1. Estuaries: Where Rivers Meet the Sea
Chapter 14
Estuaries: Where Rivers Meet the Sea
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2. Estuaries
Estuary = semi-enclosed area where fresh water and seawater meet and mix.
Lower species diversity (no. of species) but typically higher biomass (total mass of living organisms) than rocky shores
Rank among the environments most affected by humans

3. Types of Estuaries Drowned river valleys:
Also called coastal plain estuaries
Most common type
Formed by the drowning of low land around the mouths of rivers when sea level rose after the last ice age
Examples: Chesapeake Bay and bayous

4. Types of Estuaries Bar-built estuaries:
Built by the accumulation of sediments into sand bars or barrier islands
Example: coast by Cape Hatteras barrier islands; common along the Florida and Texas Gulf coasts

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

6. Types of Estuaries Fjords:
Deep valleys cut on the coast as a result of retreating glaciers
Example: coasts of Alaska, Norway, New Zealand, Chile

7. Development of Estuaries
Estuaries are best developed in areas where the coastal plain is flat and the continental shelf is wide.
Areas with steep, narrow continental shelves and coastal plains restrict the formation of estuaries.

8. Physical Characteristics of Estuaries
Salinity:
5-30‰ and more; it varies according to distance from seawater (tides) and fresh water input (rivers).
Depth contributes to salinity profile.
Salinity is not uniform (saltwater is heavier and sinks below fresh water) as water is brought in by tides: salt wedge.

9. Physical Characteristics of Estuaries
Salinity:
Mostly mud or sand as a result of low wave action/turbulence
Mud is difficult for animals to move through; mud and sand also shift, unlike hard substrates, which is also a challenge.
Particle sizes are small enough that most areas are actually anoxic (devoid of oxygen).

10. Physical Characteristics of Estuaries
Water Temperature:
Wide variation, especially at low tide: can be stressful in summer

11. Physical Characteristics of Estuaries
Water Transparency:
Very poor: suspended sediments and particles from rivers in the water
Reduced clarity makes it more difficult for photosynthesis in autotrophs in the water
Most primary production is the result of flowering plants in the salt marsh.

12. Living in an Estuary: Adapting to Salinity Changes
Organisms in the estuaries are normally euryhaline (tolerate a wide range of salinities).
Some are osmoregulators and have mechanisms for keeping their internal concentration stable.
Others are osmoconformers and have internal concentrations that vary with that of their surroundings.

15. Adapting to Salinity Changes
Flowering plants in estuaries:
expel excess solutes - salt glands in cordgrass (Spartina) and mangroves, or
concentrate solutes in tissues as in pickleweed (Salicornia).

16. Estuarine Communities
Open water
Dominant organisms include plankton and fishes.
Many marine species spend at least a portion of their lives in an estuary, mostly as larvae.

17. Estuarine Communities
Salt Marshes
Cordgrass (Spartina species) dominates along water edge in most salt marshes
High primary production
Nursery for young of many species
Contribute detritus to the estuary

18. Representative Inhabitants of Estuarine Mudflats

19. Mangrove Forests (Mangals)
Mangrove trees
Found in tropical and subtropical coasts
Typically replace salt marshes in these areas
High primary production
Species-rich community (marine as well as terrestrial species)
Protect coasts against wave action from storms

20. Geographical Distribution of Salt Marshes and Mangrove Forests

21. Mangrove Forests

22. Mangrove Forests
Some mangrove species possess pneumatophores, vertical root extensions that help in the exchange of gases (sediments often anoxic).

23. Other Estuarine Communities
Seagrass beds
Can be found in some estuaries if water clarity permits
High primary production
Help stabilize substrate

24. Other Estuarine Communities
Oyster reefs (shellfish reefs)
Are found in many estuaries
Beds of oysters on which many other species grow and thrive
May provide the only hard substrates in many estuaries

25. Generalized Food Webs in Estuarine Ecosystems