Which one is the answer?

Phytoplankton

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.

kelp are large brown algae that live in cool, relatively shallow waters close to the shore. they grow in dense groupings much like a forest on land. these underwater towers of kelp provide food and shelter for thousands of fish, invertebrates, and marine mammal species.

e) all of the above

and after water is usually warmed by the sun the water temperature raises and then evaporates .