Question

what two effects might occur if most of the phytoplankton living in the world's oceans were destroyed?

Answer 1

Well for one the Oceans food chain starts with the sun and then microscopic organisms such as phytoplankton, and if they were destroyed the fish and other small creatures would eventually die off and then larger animals would too. Then that could lead to not much sea life, or sea food left for other people and animals

Answer 2

Reason #1: All the animals who feed on it could die 
Reason #2: We wouldn't have half of the oxygen we breathe since phytoplankton releases oxygen into the air and if they died off, the oxygen they used to make will cease to exist.