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Unit 12 Textbook Answer Key

Chapter 12
1. What are estuaries?

Estuaries are semi-enclosed areas where fresh water and sea water meet and mix. This typically occurs where rivers enter the ocean.
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 face?
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 origins?
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 margins?
Estuaries are more common and more extensive along passive continental margins.
11. Review: What are passive continental margins? What are active continental margins?
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 estuaries?
Salinity and substrate are the two most important physical factors affecting estuaries.
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 estuaries?
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 salinity.
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 osmotic balance?
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 muds.
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 more common.
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 estuaries?
The most important predators on the mudflats in estuaries are fishes and birds.
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.
Chapter 13

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 to land.
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 subtidal zone.
55. How do waves and currents on the continental shelf affect subtidal organisms?
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 layer.
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 zone.
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 patchy distributions.
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 by phytoplankton.
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 subtidal communities.
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 bed.
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 a community.
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 communities?
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 the tropics.
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 kelp communities?
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 kelp.
90. 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.

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