OG-34,问什么珊瑚礁会减少？

珊瑚礁为什么会下降？以下红色为文章reasoning line, 但是我有个问题。。。

Passage 34

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.

人类活动---〉水环境的富营养化---〉海藻的繁殖以及食草类动物的改变---〉珊瑚礁减少（其中包括由于珊瑚礁对营养的细微变化的敏感性）

我的问题是：海藻类繁殖上升，导致珊瑚礁下降，是因为

1，海藻类有大量食物，而不需要用珊瑚的新陈代谢物为其包括珊瑚制造食物，由于食物短缺导致珊瑚下降呢？还是

2，海藻类繁殖上升导致水中氧气减少，由于缺氧导致海生物包括珊瑚死亡呢？