Estuaries: Where Rivers Meet the Sea

2. Estuaries:
Origins and Types of Estuaries
Physical Characteristics of Estuaries (Salinity, Substrate,
Other Physical Factors)
Estuaries as Ecosystem (Types of Communities, Feeding
Interactions)
Human Impact on Estuarine Communities

3. http://www.earthgauge.net/wp-content/images/Estuaries_Low.jpg
Estuaries are partially closed bodies of water
where freshwater rivers and streams meet
and mix with the salt water of the ocean.
Over 60% of coastal rivers and bays in the
continental U.S. are degraded by nutrient
pollution from things like fertilizers and pet
waste
75% of commercial aquatic species – like
salmon, oysters and horseshoe crabs – depend
on estuaries for their primary habitat,
spawning grounds and nursery areas.
Estuaries come in all shapes and sizes. They
can be called bays, lagoons and inlets.

4. • Drowned River Valleys /
Coastal Plain Estuaries
Melting of ice rose sea
level = sea invaded
lowlands and river mouths
Most common
Ex. Chesapeake Bay,
mouth of Delaware River,
St. Lawrence River, mouth
of River Thames
←Delaware River
Chesapeake Bay
← Cape Hatteras

5. • Bar-built Estuaries
Accumulation of sediments
along the coast =sand bars
& barrier islands
acts as walls between
fresh water and ocean.
Ex. Cape Hatteras , Texas Coast
of Gulf Mexico
←Delaware River
Chesapeake Bay
← Cape Hatteras

6. • Tectonic Estuaries
Land sank or subsided,
Result of movements of
crust

7. • Fjords
Retreating glaciers cut
deep valleys along the
coast.
Submerged when sea
level rose and rivers flow
Common in Southeastern
Alaska, British Columbia,
Norway, New Zealand

8. • SALINITY
Seawater 35% salinity + Freshwater 0% salinity
↑ Salinity = ↑ Depth 35% 25% 15% 5% 0%

9. • SALINITY
Diurnal tide –
organisms are
subjected to 2
shifts in
salinity
Semidiurnal
Tide- 4 shifts
in salinity
35% 25% 15% 5% 0%
25% 15% 5% 0%
35%

10. • SALINITY
Distribution of Salinity is influenced by:
the shape of estuary & its bottom, wind, evaporation of
water, and tide
Seasonal variations in freshwater runoff from rivers by
rainfall patterns or snowmelt
Little freshwater runoff + high evaporation = high salinity
estuaries
May reach 50% -100% during dry spells = negative estuaries

11. • SALINITY
Current – tide rushes in creating strong tidal currents
In few places tide comes in a nearly vertical wall of water
tidal bore - as high as 6m in Qiantang River, China
Greatly affects salinity

12. • SALINITY
Coriolis Effect
Northern hemisphere– fresh water toward the sea is
deflected to the right
Southern hemisphere– the flow is to the left

13. • SUBSTRATE
Large amount of sediments & other materials are from
river

14. • SUBSTRATE
Sand & Coarse
material settle out in
upper reaches
Fine, muddy particles
are carried down the
estuary in quiet
waters
Substrate of most
estuaries is sand or
soft mud

15. • SUBSTRATE
Mud
Combination of silt and clay , rich in
organic material
Decay bacteria uses up oxygen in
interstitial water
Water cannot easily flow through
fine sediments to replenish oxygen
As result, sediments in estuaries are
devoid of oxygen = anoxic

16. • SUBSTRATE
Mud
Black color and rotten-egg smell
Accumulation of Hydrogen sulfide
H2S → toxic to most organisms
Anaerobic bacteria thrive under
these conditions

17. • SUBSTRATE
There is plenty of oxygen
in unimpeded tidal flow
Sill restricts water
circulation
Stagnant deep water may
become depleted in
oxygen ↔ decomposition
of organic matter

18. • OTHER PHYSICAL FACTORS
Water temperature is affected by depths and
surface area
Organisms exposed at low tide face drastic
temperature fluctuations
Suspended sediments reduces water clarity
Can clog surfaces of filter feeders → death

19. Estuaries are tremendously productive and are home to
large numbers of organisms
Provide breeding & feeding grounds for birds, fishes,
shrimps, etc.
Estuarine ecosystems consist of several distinct
communities

20. Need to adapt to extremes in salinity, temperature, and
other physical factors.
No other marine environments changes so rapidly in many
ways as an estuary.
Few species have adapted to estuarine conditions
• LIVING IN AN ESTUARY

21. Coping with Salinity
Fluctuations
Euryhaline species
tolerate wide range of
salinities
Stenohaline species are
limited to upper/lower
ends of estuary
Rarely penetrate
estuary proper
Can be marine or
freshwater origin
• LIVING IN AN ESTUARY

22. Coping with Salinity
Fluctuations
Brackishwater – water
of intermediate salinity
Brakishwater,
stenohaline, and
freshwater marine
species.
• LIVING IN AN ESTUARY

23. Coping with Salinity
Fluctuations
Some undergo osmosis
Some animals adapt by
hiding in mud burrows,
close their shells, or swim
away
Most organism rely on
other mechanism
• LIVING IN AN ESTUARY

24. Coping with Salinity
Fluctuations
Osmoconformers –
maintain osmotic balance
by changing salinity of body
fluids.
Soft-bodied estuarine
animals, mollusks,
polychaetes worms.
• LIVING IN AN ESTUARY

25. Coping with Salinity
Fluctuations
Osmoregulators – keep the
salinity of body fluids
constant.
Via Active transport
Accomplished by gills,
kidneys & other structures
Bony fishes osmoregulate
- lower salinity of body
fluid
Salmon & freshwater eel -
active transport in gills
and kidney
• LIVING IN AN ESTUARY

26. Coping with Salinity
Fluctuations
Perfect Osmoconformers or
Perfect Osmoregulators –
Invertebrates
Osmoregulate = low
salinities
Osmoconform =high
salinities
• LIVING IN AN ESTUARY

27. Coping with Salinity Fluctuations
Estuarine Plants
Grasses & some Salt-marsh plants have high
salt tolerance
Absorb salts & concentrate sugars to prevent
water from leaving tissues
Opposite to marine organisms in estuaries
• LIVING IN AN ESTUARY

28. Coping with Salinity Fluctuations
Cordgrasses, other Salt-marsh plants,
and Mangroves
Excrete excess salts by salt glands in
their leaves
• LIVING IN AN ESTUARY
Cordgrass (Spartina)
Black Mangrove leaf

29. Coping with Salinity Fluctuations
Pickleweed (Salicornia) – succulents
Absorb water to dilute salts
• LIVING IN AN ESTUARY

30. • LIVING IN AN ESTUARY
Adapting to the Mud
Problem:
Nothing to hold on to → most animals
burrow or live in permanent tubes
Clams extend their siphons for food
and oxygen
Difficult to move on mud → inhabitants
tend to be stationary/ slow-moving
Depletion of oxygen → burrowers pump
oxygen-rich water into their burrows
→ Some have blood with hemoglobin

31. • LIVING IN AN ESTUARY
Adapting to the Mud
Benefit:
Salinity fluctuation is less drastic