The correct answer is - nutrients.
The cold water is very rich in nutrients, and those nutrients are crucial for the survival of the small organisms as they are their primary food source. When El Nino occurs though, the water is becoming warmer and stops the up-welling of the cold water, which also means that the nutrients remain deep with it. As a result, the small organisms are left without their primary food sources, thus they struggle to survive, and even die out in the region on a large scale.
