An increase in the number of phytoplankton in an aquatic ecosystem is always beneficial.

An increase in phytoplankton is not always beneficial. Too many phytoplankton can form algal blooms that block sunlight and, when they die and decompose, reduce dissolved oxygen in the water. This can harm or kill other aquatic organisms, so the statement is false; the correct choice is “F” (false).

AI-written and checked against the verified answer.