1. Estuaries: Where Rivers Meet the Sea

2. What is an Estuary
An Estuary is where freshwater from rivers merges with the ocean and is bordered by extensive wetlands (mudflats or saltmarshes)

3. Characteristics of Estuaries
Salinity varies spatially and over the course of a day as the tides move in and out
Nutrients from the rivers enrich estuarine waters making them one of the most biologically productive environments on earth
Salt marsh grasses, algae, and phytoplankton are the major producers
Many organisms are found in the muddy bottom
Estuaries rank among the environments most affected by humans

5. Functions and Values of Estuaries
Provide open space and recreation
Provide natural flood control
Purify the water
Produce oxygen
Are outdoor laboratories for scientists and educators
Provide sediment traps and erosion control
Serve as nurseries for marine fishes
Provide homes for endangered species
Provide habitat for migrating, wintering and breeding birds
Serve as repositories for native plants and animals

6. Types of Estuaries – Four basic groups based on their origins
Drowned River Valley or Coastal Plain – Fig – formed as a result of the sea invading lowlands and river mouths. This is the most common type of estuary.

7. Types of Estuaries
Bar-built estuary – Fig – formed when a barrier island or sand bar separates a section of the coast where freshwater enters. (Pictured below is an aerial view of Cape Hatteras, North Carolina.)

8. Types of Estuaries
Tectonic Estuary – formed as a result from sinking land due to movements of the earth’s crust. San Francisco Bay formed in this way.

9. Types of Estuaries
Fjords – Fig – formed in a deep valley created by a retreating glacier. (Kenai Fjord National Park, Alaska)

10. Physical Characteristics of Estuaries – unique because of mixing of salt and freshwater
Salinity fluctuates as the tides move seawater in and out of an estuary.
Organisms are subjected to these changes every day and therefore have adapted to these conditions.
Other factors that influence salinity include; the shape of the estuary, the bottom, wind and evaporation, and freshwater runoff

11. Physical Characteristic - Salinity
Salt Wedge – a layer of denser, saltier seawater that flows along the bottom
Salt Wedge

12. Physical Characteristic - Substrate
Sand and other coarse material settle out first near the river mouth
Finer particles settle out into “main” body of water, therefore most estuaries are soft mud (silt & clay) which is very dense and rich in organic material
Decomposition by bacteria depletes oxygen in the substrate and produces hydrogen sulfide (rotten egg smell)
Tidal flow keeps water in estuaries oxygen rich

13. Close up of Estuary Sand

14. Physical Characteristics
Temperature – fluctuates widely because of tides, organisms that are exposed at low tides also face seasonal temperature changes
Turbidity – estuaries usually have large amounts of suspended sediments which can reduce water clarity

15. Coping with Salinity
Maintaining the proper salt and water balance is essential for all organisms.
Organisms that live in estuaries must cope with rapid changes in salinity.
Most estuarine organisms are marine species that have evolved to tolerate low salinities.

16. Euryhaline species can tolerate a wide range of salinities.

17. Stenohaline species can only tolerate a narrow range of salinities (yellow fin tuna)

18. Osmoconformers (molluscs and worms) – osmotic balance is maintained by changing with the salinity of the water

19. Osmoregulators keep internal salt balances constant despite salinity levels of the water in which they live

20. Osmoconformer vs Osmoregulator

21. Osmoconformer vs Osmoregulator

22. Coping with Salinity
Marine fish – have a lower salt concentration than seawater, therefore tend to loose water; adaptations to maintain homeostasis include (Fig. 4.14)
Drinks seawater
Excretes excess salt through gills
Excretes small volumes of concentrated salty urine

23. Coping with Salinity Adaptations of other marine organisms
Sea turtles/sea gulls/sea lions have glands near the eyes that excrete “salty” tears (Fig. 4-15)
Mangroves/spartina grass – excrete excess salt through leaves
Salt crystals on spartina grass

24. Coping with Salinity – Pickleweed accumulates large amounts of water to dilute the salts they take up.

25. Adapting to the Mud
Most animals burrow or live in permanent tubes beneath the sediment
Inhabitants tend to be stationary or slow moving
Salinity fluctuations are less drastic than in the water column
Organisms overcome the depletion of oxygen by:
Pumping oxygen rich water into their burrows
Some have blood that contains hemoglobin, an oxygen carrying molecule

26. Adapting to the Mud

27. Estuarine Communities
Different types of communities are associated with various estuaries
Open Water community includes those organisms that come and go with the tide (plankton, fishes, jellies).
Other types of communities are permanent parts of the ecosystem, these include:
Mudflats
Salt Marshes
Mangrove Forests
Sea-grasses
Oyster Reefs

28. Open Water Community
Organisms in this type of community include phytoplankton, zooplankton, and marine fish that are flushed in and out by the tides.
Many types of marine fish and shrimp use estuaries as nurseries for their young.
Some fish migrate through estuaries.
Anadromous – marine fish that live their lives in the sea but return to freshwater to breed (salmon)
Catadromous – freshwater fish that migrate to the sea to breed (freshwater eels)

29. Open Water Community Zooplankton

30. Open Water Community Phytoplankton

31. Open Water Community Anadromous

32. Open Water Community Catadromous

33. Mudflat Communities
Low tides expose organisms to desiccation, fluctuating temperatures and salinity, and predation.
Primary produces consist of diatoms, bacteria, and some algae
Consumers are typically burrowing deposit and suspension feeders that feed on detritus

34. Mudflat Communities

35. Mudflat Communities

36. Mudflat Communities

37. Mudflat Communities

38. Mudflat Communities

39. Mudflat Communities

40. Mudflat Communities

41. Mudflat Communities

42. Salt Marsh Communities
Salt marshes are grassy areas that extend along the shores of estuaries and sheltered coasts in temperate and subpolar regions.
Salt marshes are subject to the same fluctuations in salinity, temperature, and tidal changes that affect mudflats.
Pronounced zonation of vegetation in salt marsh communities is a result of fluctuations in salinity, height relative to the tide, and rate of evaporation
Cordgrass (Spartinia alterniflora) is the most common plant found where the marsh meets the mudflat. Helps to stabilize the bottom.

43. Salt Marsh Communities
Zonation depends on several factors: two important ones are salinity and nutrient availability.
Salinity changes depending on these conditions:
a. frequency of tidal inundation b. rainfall c. tidal creeks and drainage d. soil texture e. vegetation f. depth of water table g. freshwater inflow
Nutrient availability varies considerably, especially the supply of usable nitrogen and phosphorous. Often there is not enough oxygen present in marsh soils to combine into useful nitrates and phosphates.

44. Salt Marsh Communities

45. Salt Marsh Communities

46. Salt Marsh Communities

47. Mangrove Forest Communities
Mangrove forests are formed by mangroves, tropical and subtropical trees and shrubs adapted to inundation by seawater.
Grow on protected coasts where muddy sediments accumulate.
The red mangrove is found along the coast of Florida. Can be identified by its prop roots.

48. Mangrove Forest Communities

49. Mangrove Forest Communities

50. Mangrove Forest Communities

51. Mangrove Forest Communities
Coral growing on a
mangrove root.

52. Mangrove Forest Communities
Snapper living among the mangrove roots.

53. Mangrove Forest Communities
Cattle egret and nest in mangrove.

54. Sea Grass Communities
Roots help to stabilize the sediment and provide shelter to organisms
Florida lobster
(yummy)

55. Oyster Reef Communities

56. Oyster Reef Communities
Friends of Hunting Island help resort oyster reefs.