14. many pacific atolls consist of a ring of low coral islands surrounding a central ________.
15. narrow ocean bays fed by a large freshwater river at one end become more ________ as one approaches the ocean end of the bay.
16. in general, seawater temperature ________ from the surface to the bottom.
17. in general, the speed of sound in seawater ________ from the surface to the bottom.
18. because clams, corals, and anemones are attached to or live on the ocean floor, they are considered ________ organisms.
19. organism that are not attached to or do not live on the ocean floor are considered ________ organisms.
20. below the photic zone in the ocean, there are no organisms that use ________ for nourishment.
21. carbon in the atmosphere is typically in the form of ________.
22. cyanobacteria in the oceans are a key part of the ________ cycle.
23. the main natural source of fixed nitrogen that does not involve living things is ________.

Question

Accepted Answer

### Question 14
# Brief Explanations:
Pacific atolls typically have a ring of coral islands around a central lagoon. This is a key feature of atoll geography in oceanic environments.
# Answer:
lagoon

### Question 15
# Brief Explanations:
A narrow bay fed by a freshwater river will have less salt (more brackish, but as we approach the ocean end, the salinity increases? Wait, no—wait, the bay is fed by a freshwater river at one end, so at the river end, it's fresh or brackish, and as we move toward the ocean end, the water becomes more saline (salty) because of the ocean's influence. Wait, but the question is "become more ______ as one approaches the ocean end of the bay". So the water at the ocean end is more saline (salty) than at the river end. So the answer is "saline" or "salty". But maybe "saline" is the term. Alternatively, maybe "turbid"? No, salinity is the key here. Wait, the bay is narrow, fed by a freshwater river (so low salinity at river end), and as we go toward the ocean, the ocean water mixes in, so salinity increases. So the blank is "saline" (or "salty").
# Answer:
saline (or salty)

### Question 16
# Brief Explanations:
In general, seawater temperature decreases from the surface to the bottom (the thermocline is where the temperature drops rapidly, but overall, from surface to deep, temperature decreases because the surface is heated by the sun, and deeper waters are colder).
# Answer:
decreases

### Question 17
# Brief Explanations:
The speed of sound in seawater generally increases with depth (because pressure increases, and sound speed in water is affected by temperature, salinity, and pressure. As depth increases, pressure increases, and even though temperature may decrease, the effect of pressure can lead to an overall increase in sound speed with depth, especially below the thermocline). Wait, actually, the speed of sound in seawater is given by the formula $ c = 1449.2 + 4.6T - 0.055T^2 + 0.00029T^3 + (1.34 - 0.01T)(S - 35) + 0.016D $, where $ T $ is temperature, $ S $ is salinity, and $ D $ is depth (in meters). So as depth $ D $ increases, the term $ 0.016D $ increases the speed. So in general, from surface to bottom (increasing depth), the speed of sound in seawater increases. Wait, but I thought earlier that temperature decreases with depth. But the formula shows that depth (pressure) also affects it. So the correct answer is that the speed of sound in seawater increases from the surface to the bottom (due to increasing pressure, even with decreasing temperature, the pressure effect dominates in deeper waters). Wait, but maybe in the upper layers, temperature is the main factor, but overall, from surface to bottom (deep), the speed increases. So the answer is "increases".
# Answer:
increases

### Question 18
# Brief Explanations:
Organisms attached to or living on the ocean floor are called benthic organisms (benthos). So clams, corals, anemones attached to the ocean floor are benthic.
# Answer:
benthic

### Question 19
# Brief Explanations:
Organisms not attached to or not living on the ocean floor are called pelagic organisms (they live in the water column, not on the bottom).
# Answer:
pelagic

### Question 20
# Brief Explanations:
The photic zone is where light penetrates, so organisms in the photic zone can use photosynthesis (using light energy) for nourishment (autotrophs). Below the photic zone, there is no light, so no organisms can use photosynthesis for nourishment (since photosynthesis requires light).
# Answer:
photosynthesis (or light)

### Question 21
# Brief Explanations:
Carbon in the atmosphere is typically in the form of carbon dioxide ($ \text{CO}_2 $).
# Answer:
carbon dioxide (or $ \text{CO}_2 $)

### Question 22
# Brief Explanations:
Cyanobacteria in the oceans are key in the nitrogen cycle (they can fix atmospheric nitrogen, converting $ \text{N}_2 $ to ammonia, etc.) or the carbon cycle? Wait, cyanobacteria are photosynthetic, so they are part of the carbon cycle (fixing carbon via photosynthesis), but also the nitrogen cycle (nitrogen fixation). But the key part—cyanobacteria in oceans are a key part of the nitrogen cycle (they are important nitrogen fixers in the ocean). Wait, but also carbon. But the most common is nitrogen cycle, because they perform nitrogen fixation. Wait, no—cyanobacteria are photosynthetic, so they take in $ \text{CO}_2 $ and release $ \text{O}_2 $, so carbon cycle. But some cyanobacteria also fix nitrogen. However, the question is "key part of the ______ cycle". The answer is likely "nitrogen" (because of nitrogen fixation) or "carbon". Wait, let's think: Cyanobacteria in oceans: they are primary producers, so carbon cycle (fixing carbon via photosynthesis). But they also fix nitrogen (converting $ \text{N}_2 $ to ammonia), so nitrogen cycle. Which is more key? The nitrogen cycle, because they are important nitrogen fixers in the ocean (especially in oligotrophic waters). So the answer is "nitrogen" or "carbon". Wait, maybe nitrogen. Let me check: Cyanobacteria (like Trichodesmium) are important nitrogen fixers in the ocean, so they are key in the nitrogen cycle. So the answer is "nitrogen".
# Answer:
nitrogen (or carbon, but nitrogen is more specific for cyanobacteria's nitrogen fixation)

### Question 23
# Brief Explanations:
The main natural source of fixed nitrogen (nitrogen in a form usable by organisms, like ammonia, nitrate) that does not involve living things is lightning (which fixes atmospheric $ \text{N}_2 $ into nitrogen oxides, which then dissolve in rain and become fixed nitrogen in the soil or ocean). Alternatively, volcanic activity, but lightning is the main natural non-biological source of fixed nitrogen.
# Answer:
lightning

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