Ecology

Step 1: Understanding El Niño phenomenon.
El Niño is a climatic phenomenon that affects the Pacific Ocean's currents, particularly the Peru current. During this event, the normal cold water of the north-flowing Peru current is replaced by warmer water moving southward, disrupting marine ecosystems and weather patterns.

Step 2: Analyzing the options.
(A) cold water of the north flowing Peru current is displaced by a low-nutrient warm southward current: Correct — This is the essence of El Niño, where warm waters replace cold waters, leading to nutrient loss in marine ecosystems.
(B) warm water of the north flowing Peru current is displaced by a low-nutrient cold southward current: Incorrect — This does not describe the typical El Niño event.
(C) cold water of the south flowing Peru current is displaced by a warm northward current rich in nutrients: Incorrect — This is not the pattern of water displacement during El Niño.
(D) warm water of the south flowing Peru current is displaced by a cold northward current rich in nutrients: Incorrect — This does not describe El Niño's effect.

Step 3: Conclusion.
The correct answer is (A) cold water of the north flowing Peru current is displaced by a low-nutrient warm southward current.