Coral reefs are one of the most fragile, biologicallycomplex, and diverse marine ecosystem

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.

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