1. What are estuaries?
Estuaries are semi-enclosed areas where fresh water
and sea water meet and mix. This typically occurs where rivers enter
2. Describe the diversity and productivity of estuaries.
Estuaries typically are less diverse than rocky shore environments.
That is to say, there are fewer species present in estuaries compared
to rocky shore environments. Estuaries, however, are among the most
productive environments on earth—populated by salt-marsh grasses,
mangroves, sea weeds, and photosynthetic bacteria.
3. What are some of the human-caused impacts that estuaries
Estuaries are one of the environments most impacted by humans, mostly
due to the proximity or co-location of cities and communities. Loss
of habitat, due to dredging and filling, pollution, and habitat degradation
are just a few of the human-caused impacts faced by estuaries.
4. What are three synonyms for estuaries?
Lagoons, sloughs, or bays
are all terms that are equivalent to estuary.
5. What are the four types of estuaries, as defined by their
The four types of estuaries, as defined by their origins, are drowned
river valleys, bar-built estuaries, tectonic estuaries, and fjords.
6. Briefly describe drowned river valleys and give an example.
Drowned river valleys, also called coastal
plain estuaries, are the most common type of estuary. They formed
when sea level rose at the end of the last ice age about 18,000 years
ago. The sea flooded lowlands and river mouths, creating estuaries.
An example of a drowned river valley estuary is Chesapeake Bay.
7. Briefly describe bar-built estuaries and give an example.
Bar-built estuaries occur where the accumulation of
sediments along the coast builds up sand bars and barrier islands that
act as a wall between the ocean and fresh water from rivers. Bar-built
estuaries are common along the coast of the Gulf of Mexico and any coastline
with barrier islands.
8. Briefly describe tectonic estuaries and give an example.
Tectonic estuaries form when the earth sinks or subsides,
due to movements of the earth’s crust (due to plate tectonics
and/or volcanism). San Francisco Bay is an example of a tectonic estuary.
9. Briefly describe fjords and give an example.
Fjords are created by retreating (melting) glaciers,
which cut deep, narrow valleys along the coast. They are common in southeastern
Alaska, Norway, Chile, and New Zealand.
10. Where are estuaries more common: on passive or active continental
Estuaries are more common and more extensive along passive continental
11. Review: What are passive continental margins? What are active
Passive continental margins occur along ocean basins
with sea-floor spreading. Active continental margins occur near subduction
zones, where one lithospheric plate is colliding with another.
12. What are the two most important physical factors affecting
Salinity and substrate are the two most important physical factors affecting
13. Describe how salinity varies from the mouth to the head
of an estuary.
Salinity in an estuary is closest to the salinity of sea water (35‰)
near its mouth—where it is closest to the ocean.
Near its head—where it is closest to the river
the empties into it—the salinity is much lower. Values might be
10–20‰. Water that is intermediate between salt water and
fresh water salinity is brackish. Up river, outside of the tidal influence
that brings salt water upstream in the river, the salinity is 0‰
(e.g., it is fresh water).
14. How does salinity vary with depth in an estuary?
The higher the salinity of water, the greater its density. High salinity,
dense water sinks to the bottom of the estuary, whereas fresher, lower
salinity water floats on top.
15. What is a salt wedge?
A salt wedge is the dense, high salinity water that
flows along the bottom of an estuary. It moves up the estuary, towards
its head, as the tide rises towards high tide and moves down the estuary,
towards its mouth, as the tide falls and becomes low tide.
16. What are four factors that influence the distribution of
salinity in an estuary?
The shape of an estuary and its bottom, the wind, evaporation of water
from the surface, and changes in the tide all influence the distribution
of salinity in an estuary.
17. What is a tidal current?
A tidal current occurs when the tide rushes in and
out of an estuary.
18. What is a negative estuary?
In estuaries with very little or no river flow and high rates of evaporation,
the salinity at the head of the estuary may actually be higher than
the salinity at the mouth. This type of estuary is called a negative
estuary. Estuaries (called esteros) on the mainland side of
the northern Gulf of California often are negative estuaries.
19. Describe the typical substrate in estuaries?
The typical substrate or type of bottom in an estuary is sand or soft
mud. The mud often is organic rich, and organisms that live in the organic
rich muds of an estuary face the same challenges as those faced by soft-bottomed
intertidal organisms—reduced or no oxygen.
20. How do organisms in organic rich muds in estuaries deal
with reduced or no oxygen?
In anoxic muds, anaerobic bacteria thrive. In oxygen-poor muds, organisms
have adaptations that allow them to either live in low-oxygen conditions
or to pump oxygen-rich water from above into their burrows.
21. What are two other physical factors that are important in
Variability in water temperature and reduced visibility due to large
amounts of suspended sediments are two physical factors that influence
estuaries and the organisms that live in them.
22. Describe how estuarine organisms are adapted to deal with
Maintaining internal salinity balance is one of the major challenges
faced by estuarine organisms. Most estuarine organisms are euryhaline—they
can tolerate a wide range of salinities. A small number of stenohaline
species—species that can tolerate only a narrow range of salinities—inhabit
estuaries, but are usually very limited in their distribution.
23. Describe osmoconformers in estuaries.
Osmoconformers maintain osmotic balance simply by allowing
their body fluids to match the salinity of the surrounding water. Examples
include many molluscs and polychaete worms.
24. Describe how osmoregulators use active transport to maintain
Many estuarine organisms are osmoregulators—they
keep the salt concentration of their body fluids more or less constant
regardless of the water’s salinity. When the salinity of the water
is lower than that of their body fluids, they get rid of excess water
and actively transport solutes from the surrounding water.
25. Review: What is osmosis? What is active transport?
Osmosis is the movement of water from high to low concentration
across a membrane. Active transport is the transfer
of substances across membranes by a cell against a concentration gradient.
26. Describe four adaptations used by estuarine organisms to
deal with living in the mud.
Most estuarine organisms either burrow or live in permanent tubes beneath
the sediment surface. Clams and other bivalves extend their siphons
to the surface to get water for food and oxygen. Some organisms have
special hemoglobins so that they can survive in oxygen-poor sediments.
27. List the six types of estuarine communities.
The six distinctive communities associated with estuaries are open water,
mudflats, salt marshes, mangroves, sea grasses, and oyster beds.
28. Describe open water communities in estuaries.
The open water community is the plankton, fishes, and
other organisms that come in to and leave the estuary with the tides.
The plankton are mostly marine species. Estuaries are nurseries, where
many species of commercially important fishes and shellfish take advantage
of the abundant food and grow into adults. Very few fishes spend their
entire lives in estuaries.
29. Describe the physical conditions of mudflats in estuaries.
Mudflats are the intertidal, soft-bottomed areas of
estuaries that are exposed at low tide. They are especially extensive
in estuaries with a large tidal range.
30. Review: What is the tidal range?
Tidal range is the difference in water level between
successive high and low tides.
31. How are mudflat communities in estuaries similar to those
on muddy shorelines?
Similar to muddy shoreline communities, mudflat communities are exposed
to desiccation, wide variations in salinity and temperature, and predation
at low tide.
32. Describe primary producers on mudflats in estuaries.
Primary producers on mudflats in the intertidal include a few types
of sea weeds, large numbers of benthic diatoms, and photosynthetic bacteria.
33. Review: What are diatoms?
Diatoms are unicellular, eukaryotic organisms in the
Kingdom Protista that live mostly as part of the plankton. A silica
shell is their most distinctive feature.
34. Describe the bacteria on mudflats in estuaries.
Bacteria are extremely abundant on mudflats in estuaries. They primarily
are decay bacteria that decompose the abundant organic matter in the
35. Describe the infauna on mudflats in estuaries.
Infauna are the dominant animals on mudflats in estuaries. They feed
on the abundant detritus in the sediment and water. Most are either
deposit feeders or suspension feeders, although deposit feeders are
36. Describe the meiofauna on mudflats in estuaries.
Meiofauna are organisms that live in the water in the
spaces between grains of sediment. Protozoans, nematodes, and many other
organisms make up the meiofauna on mudflats in estuaries
37. What are four common animals that live on mudflats in estuaries?
Bivalves, crustaceans, worms, and snails are four common animals that
live on mudflats in estuaries.
38. What are the most important predators on the mudflats in
The most important predators on the mudflats in estuaries are fishes
39. Describe salt marshes in estuaries.
Extensive grassy areas called salt marshes that extend
inland from the mudflats usually border estuaries in temperate and subarctic
regions. They are partially flooded at high tide. Tidal creeks, freshwater
streams, and shallow pools cut through the marsh.
40. Where in North America are salt marshes common?
In North America salt marshes are common along the Atlantic and Gulf
of Mexico coasts, because they typically have gently sloping shorelines.
41. Describe the dominant organisms in salt marshes.
Grasses and other marsh plants dominate salt marshes. Bacteria in the
mud decompose dead plant material and contribute a large portion of
the detritus in the estuary.
42. Describe mangrove forests in estuaries.
Although mangrove forests are not limited to estuaries,
they are basically the tropical and subtropical equivalents of salt
marshes. Various species of mangroves have different tolerances to immersion
by high tide. The Indo-Pacific region has the world’s most extensive
mangrove forests and the largest number of mangrove species. In North
America, the Atlantic, Gulf of Mexico, Gulf of California, and Caribbean
coasts have mangrove forests.
43. Review: What are mangroves?
Mangroves are flowering land plants adapted to live
in the intertidal.
44. Describe sea grass beds.
The muddy bottoms of estuaries below low-tide level may be covered by
beds, or meadows, of grass-like flowering plants known as sea
grasses. The roots of sea grasses help to stabilize the sediment,
and the beds provide shelter for many organisms. Like mangrove forests,
sea grass beds are not limited to estuaries.
45. Describe oyster reefs.
In temperate waters, oysters may form extensive beds or reefs on the
muddy bottoms of estuaries. The oysters build up over time, growing
on the dead shells of their predecessors, until they form a reef. Many
other organisms inhabit oyster reefs.
46. Why do estuaries have high primary productivity?
There are several reasons. Estuaries are nutrient-rich, with rivers
and tides bringing in nutrients, which are added to the nutrients released
by the decomposition of detritus and nitrogen-fixing bacteria. Estuarine
producers are particularly adapted to live on muddy bottoms. Also, very
few organisms graze on primary producers like salt-marsh grasses.
47. What is outwelling?
Outwelling occurs when surplus detritus in estuarine
systems is carried or exported to the open ocean and neighboring ecosystems
by currents and tides.
48. What types of human activities are detrimental to estuaries?
Examples include dredging (to make marinas, barbors, and seaports,
to create new "land" for development, and to keep navigation
channels open), building dams on rivers (which changes the salinity
of estuaries), clearing mangrove forests to create shrimp mariculture
facilities and to provide space for urban development.
49. About what percent of estuaries in the United States have been destroyed
due to human impacts?
About 1/3rd (33%) of all estuaries in the United States have
disapeared altogether. This level of destruction is probably characteristic
of estuaries globally.
50. Review: What is the continental shelf?
The continental shelf is the submerged edge of the
continents. It is the zone between the end of the intertidal and the
shelf break, which typically is at about 400 to 600 feet in depth.
51. Why is the continental shelf considered the richest part
of the ocean?
The continental shelf is considered the richest part of the ocean because
it includes the world’s most important fishing grounds, which
yield about 90% of the global catch. Also, oil and mineral resources
abound on the continental shelf and are more easily extracted from its
relatively shallow waters than, for example, from the deep ocean bottom.
52. Review: What is the subtidal zone? What is the shelf break?
The subtidal zone is the part of the continental shelf
that is never exposed at low tide. It is also called the sublittoral
zone. The subtidal zone ends at the shelf break—the
outer edge of the continental shelf, where there is an abrupt change
in slope as the continental shelf transitions to the deep-ocean basin.
53. What are the two fundamental physical characteristics of
the continental shelf that affect subtidal organisms?
The two physical characteristics of the continental shelf that affect
subtidal organisms are its relatively shallow depth and its proximity
54. How does temperature variability on the continental shelf
affect subtidal organisms?
Because of its shallow depth, temperature varies from place to place
in the subtidal zone, compared to the deep ocean. This is important
because temperature is one of the most important factors determining
the distribution of marine organisms. As a result, the kinds of organisms
found in the subtidal zone vary tremendously from pole to equator. Additionally,
there are many more individuals of any given species in the equatorial
55. How do waves and currents on the continental shelf affect
Because of its shallow depth, waves and currents have a much bigger
impact in the subtidal zone than they do in the deep ocean. Tidal currents
and wind waves affect the bottom, to depths of over 600 feet. They also
prevent stratification of the water column. Lack of stratification keeps
salinity, temperature, and nutrients relatively constant from top to
bottom on the continental shelf.
56. Review: What is stratification?
Stratification is the separation of the water column
into layers, with the densest, coldest water at the bottom, which prevents
the mixing of nutrient-rich deep water with the less dense, warmer upper
57. How does nutrient availability affect subtidal organisms?
Because of the shallow depth, nutrients do not concentrate on the bottom
and are available to organisms throughout to water column and on the
bottom. Because of the proximity of the continental shelf to the continents,
nutrient levels are high. Rivers and flooding from rainfall bring nutrients
into the subtidal zone. As a result, the subtidal zone is very productive.
58. What is sedimentation?
Sedimentation is the settling of sediment particles
from the water on to the bottom. Most of the sediment on the continental
shelf is lithogenous (sediment that comes from the
physical or chemical breakdown of rocks on land).
59. How do the abundant phytoplankton and sediment in the water
in the subtidal zone affect subtidal organisms?
The abundant phytoplankton and sediment in the water in the subtidal
zone makes the water murky—it is responsible for the greenish
color of continental shelf water (compared to the deep blue of the deep
ocean). Light does not penetrate as deeply through the murky water,
which reduces the depth at which primary producers can carry out photosynthesis.
60. What are the two types of subtidal communities that are
determined by substrate type?
The two types of subtidal communities that are determined by substrate
type are soft-bottom and rocky (or hard-bottom) communities.
61. Which are more common on the continental shelf: soft-bottom
or rocky bottom substrates?
Soft-bottom substrates are more common on the continental shelf.
62. What are the factors that influence soft-bottom communities
in the subtidal zone?
Sediment particle size, the stability of the sediments, light, and temperature
all are important influences on soft-bottom communities in the subtidal
63. What type of organisms dominates soft-bottom communities
in the subtidal zone?
Infauna dominate soft-bottom communities in the subtidal zone. There
are some mobile epifauna, but sessile epifauna are extremely rare due
to the lack of hard substrates for attachment.
64. How does the number of species (the diversity) of soft-bottom
subtidal communities compare with soft-bottom intertidal communities?
Mostly because conditions are less extreme and less variable in the
subtidal zone, the number of species in soft-bottom subtidal communities
is higher than the number of species in soft-bottom intertidal communities.
65. What is a patchy distribution?
When organisms occur in distinct clumps or patches they are said to have
a patchy distribution. In the soft-bottom subtidal
zone, there often are patches of different types of sediment that create
slightly different habitats for organisms and are responsible for the
66. What is an unvegetated community? Describe primary production
in unvegetated communities.
Communities that lack significant amounts of sea weeds or sea grasses
are unvegetated communities. This is a defining characteristic
of these communities. Primary producers in unvegetated communities typically
are diatoms and other microscopic algae and bacteria that grow on soft
sediments. Benthic primary production in unvegetated communities typically
is very low. Nearly all the primary production occurs in the water column
67. Why is detritus an important food source for benthic organisms
in the unvegetated subtidal communities?
Because there is little benthic primary production, most organisms depend
on detritus as a food source. The meiofauna and larger benthic organisms
feed on detritus. Most of the larger benthic organisms are deposit feeders,
with worms being the most common. Other deposit feeders include echinoderms
such as sea urchins, heart urchins, and sand dollars. Suspension feeders
also depend on detritus suspended in the water column and include many
different types of clams, worms, and amphipods.
68. What factors influence the distribution of suspension and
deposit feeders in the unvegetated soft-bottom subtidal zone?
Substrate and competitive exclusion influence the distribution of suspension
and deposit feeders in the unvegetated soft-bottom subtidal zone. Deposit
feeders are more common in muddy substrates, whereas suspension feeders
are more common in sandier sediments. The types of organisms already
present also influence the establishment of new types of organisms.
For example, deposit feeders, which are constantly disturbing or bioturbating
the muddy sediment as they feed, exclude suspension feeders, which mostly
live in tubes (that are destroyed by the bioturbation).
69. Briefly describe the predators in unvegetated, soft-bottom
Predators in unvegetated, soft-bottom subtidal communities include snails,
sea stars, crabs, and many other invertebrates that typically either
burrow through the sediment to get their prey or catch it on the surface.
Demersal (or bottom-dwelling) fishes also are important predators and
include rays, skates, flounders, halibuts, and many others. Pelagic
(or swimming) fishes and squid and marine mammals such as grey whales
and walruses are also important predators.
70. What is a seagrass bed? Describe primary production in a
Seagrass beds are soft-bottomed areas that are carpeted
by dense forests of sea grasses. Sea grasses are grass-like, flowering
plants that are adapted to a marine lifestyle. Seagrass beds develop
best in sheltered, shallow water close to the coast. Seagrass beds contain
a very high plant biomass and have the highest benthic primary production
in the subtidal zone.
71. Review: What is biomass?
Biomass is the total weight of living organisms in
72. Describe how sea grasses grow in soft-bottom areas.
Sea grasses have true roots and a network of underground stems that
anchor them in place. The roots and stems help to stabilize the soft
sediment and reduces turbulence and erosion. This helps to increase
the clarity of the water, allowing for high rates of photosynthesis.
73. Describe epiphytes in seagrass beds.
Epiphytes are small algae that grow on the surface
of seagrass leaves (and other surfaces). They are photosynthetic and
contribute to the high levels of primary production in seagrass beds.
Some epiphytic cyanobacteria are nitrogen fixers that release nutrients
in the form of nitrogen compounds.
74. Describe herbivores in seagrass beds.
Herbivores in seagrass beds include sea turtles, manatees, sea urchins,
and some fishes.
75. Which is a more important food source in seagrass beds:
plants or detritus?
The plants in seagrass beds are not heavily grazed but produce a lot
of detritus that is used by deposit feeders and exported to other communities.
76. What are the two types of communities associated with hard
bottom areas in the subtidal?
The two types of communities associated with hard-bottom areas in the
subtidal are rocky bottom communities and kelp forest communities.
77. What are the most conspicuous inhabitants of rocky bottom
Seaweeds of all types, shapes, and colors are the most conspicuous inhabitants
of rocky bottom communities in the subtidal. Many are encrusting or
have a holdfast to anchor them in place.
78. Review: What is a holdfast? What is an encrusting seaweed?
The holdfast is a root-like structure in seaweeds that
anchors the body of the seaweed (the thallus) to the
substrate. Encrusting seaweeds are those that grow
as a thin layer over rocks, such as some of the coralline red algae.
79. What is the limiting factor in rocky bottom communities
in the subtidal?
Space is the limiting factor in rocky bottom communities in the subtidal,
with intense competition between and among seaweeds and sessile organisms.
80. Review: What are kelp?
Kelp are a group of large brown seaweeds that live
in relatively cold water and are restricted to temperate and subpolar
regions. When they occur in dense numbers and their fronds float flat at the
surface in a thick mat, the result is a kelp forest.
81. What three physical factors have a major influence in kelp communities?
Temperature is a very important physical factor in kelp communities.
Nutrients and depth are also important.
82. Describe the optimal temperature conditions for kelp.
Kelp prefer cold, nutrient-rich water and are found mostly along the
sides of continents that have currents that flow from the poles towards
83. Describe the optimal depth for kelp.
Kelp will grow in water as deep as light availability allows. This can
be up to 40 m for some species. Their holdfast anchors them to the bottom
and their fronds float at the sunlit surface.
84. Why are kelp fragile?
Because they are so large and so long, kelp are easily torn from the
bottom. Kelp do not do well in areas with heavy wave action and may
be damaged by storms.
85. How fast do kelp grow?
Kelp can grow very fast, as fast as 50 cm/day (20 in/day).
86. Describe the primary productivity in kelp forests.
Kelp forests are very productive, with rates of primary production reaching
levels of close to 2,000 gC/m2/year.
87. What effect do sea urchin “outbreaks” have on
Sea urchins are the most important grazers in kelp communities. Occasionally,
sea urchin populations “explode,” and the sea urchins completely
clear away large areas of kelp, leaving behind “urchin deserts.”
88. How might sea otter populations be tied to sea urchin “outbreaks”?
Sea otters eat sea urchins. The decline in sea otter populations in
some areas, such as the Aleutian Islands, may have allowed sea urchin
populations to explode in numbers.
89. How might orca populations be tied to sea urchin "outbreaks"?
Declining sea otter populations in some areas may be related
to increased predation by orca. Orca prefer seals and sea lions, but
they have become scarce due to overfishing of their food supply (bottom
fishes such as pollock). So, orca have switched to sea otters; fewer
sea otters leads to explosions in sea urchin populations and then less
How might the decrease in drifting algae off the coast of southern California
be tied to sea urchin “outbreaks”?
Sea urchins prefer to feed on drifting seaweeds. Sewage pollution and
warmer water temperatures off the coast of southern California has led
to a decline in drifting seaweeds. This may have caused the sea urchins
to switch to feeding on kelp.
91. What are three other possible explanations for declines
in kelp forests?
Removal of abalones, which compete with sea urchins for space, may lead
to sea urchin “outbreaks”; high survival rates for sea urchin
larvae due to favorable temperatures or abundance of food may lead to
sea urchin “outbreaks”; and climate variability, such as
El Niño and La Niña, is associated with storms and warm
water temperatures, both of which may cause kelp mortality.
92. See the map at the back of your textbook.