oceanservice.noaa.gov/education 1

ESTUARY SUBJECT REVIEW

Fill-in-the-Blank

1. An __________ is a partially enclosed body of water and its 

adjacent habitats where saltwater from the ocean mixes 

with freshwater from rivers or streams. 

2. Waters whose salt content is between that of freshwater and 

that of seawater are called __________.

3. Of the 32 largest cities in the world, 22 are located on 

__________.

WORD BANK stenohaline

euryhaline

energy

pneumatophores

propagules

anaerobic

zoea

megalope

anthropogenic

storms

winter ice

anoxic

glaciers

high tides

bar built

barrier

delta

tectonic

smaller

unique

geology

estuary

brackish

estuaries

salinity

variable

tides

two 

freshwater 

nurseries

pollutants

habitat

fish

ecosystem services

ice age

decreases 

lagoons

sediment 

earthquakes

fjords

tidal creeks

pannes

mudflats

nutrients

watershed

SWMP

polluted

buffers

coastal elevation 

coastal plain 

halophytes

smooth cordgrass

mangrove

prop roots

salinity

dense

tidal range

isohalines

highest 

mixing

salt wedge

slightly stratified

vertically-mixed 

storms

peat

red mangrove

flooded

salinity

wrack

conversion

pollution

biomagnification

sediments

eutrophication

nutrient

pathogens

combined sewage 

 overflows 

invasives

NERRS

increases

dissolved oxygen

photosynthesis

organic matter

turbidity

pH

chlorophyll 
Subject Review Estuaries

2 oceanservice.noaa.gov/education

4. In the ocean the concentration of salts, or __________, averages about 35 parts per thousand. 

5. The salinity of estuarine water is __________ from one day to 

the next.

6. __________ are a major influence on many estuaries.

7. Most areas of the earth experience __________ high and low 

tides each day. 

8. Tides at broad mudflats might appear to be __________ than 

tides at the end of a long, narrow inlet.

9. Every estuary is __________.

10. Estuaries are typically classified based on how saltwater 

and freshwater mix in the estuary and on their __________.

11. Estuaries formed by rivers or streams entering massive 

lakes are called __________ estuaries. 

12. Estuaries are often called __________ of the sea. 

13. Estuaries filter out __________ from rivers and streams 

before they flow into the oceans.

14. Estuaries provide critical __________ for many birds, fish, 

amphibians, insects, and other wildlife.

15. Estuaries provide habitat for more than 75 percent of the 

__________ harvested in the United States. 

16. __________ are fundamental life support processes upon 

which all organisms depend. [two words]

17. __________ is a spongy matrix of live roots, decomposing 

organic material, and soil that helps filter pollutants out of 

the water.
oceanservice.noaa.gov/education 3

Estuaries Subject Review

18. One reason that estuaries are so productive is that the 

water filtering through them brings in __________ from the 

surrounding watershed. 

19. The entire land area that drains into a particular body of 

water, like a lake, river or estuary is called a drainage basin 

or __________. 

20. Estuaries are some of the most fertile ecosystems on earth, 

yet they may also be one of the most __________.

21. Estuaries and their surrounding wetlands act as _______ 

that stabilize shorelines and protect coastal areas from 

floods, storm surges, and excessive erosion.

22. The __________ determines the rate of freshwater that flows 

into an estuary from rivers and streams. [two words]

23. __________ estuaries are formed when rising sea levels 

flood existing river valleys. [two words]

24. __________ estuaries are characterized by barrier beaches or 

islands that form parallel to the coastline and separate the 

estuary from the ocean. [two words]

25. __________ beaches and islands are formed by the accumulation of sand or sediments deposited by ocean waves. 

26. A __________ is characterized by large flat fan-shaped 

deposits of sediment at the mouth of a river.

27. __________ estuaries are formed when the earth’s tectonic 

plates run into or fold-up underneath each other.

28. __________ are steep-walled river valleys created by 

advancing glaciers that later became flooded with seawater 

as the glaciers retreated.

29. Most coastal plain estuaries in North America were formed 

at the end of the last__________. [two words]
Subject Review Estuaries

4 oceanservice.noaa.gov/education

30. A protected area of calm water between the coast and a 

barrier island is called a __________. 

31. Bar-built estuaries and deltas both have large deposits of 

__________.

32. The first stage in the formation of a tectonic estuary typically occurs during __________. 

33. __________ tend to have a moderately high input of freshwater, but very little inflow of seawater.

34. When a sill prevents deep waters in an estuary from mixing with deep waters of the sea, poor water exchange 

causes __________ (low oxygen) water to build up on the 

bottom of the estuary.

35. Estuaries with a sill are found in areas that were once covered with __________. 

36. __________ create saltwater currents that move seawater 

into estuaries. [two words]

37. The daily mixing of freshwater and saltwater in estuaries 

leads to variable and dynamic chemical conditions, especially __________. 

38. Because freshwater flowing into the estuary is less 

__________ than water from the ocean, it often floats on top 

of the heavier seawater. 

39. The difference between the average low tide and the average high tide is the __________. [two words]

40. __________ are areas in water that have equal salt concentrations.

41. The shape of the isohalines indicates the amount of 

__________ that is occurring, and may provide clues about 

the estuary’s geology.
oceanservice.noaa.gov/education 5

Estuaries Subject Review

42. __________ estuaries occur when a rapidly-flowing river 

discharges into the ocean where tidal currents are weak. 

[two words]

43. In __________ estuaries, saltwater and freshwater mix at all 

depths. [two words]

44. A __________ estuary occurs when river flow is low and tidally generated currents are moderate to strong. [two words]

45. Freshwater estuaries are driven by __________.

46. Salt marshes are a mosaic of snaking channels called 

__________. 

47. Salt __________ are shallow depressions that contain very 

high concentrations of salt. 

48. Burrowing organisms such as clams, mussels, oysters, fiddler crabs, sand shrimp, and blood worms are typical of 

__________.

49. Salt marshes are covered with salt-tolerant plants called 

__________.

50. __________ is one of the main components of peat and 

dominates the low marsh all the way up to the estuary’s 

edge. [two words]

51. __________ trees grow at tropical and subtropical latitudes, 

and can grow in anoxic soils where slow moving waters 

allow fine sediments to accumulate. 

52. Many mangals can be recognized by their dense tangle of 

__________ that make the trees appear to be standing on 

stilts above the water. [two words]

53. The __________ colonizes the seaward side of mangals. 

[two words]

54. The zones where white mangrove and buttonwood trees 

grow are almost never __________.
6 oceanservice.noaa.gov/education

55. Plants and animals living in estuaries must be able to 

respond quickly to drastic changes in __________.

56. Plants and animals that can tolerate only slight changes in 

salinity are called __________.

57. Plants and animals that can tolerate a wide range of salinities are called __________. 

58. It requires a lot of __________ to adapt to constantly changing salinities. 

59. Many species of mangrove trees have aerial roots called 

__________ that take up oxygen from the air for the roots. 

60. Mangrove seeds that begin growing while still attached to 

the parent plant are called __________.

61. During low tides, oysters close up their shells, stop feeding, and switch to __________ respiration.

62. Blue crab larvae called __________ require water with a 

salinity over 30 ppt for optimal development.

63. As they develop, blue crabs eventually return to the estuary as young crabs called __________.

64. __________ disturbances are caused by humans. 

65. Large __________ are especially destructive to estuaries.

66. A common natural disturbance to estuaries in non-tropical 

regions is __________. [two words]

67. A natural disturbance in salt marshes caused by the burial 

of vegetation by rafts of dead floating plant material, is 

called __________. 

68. The greatest human-caused threat to estuaries is their 

large-scale __________.

Self Test | Fill-in-the-Blank Estuaries
oceanservice.noaa.gov/education 7

69. __________ is probably the most important threat to water 

quality in estuaries. 

70. __________ is a process through which toxic substances can 

accumulate in the tissues of organisms that consume these 

substances.

71. Some toxic substances become attached to __________ that 

flow down rivers and get deposited in estuaries. 

72. __________ is a type of pollution in which excess nutrients 

stimulate an explosive growth of algae which depletes 

the water of oxygen when the algae die and are eaten by 

bacteria. 

73. __________ pollution is the single largest pollution problem affecting coastal waters of the United States.

74. __________ are disease-causing organisms. 

75. In most estuaries, the largest contributor of bacteria and 

viruses is probably __________. [three words]

76. __________ are plants and animals that have found their 

way into areas outside their normal range. 

77. Non-native species are often introduced to estuaries in the 

__________ water of ships. 

78. __________ is a partnership program between NOAA and 

U.S. coastal states that protects more than one million acres 

of estuarine land and water. 

79. The health of every National Estuary Reserve is continuously monitored by the __________.

80. As water temperature increases, the amount of oxygen that 

can dissolve in the water __________.

81. In estuaries, salinity levels are generally ___________ near 

Estuaries Self Test | Fill-in-the-Blank
Subject Review Estuaries

8 oceanservice.noaa.gov/education

the mouth of a river where the ocean water enters.

82. The amount of oxygen that can dissolve in water _________

as salinity decreases.

83. The amount of __________ in an estuary’s waters is the 

major factor that determines the type and abundance of 

organisms that can live there. [two words]

84. Oxygen enters the water through two natural processes: 

diffusion from the atmosphere, and __________. 

85. Bacteria, fungi, and other decomposer organisms reduce 

DO levels in estuaries because they consume oxygen while 

breaking down __________. [two words]

86. __________ is related to the amount of sediment and other 

solids suspended in water.

87. The chemical components in seawater resist large changes 

to __________.

88. Scientists can determine the density of phytoplankton and 

the amount of primary productivity by measuring ________.

8 oceanservice.noaa.gov/education 
oceanservice.noaa.gov/education 9

ESTUARY REVIEW

Crossword Puzzle

4 5

1 2

12 13

9

16

17

18

20

24

28

25

23

31

34 35

32

29

38

3

6

7

8

10

14

11

15

19

22

26

33

39

41

44 45

27

30

47

50

40

46

51

43

49

52

57

59

66

70

48

58

54

63 64

65

67 68

55

53

56

36

42

37

60

69

61 62

72

78

80

71

76

82

73 74

77

81

75

79

83

84
Subject Review Estuaries

10 oceanservice.noaa.gov/education

Across

6. As water temperature increases, the amount of oxygen that 

can dissolve in the water __________.

7. During low tides, oysters close up their shells, stop feeding, 

and switch to __________ respiration.

8. The zone where white mangrove and buttonwood trees 

grow are almost never __________.

12. Estuaries filter out __________ from rivers and streams 

before they flow into the oceans.

14. __________ are steep-walled river valleys created by 

advancing glaciers that later became flooded with seawater 

as the glaciers retreated.

15. __________ estuaries are characterized by barrier beaches or 

islands that form parallel to the coastline and separate the 

estuary from the ocean. [two words]

16. __________ is a partnership program between NOAA and 

U.S. coastal states that protects more than one million acres 

of estuarine land and water.

17. Estuaries formed by rivers or streams entering massive 

lakes are called __________ estuaries.

18. Salt __________ are shallow depressions that contain very 

high concentrations of salt.

19. __________ are a major influence on many estuaries.

20. Oxygen enters the water through two natural processes: 

diffusion from the atmosphere, and __________.

22. Of the 32 largest cities in the world, 22 are located on 

__________.

25. __________ trees grow at tropical and subtropical latitudes, 

and can grow in anoxic soils where slow moving waters 

allow fine sediments to accumulate.

26. Burrowing organisms such as clams, mussels, oysters, fiddler crabs, sand shrimp, and blood worms are typical of 

__________.

28. __________ is dead floating plant material, called that may 

bury vegetation in salt marshes.

29. The chemical components in seawater resist large changes 

to __________.

32. The health of every National Estuary Reserve is continuously monitored by the __________.

33. The __________ colonizes the seaward side of mangals. 

[two words]
oceanservice.noaa.gov/education 11

Estuaries Subject Review

34. In the ocean the concentration of salts, or __________, averages about 35 parts per thousand

38. Estuaries provide critical __________ for many birds, fish, 

amphibians, insects, and other wildlife.

39. Non-native species are often introduced to estuaries in the 

__________ of ships.

40. Plants and animals that can tolerate only slight changes in 

salinity are called __________.

43. Estuaries are often called __________ of the sea.

44. The entire land area that drains into a particular body of 

water, like a lake, river or estuary is called a drainage basin 

or __________.

46. Every estuary is __________.

47. Salt marshes are a mosaic of snaking channels called 

__________.

48. It requires a lot of __________ to adapt to constantly changing salinities.

50. In __________ estuaries, saltwater and freshwater mix at all 

depths. [two words]

51. As they develop, blue crabs eventually return to the estuary as young crabs called __________.

52. Bacteria, fungi, and other decomposer organisms reduce 

DO levels in estuaries because they consume oxygen while 

breaking down __________. [two words]

55. __________ is one of the main components of peat and 

dominates the low marsh all the way up to the estuary’s 

edge. [two words]

57. Scientists can determine the density of phytoplankton 

and the amount of primary productivity by measuring 

__________.

60. Estuaries are some of the most fertile ecosystems on earth, 

yet they may also be one of the most __________.

63. Blue crab larvae called __________ require water with a 

salinity over 30 ppt for optimal development.

64. An __________ is a partially enclosed body of water and 

its adjacent habitats where saltwater from the ocean mixes 

with freshwater from rivers or streams.

65. Because freshwater flowing into the estuary is less 

__________ than water from the ocean, it often floats on top 

of the heavier seawater.
Subject Review Estuaries

12 oceanservice.noaa.gov/education

66. One reason that estuaries are so productive is that the 

water filtering through them brings in __________ from the 

surrounding watershed.

67. Tides at broad mudflats might appear to be __________ 

than tides at the end of a long, narrow inlet.

70. __________ estuaries are formed when rising sea levels 

flood existing river valleys. [two words]

72. __________ is probably the most important threat to water 

quality in estuaries.

74. When a sill prevents deep waters in an estuary from mixing with deep waters of the sea, poor water exchange 

causes __________ (low oxygen) water to build up on the 

bottom of the estuary.

76. Salt marshes are covered with salt-tolerant plants called 

__________.

78. __________ is a type of pollution in which excess nutrients 

stimulate an explosive growth of algae which depletes the 

water of oxygen when the algae die and are eaten by bacteria.

79. __________ estuaries occur when a rapidly-flowing river 

discharges into the ocean where tidal currents are weak. 

[two words]

80. The first stage in the formation of a tectonic estuary typically occurs during __________.

81. Estuaries are typically classified based on how saltwater 

and freshwater mix in the estuary and on their __________.

82. A common natural disturbance to estuaries in non-tropical 

regions is __________. [two words]

83. Waters whose salt content is between that of freshwater 

and that of seawater are called __________.

84. Estuaries provide habitat for more than 75 percent of the 

__________ harvested in the United States.

Down

1. __________ are plants and animals that have found their way 

into areas outside their normal range.

2. __________ are areas in water that have equal salt concentrations.

3. Many mangals can be recognized by their dense tangle of 

__________ that make the trees appear to be standing on 

stilts above the water. [two words]
oceanservice.noaa.gov/education 13

Estuaries Subject Review

4. Mangrove seeds that begin growing while still attached to 

the parent plant are called __________.

5. Bar-built estuaries and deltas both have large deposits of 

__________.

9. The salinity of estuarine water is __________ from one day to 

the next.

10. In most estuaries, the largest contributor of bacteria and 

viruses is probably __________. [three words]

11. Plants and animals that can tolerate a wide range of salinities are called __________.

13. __________ pollution is the single largest pollution problem effecting coastal waters of the United States.

15. Estuaries and their surrounding wetlands act as __________ 

that stabilize shorelines and protect coastal areas from 

floods, storm surges, and excessive erosion.

23. The __________ determines the quantity and rate of freshwater that flows into an estuary from rivers and streams. 

[two words]

24. Most coastal plain estuaries in North America were formed 

at the end of the last __________. [two words]

27. __________ is a process through which toxic substances can 

accumulate in the tissues of organisms that consume these 

substances.

29. __________ is a spongy matrix of live roots, decomposing 

organic material, and soil that helps filter pollutants out of 

the water.

30. __________ are disease-causing organisms.

31. The shape of the isohalines indicates the amount of 

__________ that is occurring, and may provide clues about 

the estuary’s geology.

35. The amount of oxygen that can dissolve in water 

__________ as salinity decreases.

36. __________ is related to the amount of sediment and other 

solids suspended in water.

37. __________ create saltwater currents that move seawater 

into estuaries. [two words]

41. Most areas of the earth experience __________ high and 

low tides each day.

42. A __________ estuary occurs when river flow is low and 

tidally generated currents are moderate to strong.

45. Freshwater estuaries are driven by __________.
Subject Review Estuaries

14 oceanservice.noaa.gov/education

49. __________ substances are chemicals and metals that can 

causing serious illness or death. They may be poisonous, 

carcinogenic (cancer-causing) or harmful in other ways to 

living things. Pesticides, automobile fluids like antifreeze, 

oil or grease, and metals such as mercury or lead have 

all been found to pollute estuaries. These substances can 

enter an estuary through industrial discharges, yard runoff, 

streets, agricultural lands, and storm drains.

53. __________ beaches and islands are formed by the accumulation of sand or sediments deposited by ocean waves.

54. Estuaries with a sill are found in areas that were once covered with __________.

56. Some toxic substances become attached to __________ that 

flow down rivers and get deposited in estuaries.

58. Many species of mangrove trees have aerial roots called 

__________ that take up oxygen from the air for the roots.

59. __________ estuaries are formed when the earth’s tectonic 

plates run into or fold-up underneath each other

61. The difference between the average low tide and the average high tide is the __________. [two words]

62. The amount of __________ in an estuary’s waters is the 

major factor that determines the type and abundance of 

organisms that can live there. [two words]

68. A protected area of calm water between the coast and a 

barrier island is called a __________.

69. __________ are fundamental life support processes upon 

which all organisms depend. [two words]

71. __________ disturbances are caused by humans.

73. A __________ is characterized by large flat fan-shaped 

deposits of sediment at the mouth of a river.

75. The greatest human-caused threat to estuaries is their 

large-scale __________.

77. In estuaries, salinity levels are generally ___________ near 

the mouth of a river where the ocean water enters.