Coral reefs are one of the most fragile, biologically complex, and diverse marine ecosystem on Earth. This ecosystem is one of the fascinating paradoxes of the bio- sphere: how do clear, and thus nutrient-poor, waters sup- (5) port such prolific and productive communities? Part of the answer lies within the tissues of the corals themselves. Symbiotic cells of algae known as zooxanthellae carry out photosynthesis using the metabolic wastes of the coral thereby producing food for themselves, for their corals, (10) hosts, and even for other members of the reef community. This symbiotic process allows organisms in the reef com- munity to use sparse nutrient resources efficiently.

Unfortunately for coral reefs, however, a variety of human activities are causing worldwide degradation of (15) shallow marine habitats by adding nutrients to the (water. Agriculture, slash-and-burn land clearing, sewage disposal and manufacturing that creates waste by-products all increase nutrient loads in these waters. Typical symptoms of reef decline are destabilized herbivore populations and (20) an increasing abundance of algae and filter-feeding animals. Declines in reef communities are consistent with observa- tions that nutrient input is increasing in direct proportion to growing human populations, thereby threatening reef com- (25) munities sensitive to subtle changes in nutrient input to their waters.

1. The passage is primarily concerned with

(A) describing the effects of human activities on algae in coral reefs

(B) explaining how human activities are posing a threat to coral reef communities

(C) discussing the process by which coral reefs deteriorate in nutrient-poor waters

(D) explaining how coral reefs produce food for themselves

(E) describing the abundance of algae and filter-feeding animals in coral reef areas
2. The passage suggests which of the following about coral reef communities?

(A) Coral reef communities may actually be more likely to thrive in waters that are relatively low in nutrients.

(B) The nutrients on which coral reef communities thrive are only found in shallow waters.

(C) Human population growth has led to changing ocean temperatures, which threatens coral reef communities.

(D) The growth of coral reef communities tends to destabilize underwater herbivore populations.

(E) Coral reef communities are more complex and diverse than most ecosystems located on dry land

3. The author refers to "filter-feeding animals" (line 20) in order to

(A) provide an example of a characteristic sign of reef deterioration

(B) explain how reef communities acquire sustenance for survival

(C) identify a factor that helps herbivore populations thrive

(D) indicate a cause of decreasing nutrient input in waters that reefs inhabit

(E) identify members of coral reef communities that rely on coral reefs for nutrients

4. According to the passage, which of the following is a factor that is threatening the survival of coral reef communities?

(A) The waters they inhabit contain few nutrient resources.

(B) A decline in nutrient input is disrupting their symbiotic relationship with zooxanthellae

(C) The degraded waters of their marine habitats have reduced their ability to carry out photosynthesis

(D) They are too biologically complex to survive in habitats with minimal nutrient input.

(E) Waste by-products result in an increase in nutrient input to reef communities.

5. It can be inferred from the passage that the author describes coral reef communities as paradoxical most likely for which of the following reasons?
　   (A) They are thriving even though human activities have depleted the nutrients in their environment.

(B) They are able to survive in spite of an over- abundance of algae inhabiting their waters.

(C) They are able to survive in an environment with limited food resources.

(D) Their metabolic wastes contribute to the degra- dation of the waters that they inhabit.

(E) They are declining even when the water sur- rounding them remains clear