| 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 34Coral 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？ | 
