Question

how are phytoplankton, tiny plants that float in the near - surface waters of the ocean, affected by el nino conditions?
a downwelling decreases their numbers.
b upwelling increases their numbers.
c changes in wind direction push them to the far western side of the ocean.
d changes in wind direction push them southward.

Explanation:

El Niño causes reduced upwelling (since trade winds weaken, less cold, nutrient - rich water from deep rises). Phytoplankton rely on nutrients from upwelling. Downwelling (opposite of upwelling, where surface water moves down) would decrease nutrient availability, thus decreasing phytoplankton numbers. Option A says downwelling decreases their numbers, which matches El Niño's impact on phytoplankton. Option B is wrong because upwelling usually increases phytoplankton (but El Niño reduces upwelling). Options C and D are about wind - driven movement direction, not the main factor for phytoplankton numbers affected by El Niño (which is nutrient availability via upwelling/downwelling).

Answer:

A. Downwelling decreases their numbers.