1. Chapter 12
Estuaries

2. Types of Estuaries Drowned river valleys Most common type of estuary
They were formed by the “drowning” of low land around the mouths of rivers when sea level rose after the last ice age
Ex: Chesapeake Bay

4. Types of Estuaries Bar built estuary-
Built by the accumulation of sediments into sand bars or barrier islands
Ex: North Carolina barrier islands (seen in lower part of the photo to the right near Cape Hatteras)

5. Types of Estuaries Tectonic estuaries –
Formed as a result of land sinking due to movements of the Earth’s crust
Ex: San Francisco Bay

6. Types of Estuaries Fjords –
Deep channels cut in the coastal zone as a result of retreating glaciers
Ex: Alaska, Norway

7. Physical Characteristics of Estuaries
Salinity-
Can vary from 5 – 30 ppt
Salinity varies according to distance from saltwater (tides) or freshwater (river) input
Can also vary as a result of storms
Depth also contributes to salinity
The diagram to the right illustrates that the salinity is not uniform (saltwater is heavier and sinks below freshwater) – this is known as a salt wedge

8. Dealing with Salinity Changes
Organisms in the estuaries are normally ________ (can tolerate a wide variety of salinities)
Some are osmoregulators that have mechanisms for keeping their internal concentration stable
Others are osmoconformers with internal concentrations that vary with their surroundings

9. Types of species living in estuaries in relation to salinity

10. Substrate Mostly composed of sand or mud
Mud is difficult to move through
Mud or sand can also shift unlike hard substrate which is a challenge for organisms as well
Particle sizes are so small that most areas are actually anoxic (devoid of oxygen)

11. Communities within an Estuary
Open water in bays and tidal creeks
Predominant organisms include phyto- and zooplankton, floating macroalgae, fish and many larval forms (crabs, shrimp, molluscs, and fish)
At the right, this diagram shows the extreme importance of estuaries as a “nursery” area for larval species
Many marine species spend at least a portion of their life in an estuary, mainly as larvae.

12. Communities within an Estuary
Saltmarsh
The vegetated area of an estuary
Spartina species predominate near the water in most estuaries. Halophytes occur in higher elevation parts of the marsh.

13. Aransas National Wildlife Refuge
Salt marsh on Texas Coast with
endangered whooping cranes that
depend on the marsh ecosystem.
Blue crab and halophytes are their
main food sources.

14. Representative Inhabitants in a Mudflat of an Estuary
Epifauna includes
deposit feeding snails
like mudsnails and
Predators like shorebirds
and moon snails.
Infauna includes clams,
crustaceans like ghost
shrimp, and
annelids.
Meiofauna is low
due to anoxy.

15. Communities within an Estuary
Mangroves or mangals
Found in tropical and subtropical coastal areas
Normally replace saltmarshes in these areas

16. Worldwide Distribution of Saltmarshes and Mangrove Forests

17. Generalized Food Webs in Estuarine Ecosystems
Food webs for salt mashes and mangals

18. Aerial View of a Mangal

19. Communities within an Estuary
In Texas, mangrove
forest estuaries are found at Boca Chica, where the Rio Grande River enters (sometimes) the Gulf of Mexico. It is rich in tropical marine life but like many estuaries and bays is becoming more saline.

20. Communities within an Estuary
Seagrass beds can be found in some estuaries if the water clarity permits
Water clarity must be better than the average estuary to support this growth

21. Communities within an Estuary
“Oyster reefs” are found in many estuaries
These congregations of oysters are the platform on which dozens of other species grow and thrive
The oysters provide the only hard substrate in many estuaries