Answer:
Phytoplankton
Step-by-step explanation:
A decrease in the number of phytoplankton in this ecosystem will most significant affect the population of other organisms here.
The phytoplankton are the primary producers in an ecosystem and this implies that they produce their food using inorganic chemical sources. It is the food they produce that the primary consumers and other forms of consumers will depend on for their nourishment.
A decline in the population of the phytoplankton will adversely affect the population of organisms in this ecosystem.