Primary producers are the largest numbers of organisms in a biosphere. Primary producers are plants. Although we tend to think of plants as the trees and grass we see out our window or the tropical rainforests we see in the news, the largest number of plants are single celled plants found in the oceans.
Being primary producers, plants directly convert energy from the sun along with water and carbon dioxide to produce carbohydrates and oxygen. Consequently the largest producer of oxygen which animals need to survive come from aquatic plants. It is important to maintain aquatic ecosystems in order to protect the major oxygen producers which terrestrial creatures need. Without that oxygen, terrestrial ecosystems would eventually die off.