What would most likely happen if the amount of phytoplankton in the ocean were reduced?

The ocean food chain would shrink, as the phytoplankton are the base of the food chain

Explanation:

Atmospheric carbon dioxide levels would increase because phytoplankton are plants that use carbon dioxide in photosynthesis to grow.

Atmospheric oxygen levels would decrease since there would be more carbon dixodie (the carbon dioxide is converted to oxygen and carbohydrates by photosynthesis).

Oceanic pH levels would decrease, that is the ocean would become more acid. This would be due to higher levels of carbon dioxide, which makes carbonic acid when it dissolves.