types of Estuaries:
Drowned River Valleys or Coastal Plain Estuaries
Bar-Built Estuary
Tectonic Estuaries
Fjords
A coastal estuary off of North Carolina

3. Drowned River or Coastal Plain Estuaries
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.

4. Bar-Built 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.

5. Tectonic 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.

6. Fjords
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.

7. Physical Characteristics of Estuaries
Influenced by the tides and the mixing of fresh and salt water, estuaries have a unique combination of physical and chemical characteristics, such as:
Salinity
Substrate
Water temperature
Water clarity / light penetration
These characteristics govern the lives of the organisms that live there.

8. Salinity
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.

10. Substrate
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.

11. Other Physical Factors
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.

12. Estuaries as an Ecosystem
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.

13. Living in an Estuary
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.

14. Coping with Salinity Fluctuations
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 salinity Stenohaline species: tolerate a narrow range of salinities

15. Adapting to the Mud
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.

16. Estuarine Communities
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:
Open Water
Mudflats
Primary Producers
Salt Marshes
Mangrove Forests
Other Communities

17. Open Water
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).

18. 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.

19. Mudflats
Mudflat communities in estuaries are similar to those on muddy shores.
Low tides expose organisms to desiccation
Wide variations in temperatures and predation
In estuaries however, mudflat organisms must also withstand regular variations in salinity.

20. Primary Producers on 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.

21. Primary Predators of the Mudflats
By far, the most important predators in the mudflat community are fishes and birds.
Fishes invade mudflats at high tide
Birds 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.

22. 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

23. Salt Marshes
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.

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

25. Dominant Life in Salt Marshes
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

27. Mangrove Forests
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.

28. 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

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

30. Other Communities
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.

31. OYSTER REEF
Oyster reef communities can include seaweeds, sponges, tubeworms, barnacles, and other organisms that are able to attach to the oysters hard shells

32. Human Impact on Estuarine Communities
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